Weather Definitions


Index:

Local Weather Facts

A B C D E F G H I J K L M
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Local Weather Facts:

RAIN FALL IN SOUTHERN MISSISSIPPI
In southern Mississippi, typically, the heaviest rain falls during the spring thunderstorm season in our state. But we can get the heaviest rains during hurricanes and tropical storms in the summer and fall months. In Ellisville, MS we average 57.43 inches of rain a year. The greatest rainfall in a 24 hour period was 8.00 inches. The average number of days with over .1 inch of rain is 74.35 days per year. The humidity is a big factor in the summer weather in southeast Mississippi, the average year-round humidity stands at 75.09 percent! On the other hand, we seldom have snow in this part of Mississippi. The average annual snowfall is an amazing .41 of an inch! And we only average .05 days per year of snow.

SEVERE WEATHER IN MISSISSIPPI
Most of the severe weather is seen in Mississippi is caused by micro bursts--not tornadoes. A "micro burst" is a small area of rapidly descending air beneath a thunderstorm. When the descending air hits the ground, it quickly spreads out in all directions, causing very strong, straight-line winds. These winds are commonly as strong as 40-60 mph but can exceed 100 mph at times. Micro bursts occur over a rather small space-scale, typically the area affected is less than 2.5 miles in diameter.

ELLISVILLE TORNADO HISTORY
Ellisville-area historical tornado activity is 229% greater than the overall U.S. average. On 2/28/1987, a category F4 (max. wind speeds 207-260 mph) tornado near Ellisville killed 6 people and injured 350 people and caused between $5,000,000 and $50,000,000 in damages.

HEAT RECORDS IN MISSISSIPPI
The hottest temperature ever recorded in Ellisville, MS showed that the mercury climbed to 108° on August 15, 2007. Two other days during that week in August 2007 the temperature reached 108. The hottest temperature ever recorded in Mississippi happened on July 29, 1930 in Holly Springs, MS, when the temperature topped out at 115°.

COLD TEMPS IN MISSISSIPPI
Ellisville, MS recorded its coldest temperature of 3° on December 24, 1989. The coldest temperature recorded in the state of Mississippi was -19°, set on January 30, 1966 in Corinth, MS.

Index
A:

ABSOLUTE HUMIDITY
A type of humidity that considers the mass of water vapor present per unit volume of space. Also considered as the density of the water vapor. It is usually expressed in grams per cubic meter.

ABSOLUTE INSTABILITY
When the lapse rate of a column of air is greater than the dry adiabatic lapse rate. The term absolute is used because this applies whether or not the air is dry or saturated.
Related term: instability

ABSOLUTE TEMPERATURE SCALE
A temperature scale with a freezing point of +273K (Kelvin) and a boiling point of +373K.
Related term: Kelvin Temperature Scale

ABSOLUTE ZERO
Considered to be the point at which theoretically no molecular activity exists or the temperature at which the volume of a perfect gas vanishes. The value is 0 Kelvin, -273.15 Celsius and -459.67 Fahrenheit.

ABSORPTION
The process in which incident radiant energy is retained by a substance. The absorbed radiation is then transformed into molecular energy.

ABYSSAL PLAIN
The flat, gently sloping or nearly level region of the sea floor.

ADVECTION
The horizontal transfer of any property in the atmosphere by the movement of air (wind). Examples include heat and moisture advection.

ADVECTION FOG
Fog that develops when warm moist air moves over a colder surface, cooling that air to below its dew point.
Related terms: Arctic Sea Smoke and sea fog

ADVISORY
Statements that are issued by the National Weather Service for probable weather situations of inconvenience that do not carry the danger of warning criteria, but, if not observed, could lead to hazardous situations. Some examples include snow advisories stating possible slick streets, or fog advisories for patchy fog condition causing temporary restrictions to visibility.

AFOS
Acronym for Automation of Field Operations and Services. It is the computer system that links National Weather Service offices together for weather data transmission.

AIR
This is considered the mixture of gases that make up the earth's atmosphere. The principal gases that compose dry air are Nitrogen (N2) at 78.09%, Oxygen (O2) at 20.946%, Argon (A) at 0.93%, and Carbon Dioxide (Co2) at 0.033%. One of the most important constituents of air and most important gases in meteorology is water vapor (H2O).

AIR MASS
An extensive body of air throughout which the horizontal temperature and moisture characteristics are similar.

AIR MASS THUNDERSTORM
A thunderstorm that is produced by convection within an unstable air mass through an instability mechanism. Such thunderstorms normally occur within a tropical or warm, moist air mass during the summer afternoon as the result of afternoon heating and dissipate soon after sunset. Such thunderstorms are not generally associated with fronts and are less likely to become severe than other types of thunderstorms. However, that does not preclude them from having brief heavy downpours.

ALBEDO
The ratio of the amount of radiation reflected from an object's surface compared to the amount that strikes it. This varies according to the texture, color, and expanse of the object's surface and is reported in percentage. Surfaces with high albedo include sand and snow, while low albedo rates include forests and freshly turned earth.
Related term: Dave's Dictionary

ALBERTA CLIPPER
A fast moving, snow-producing weather system that originates in the lee of the Canadian Rockies. It moves quickly across the northern United States, often bring gusty winds and cold Arctic air.

ALTIMETER
An instrument used to determine the altitude of an object with respect to a fixed level. The type normally used by meteorologists measures the altitude with respect to sea level pressure.

ALTIMETER SETTING
The pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea level of an aircraft on the ground at the location for which the value was determined.

ALTITUDE
In meteorology, the measure of a height of an airborne object in respect to a constant pressure surface or above mean sea level.

ALTOCUMULUS
Composed of flattened, thick, gray, globular masses, this middle cloud genus is primarily made of water droplets. In the mid-latitudes, cloud bases are usually found between 8,000 and 18,000 feet. A defining characteristic is that it often appears as a wavy billowy layer of cloud, giving it the nickname of "sheep" or "woolpack" clouds. Sometimes confused with cirrocumulus clouds, its elements (individual clouds) have a larger mass and cast a shadow on other elements. It may form several sub-types, such as altocumulus castellanus or altocumulus lenticularis. Virga may also fall from these clouds.

ALTOCUMULUS CASTELLANUS
A middle cloud with vertical development that forms from altocumulus clouds. It is composed primarily of ice crystals in its higher portions and characterized by its turrets, protuberances, or crenelated tops. Its formation indicates instability and turbulence at the altitudes of occurrence.

ALTOSTRATUS
This middle cloud genus is composed of water droplets, and sometimes ice crystals, In the mid-latitudes, cloud bases are generally found between 15,000 and 20,000 feet. White to gray in color, it can create a fibrous veil or sheet, sometimes obscuring the sun or moon. It is a good indicator of precipitation, as it often precedes a storm system. Virga often falls from these clouds.

AMERICAN METEOROLOGICAL SOCIETY
An organization whose membership promotes the education and professional advancement of the atmospheric, hydrologic, and oceanographic sciences.
For further information, contact the AMS.
Related term: National Weather Association

ANABATIC WIND
A wind that is created by air flowing uphill. Valley breezes, produced by local daytime heating, are an example of these winds. The opposite of a katabatic wind.

ANEMOMETER
An instrument that measures the speed or force of the wind.
Related term: Dave's Dictionary

ANEROID BAROMETER
An instrument for measuring the atmospheric pressure. It registers the change in the shape of an evacuated metal cell to measure variations on the atmospheric pressure. The aneroid is a thin-walled metal capsule or cell, usually made of phosphor bronze or beryllium copper. The scales on the glass cover measure pressure in both inches and millibars. Related term: mercurial barometer

ANOMALOUS PROPAGATION
This refers to the non-standard propagation of a beam of energy, radio or radar, under certain atmospheric conditions, appearing as false (non-precipitation) echoes. May be referred to as A.P.

ANTICYCLONE
A relative pressure maximum. An area of pressure that has diverging winds and a rotation opposite to the earth's rotation. This is clockwise the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. It is the opposite of an area of low pressure, or a cyclone.
Related term: high pressure

ANVIL
The upper portion of a cumulonimbus cloud that becomes flat and spread-out, sometimes for hundreds of miles downstream from the parent cloud. It may look smooth or fibrous, but in shape, it resembles a blacksmith's anvil. It indicates the mature or decaying stage of a thunderstorm.

APHELION
The point on the earth's orbit that is farthest from the sun. Although the position is part of a 21,000 year cycle, currently it occurs around July, when the earth is about 3 million miles farther from the sun than at perihelion. This term can be applied to any other celestial body in orbit around the sun. It is the opposite of perihelion.

APOGEE
The point farthest from the earth on the moon's orbit. This term can be applied to any other body orbiting the earth, such as satellites. It is the opposite of perigee.

ARCTIC AIR MASS
An air mass that develops around the Arctic, it is characterized by being cold from surface to great heights. The boundary of this air mass is often defined by the Arctic front, a semi-permanent, semi-continuous feature. When this air mass moves from its source region, it may become more shallow in height as it spreads southward.

ARCTIC JET
The jet stream that is situated high in the stratosphere in and around the Arctic or Antarctic Circles. It marks the boundary of polar and arctic air masses.

ARCTIC SEA SMOKE
A type of advection fog that forms primarily over water when cold air passes across warmer waters.
Related term: steam fog

ARID
A term used for an extremely dry climate. The degree to which a climate lacks effective, life-promoting moisture. It is considered the opposite of humid when speaking of climates.

ASOS
Acronym for Automated Surface Observing System. This system is a collection of automated weather instruments that collect data. It performs surface based observations from places that do not have a human observer, or that do not have an observer 24 hours a day.

ASTRONOMICAL TWILIGHT
The time after nautical twilight has commenced and when the sky is dark enough, away from the sun's location, to allow astronomical work to proceed. It ends when the center of the sun is 18 below the horizon.
Related term: twilight

ATMOSPHERE
The gaseous or air portion of the physical environment that encircles a planet. In the case of the earth, it is held more or less near the surface by the earth's gravitational attraction. The divisions of the atmosphere include the troposphere, the stratosphere, the mesosphere, the ionosphere, and the exosphere.

ATMOSPHERIC PRESSURE
The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg).
Related term: barometric pressure

AURORA
It is created by the radiant energy emission from the sun and its interaction with the earth's upper atmosphere over the middle and high latitudes. It is seen as a bright display of constantly changing light near the magnetic poles of each hemisphere. In the Northern Hemisphere, it is known as the aurora borealis or Northern Lights, and in the Southern Hemisphere, this phenomena is called the aurora australis.

AUTUMN
The season of the year which occurs as the sun approaches the winter solstice, and characterized by decreasing temperatures in the mid-latitudes. Customarily, this refers to the months of September, October, and November in the North Hemisphere and the months of March, April, and May in the Southern Hemisphere. Astronomically, this is the period between the autumnal equinox and the winter solstice.

AVHRR
Acronym for Advanced Very High Resolution Radiometer. It is the main sensor on the U.S. polar orbiting satellites.

AVIATION WEATHER CENTER
As one of the National Centers for Environmental Prediction, it is the national center for weather information that is used daily by the Federal Aviation Administration, commercial airlines, and private pilots. It is entering a new phase of service, growing to accept global forecasting responsibilities.
For further information, contact the AWC, located in Kansas City, Missouri.

AWIPS
Acronym for Advanced Weather Interactive Processing System. It is the computerized system that processes NEXRAD and ASOS data received at National Weather Service Forecast Offices.

AZORES HIGH
A semi-permanent, subtropical area of high pressure in the North Atlantic Ocean that migrates east and west with varying central pressure. Depending on the season, it has different names. In the Northern Hemispheric winter and early spring, when the Icelandic Low dominates the North Atlantic, it is primarily centered near the Azores Islands. When it is displaced westward, during the summer and fall, the center is located in the western North Atlantic, near Bermuda, and is known as the Bermuda High. Related term: North Pacific High

Index
B:

BACKING
A counterclockwise shift in the wind direction in the Northern Hemisphere at a certain location. In the Southern Hemisphere, it is clockwise. This can either happen in the horizontal or the vertical (with height). For example, the wind shifts from the northeast to the north to the northwest. It is the opposite of veering.

BACKSCATTER
A radar echo that is reflected, or scattered, at 180 degrees to the direction of the incident wave. Also the scattering of radiant energy into space before it reaches the earth's surface.

BALL LIGHTNING
A relatively rare form of lightning consisting of a luminous ball, often reddish in color, which moves rapidly along solid objects or remains floating in mid-air.
Related term: globe lightning

BAROCLINITY
The state of stratification in a fluid in which surfaces of constant pressure intersect surfaces of constant density. Also known as baroclinicity. An example is the tight temperature gradient along the East Coast of the United States during the winter that gives rise to intense cyclogenesis.

BAROGRAPH
An instrument that continuously records a barometer's reading of atmospheric pressure.
Related term: aneroid barometer

BAROMETER
An instrument used to measure atmospheric pressure. Two examples are the aneroid barometer and the mercurial barometer.

BAROMETRIC PRESSURE
The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg).
Related term: atmospheric pressure

BATHYTHERMOGRAPH
A device used to obtain a record of temperature against depth (pressure) in the ocean. May be referred to as a B.T.

BEAUFORT WIND SCALE
A system of estimating and reporting wind speeds. It is based on the Beaufort Force or Number, which is composed of the wind speed, a descriptive term, and the visible effects upon land objects and/or sea surfaces. The scale was devised by Sir Francis Beaufort (1777-1857), hydrographer to the British Royal Navy.

BERMUDA HIGH
A semi-permanent, subtropical area of high pressure in the North Atlantic Ocean that migrates east and west with varying central pressure. Depending on the season, it has different names. When it is displaced westward, during the Northern Hemispheric summer and fall, the center is located in the western North Atlantic, near Bermuda. In the winter and early spring, it is primarily centered near the Azores Islands.
Related term: Azores High

BERNOULLI'S THEOREM
A statement of the conservation of energy for a steady, non viscous, incompressible level flow. It is an inverse relationship in which pressures are least where velocities are greatest. Theorized by Daniel Bernoulli (1700-1782), a Swiss mathematician and physicist.

BIOSPHERE
The transition zone between the earth and the atmosphere within which most terrestrial life forms are found. It is considered the outer portion of the geosphere and the inner or lower portion of the atmosphere.

BLACK BLIZZARD
A local term for a violent dust storm on the south-central Great Plains that darkens the sky and casts a pall over the land.
Related term: black roller

BLACK ICE
Thin, new ice on fresh or salt water that appears dark in color because of its transparency. Also refers to thin, transparent ice on road surfaces.

BLIZZARD
A severe weather condition characterized by low temperatures, winds 35 mph or greater, and sufficient falling and/or blowing snow in the air to frequently reduce visibility to 1/4 mile or less for a duration of at least 3 hours. A severe blizzard is characterized by temperatures near or below 10°F, winds exceeding 45 mph, and visibility reduced by snow to near zero.

BLOCKING HIGH
The development of a warm ridge or cutoff high aloft at high latitudes which becomes associated with a cold high at the surface, causing a split in the westerly winds. Such a high will move very slowly, tending to move westward during intensification and eastward during dissipation. It prevents the movement of migratory cyclones across its latitudes.
Related terms: cut-off high and Omega block

BLUE NORTHER
Refers to a swift-moving cold frontal passage in the southern Great Plains, marked by a dark, blue-black sky with strong wintry winds from the northwest or north and temperatures that may drop 20°F to 30°F in a few minutes. Related term: Texas Norther

BOULDER WIND
A local name referring to an extremely strong down slope wind in the Front Range of the Rocky Mountains near Boulder, Colorado.

BOUNDARY LAYER
The lowest layer of the earth's atmosphere, usually up to 3,300 feet, or one kilometer, from the earth's surface, where the wind is influenced by the friction of the earth's surface and the objects on it.
Related terms: surface boundary layer and friction layer

BOW ECHO
A radar echo signature often associated with severe thunderstorms, especially those that produce wind damage. It is bent outward in a "bow" shape.

BROKEN
The amount of sky cover for a cloud layer between 5/8ths and 7/8ths, based on the summation layer amount for that layer.

BUBBLE HIGH
A small high that may be created by precipitation and vertical instability associated with thunderstorm activity. A product of down drafts, it is relatively cold and often has the characteristics of a different air mass. Convergence along the leading edge of a bubble high may help form additional thunderstorms.
Related term: meso high

BUYS BALLOT'S LAW
Describes the relationship of the horizontal wind direction to the pressure distribution. In the Northern Hemisphere, if one stands with one's back to the wind, the pressure on one's left is lower than the pressure on one's right. It is reversed in the Southern Hemisphere. This law was named after the Dutch meteorologist, Buys Ballot, who developed the formula in 1857.

BWER
Acronym for Bounded Weak Echo Region. Refers to radar echo signatures with low reflectivity in the center, surrounded by higher reflectivity. It is associated with strong up drafts and is found in the inflow region of a thunderstorm.
Related term: vault

Index
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CALM
Atmospheric conditions devoid of wind or any other air motion. In oceanic terms, it is the apparent absence of motion of the water surface when there is no wind or swell.

CALORIE
In meteorology, it is the amount of heat required to raise the temperature of one (1) gram of water one (1) degree Celsius. It is a unit of heat energy.

CAP
Composed of a layer of warmer, dryer air aloft which may suppress or delay the development of thunderstorms. As an air parcel rises, it becomes cooler relative to the ambient, or surrounding, air in the cap and therefore, less buoyant and unable to rise further. Also referred to as a lid.

CAPE
Acronym for Convective Available Potential Energy. The amount of energy available to create convection, with higher values increasing the possibility for severe weather.

CATALINA EDDY
A weak low pressure circulation that may form off the Southern California coast.

CEILING
The lowest cloud layer that is reported as broken or overcast. If the sky is totally obscured, then it is the height of the vertical visibility.
Related terms: measured ceiling and variable ceiling

CEILING LIGHT
An instrument consisting of a drum and an optical system that projects a narrow vertical beam of light onto a cloud base.

CEILOMETER
An instrument that is used to measure the angular elevation of a projected light on the base of a cloud. It measures the angle of the cloud base included by the observer (or machine), the ceiling light and the illuminated spot on the cloud.

CELESTIAL EQUATOR
The projection of the plane of the geographical equator upon the celestial sphere.

CELESTIAL SPHERE
The apparent sphere of infinite radius having the earth as its center. All heavenly bodies (planets, stars, etc.) appear on the "inner surface" of this sphere and the sun moves along the ecliptic.

CELSIUS TEMPERATURE SCALE
A temperature scale where water at sea level has a freezing point of 0C (Celsius) and a boiling point of +100C. More commonly used in areas that observe the metric system of measurement. Created by Anders Celsius in 1742. In 1948, the Ninth General Conference on Weights and Measures replaced "degree centigrade" with "degree Celsius."
Related term: Centigrade

CENTRAL PRESSURE
The atmospheric pressure at the center of a high or low. It is the highest pressure in a high and the lowest pressure in a low, referring to the sea level pressure of the system on a surface chart.

CENTRIFUGAL FORCE
The apparent force in a rotating system that deflects masses radially outward from the axis of rotation. This force increases towards the equator and decreases towards the poles.

CENTRIPETAL FORCE
The force required to keep an object moving in a curved or circular path. It is directed inwards toward the center of the curved path.

CHEMOSPHERE
A vaguely defined region of the upper atmosphere in which photo chemical reactions take place. It includes the top of the stratosphere, all of the mesosphere, and sometimes the lower part of the thermosphere.

CHINOOK
A type of foehn wind. Refers to the warm down slope wind in the Rocky Mountains that may occur after an intense cold spell when the temperature could rise by 20F to 40F in a matter of minutes.
Related term: Snow Eater

CHROMOSPHERE
A thin layer of relatively transparent gases above the photosphere of the sun. It is observed best during a total eclipse of the sun.

CIRCULATION
The flow or motion of a fluid in or through a given area or volume. In meteorology, it is used to describe the flow of air as it moves around a pressure system in the atmosphere. It describes smaller patterns in semi-permanent pressure systems as well as the relatively permanent global currents of air. In oceanic terms, it is used to describe a water in current flow within a large area, usually a closed circular pattern such as in the North Atlantic.

CIRCULATION CELLS
Large areas of air movement created by the rotation of the earth and the transfer of heat from the equator toward the poles. Circulation is confined to a specific region, such as the tropics, temperate, or polar, that influences the type of weather prevailing there.

CIRRIFORM
Clouds composed of small particles, mostly ice crystals. Because the particles are fairly widely dispersed, this usually results in relative transparency and whiteness, often producing a halo phenomena not observed in other clouds forms. These clouds generally have bases above 20,000 feet in the mid-latitudes, and are classified as high clouds. They include all varieties of cirrus, cirrocumulus, and cirrostratus clouds.

CIRROCUMULUS
A cirriform cloud with vertical development, appearing as a thin sheet of small white puffs which give it a rippled effect. It often creates a "mackerel sky", since the ripples may look like fish scales. Sometimes it is confused with altocumulus, however, it has smaller individual masses and does not cast a shadow on other elements. It is also the least common cloud type, often forming from cirrus or cirrostratus, with which it is associated in the sky.

CIRROSTRATUS
A cirriform cloud that develops from cirrus spreading out into a thin layer, creating a flat sheet like appearance. It can give the sky a slightly milky or veiled look. When viewed from the surface of the earth, these ice crystals can create a halo effect around the sun or moon. This cloud is a good precursor of precipitation, indicating it may occur within 12 to 24 hours.

CIRRUS
One of the three basic cloud forms (the others are cumulus and stratus). It is also one of the three high cloud types. Cirrus are thin, wispy clouds composed of ice crystals and often appear as veil patches or strands. In the mid-latitudes, cloud bases are usually found between 20,000 to 30,000 feet, and it is the highest cloud that forms in the sky, except for the tops, or anvils, of cumulonimbus, which occasionally build to excessive heights.

CIVIL TWILIGHT
The time between the moment of sunset, when the sun's apparent upper edge is just at the horizon, until the center of the sun is 6 directly below the horizon.
Related term: twilight

CLEAR
The state of the sky when no clouds or obscurations are observed or detected from the point of observation.

CLEAR AIR TURBULENCE
Name given to turbulence that may occur in perfectly clear air without any visual in warning in the form of clouds. It is often found in the vicinity of the jet stream where large shears in the horizontal and vertical are found, although this turbulence is not limited just to jet stream locale. Other areas where it may occur include near mountains, in closed lows aloft, and in regions of wind shear. May be referred to as CAT.

CLEAR ICE
A glossy, clear, or translucent ice formed by the relatively slow freezing of large super cooled in water droplets. The droplets spread out over an object, such as an aircraft wing's leading edge, prior to complete freezing and forms a sheet of clear ice.
Related term: glaze

CLIMATE
The historical record and description of average daily and in seasonal weather events that help describe a region. Statistics are generally drawn over several decades. The word is derived from the Greek klima, meaning inclination, and reflects the importance early scholars attributed to the sun's influence.

CLIMATE ANALYSIS CENTER (CAC)
The U.S. National Weather Service division that applies new technology and approaches to the analysis, diagnosis, and projection of short term climate fluctuations on a regional and global basis. For further information, contact the CAC, located in Camp Spring, Maryland.

CLIMATE PREDICTION CENTER (CPC)
A branch of the National Centers for Environmental Prediction,the Center maintains a continuous watch on short-term climate fluctuations and diagnoses and predicts them.
For further information, contact the CPC, located in Washington, D.C.

CLIMATOLOGY
The study of climate. It includes climatic data, the analysis of the causes of the differences in climate, and the application of climatic data to the solution of specific design or operational problems.

CLINOMETER
An instrument used to measure angles of inclination. Used in conjunction with a ceiling light, it determines cloud height at night, based on the angle of a projected light on the clouds, the observer, and the ceiling light.

CLOSED LOW
A region of low pressure distinguished by a center of counterclockwise circulation (in the Northern Hemisphere), and is surrounded by one or more isobars or height contours. Closed lows aloft (i.e., above the surface) may become disconnected from the primary westerly flow and thus progress eastward more slowly. It is important to note that a cutoff low is a closed low, but not all closed lows are cutoff lows.

CLOUD
A visible collection of minute particle matter, such as water droplets and/or ice crystals, in the free air. A cloud forms in the atmosphere as a result of condensation of water vapor. Condensation nuclei, such as in smoke or dust particles, form a surface upon which water vapor can condense.

CLOUD BANK
A well-defined cloud mass that can be observed at a distance. It covers the horizon, but is not directly overhead.

CLOUDBURST
A sudden, heavy rainfall of a showery nature.
Related term: down burst

COALESCENCE
The merging of two water drops into a single larger drop.

COLD
A condition marked by low or decidedly subnormal temperature. The lack of heat.

COLD ADVECTION
The horizontal movement of colder air into a location. Contrast with warm advection.

COLD AIR FUNNEL
Funnel clouds, usually short-lived, that develop from relatively small showers or thunderstorms when the air aloft is very in cold. Cold air funnels may touch down briefly, but in general are less violent than most other types of tornadoes.

COLD CORE THUNDERSTORMS
Thunderstorms formed primarily due to steep lapse rates, especially when very cold air aloft overlies warmer surface air.

COLD FRONT
The leading edge of an advancing cold air mass that is under running and displacing the warmer air in its path. Generally, with the passage of a cold front, the temperature and humidity decrease, the pressure rises, and the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere). Precipitation is generally at and/or behind the front, and with a fast-moving system, a squall line may develop ahead of the front.
Related terms: occluded front and warm front

COLD HIGH
A high pressure system that has its coldest temperatures at or near the center of circulation, and horizontally, is thermally barotropic. It is shallow in nature, as circulation decreases with height. Associated with cold Arctic air, it is usually stationary. Also known as a cold core high. Contrast with a warm high.

COLD LOW
A low pressure system that has its coldest temperatures at or near the center of circulation, and is thermally barotropic with respect to a horizontal plane. Also known as a cold core low. A cut off low is an example, where an isolated pool of colder air is located south of the main westerlies.

COLD WAVE
A rapid fall in temperature within twenty-four hours to temperatures requiring substantially increased protection to agriculture, industry, commerce, and social activities. National Weather Service criteria includes the rate of temperature fall and the minimum to which it falls, depending on the region of the country and time of the in year. The Weather Channel uses the following criteria for a cold wave: a cold spell of two days or more with below normal temperatures in at least fifteen states, with at least five of them more than fifteen degrees below normal.

COLLADA
A strong, steady wind blowing from the north or northwest in the upper part of the Gulf of California and from the northeast in the lower part.

COLORADO LOW
A low pressure disturbance that forms in the lee of the Rocky Mountains, usually in southeastern Colorado.

COMMA CLOUD
A feature seen on satellite images with a distinctive comma-shape. This is indicative of a synoptic cloud pattern associated with large, well-developed low pressure systems.

CONDENSATION
The process by which water vapor undergoes a change in state from a gas to a liquid. It is the opposite physical process of evaporation.

CONDENSATION FUNNEL
A funnel-shaped cloud consisting of condensed water drops that has possible rotation.

CONDENSATION NUCLEI
A particle upon which condensation of water vapor occurs. It may be either in a solid or liquid state.

CONDITIONAL INSTABILITY
Stable unsaturated air that will result in instability in the event or on the condition that the air becomes saturated. If the air is saturated, it is considered unstable; if air is unsaturated, it is considered stable.

CONDUCTION
The transfer of heat through a substance by molecular action or from one substance by being in contact with another.

CONFLUENCE
A rate at which wind flow comes together along an axis oriented normal to the flow in question. The opposite of diffluence.

CONSTANT PRESSURE CHART
A chart of a constant pressure surface in which atmospheric pressure is uniform everywhere at any given moment. Elements may include analyses of height above sea level, wind, temperature, and humidity.

CONSTANT PRESSURE SURFACE
A surface along which the atmospheric pressure is equal everywhere.

CONTINENT
A large land mass rising abruptly from the deep ocean floor, including marginal regions that are shallowly submerged. Continents constitute about one-third of the earth's surface.

CONTINENTAL AIR MASS
An air mass with continental characteristics. It is a secondary characteristic of an air mass classification, signified by the small "c" before the primary characteristic, which is based on source region. For example, cP is an air mass that is continental polar in nature.

CONTINENTAL SHELF
The zone around the continents extending from the low-water mark seaward, typically ending in steep slope to the depths of the ocean floor.

CONTRAIL
Acronym for CONdensation TRAIL. A cloud-like streamer or trail often seen behind aircraft flying in clear, cold, humid air. A vapor trail is created when the water vapor from the engine exhaust gases are added to the atmosphere.
Related term: vapor trail

CONVECTION
Motions in a fluid that transport and mix the properties of the fluid. These properties could be heat and/or moisture. When used to imply only upward vertical motion, it is then the opposite of subsidence.

CONVECTIVE CONDENSATION LEVEL (CCL)
The height at which a parcel of air, if heated sufficiently from below, will rise adiabatically until it is just saturated.

CONVERGENCE
Wind movement that results in a horizontal net inflow of air into a particular region. Convergent winds at lower levels are associated with upward motion. Contrast with divergence.

COOLING DEGREE DAY
A cooling degree day is given for each degree that the daily mean temperature departs above the baseline of 75 degrees Fahrenheit. It is used to estimate the energy requirements and is an indication of fuel consumption for air conditioning or refrigeration.
Related terms: degree day and heating degree day

CORIOLIS EFFECT
A force per unit mass that arises solely from the earth's rotation, acting as a deflecting force. It is dependent on the latitude and the speed of the moving object. In the Northern Hemisphere, air is deflected to the right of its path, while in the Southern Hemisphere, air is deflected to the left of its path. It is greatest at the poles, North and South, and almost nonexistent at the equator.

CORONA
A pastel halo around the moon or sun created by the diffraction of water droplets. The droplets in the cloud, such as cirrostratus, and the cloud layer itself must be almost perfectly uniform in order for this phenomena to occur. The color display sometimes appears to be iridescent.

CORPOSANT
A luminous, sporadic, and often audible, electric discharge. It occurs from objects, especially pointed ones, when the electrical field strength near their surfaces attains a value near 1000 volts per centimeter. It often occurs during stormy weather and might be seen on a ship's mast or yardarm, aircraft, lightning rods, and steeples. Related terms: corona discharge and St. Elmo's Fire

CREPUSCULAR RAYS
Contrasting, alternating bright and dark rays in the sky. Sunlight is scattered by molecules and particles rendering these bright rays visible. Contrast is enhanced by haze, dust, or mist. These rays are more likely to be seen in the late afternoon, as clouds come between the sun and the observer. A similar effect occurs when the sun shines though a break in a layer of clouds.

CRYSTALLIZATION
The process of a substance going directly from a vapor form (water vapor) to a solid (ice) at the same temperature, without going through the liquid phase (water). The opposite of sublimation.

CUMULIFORM
Clouds composed of water droplets that exhibit vertical development. The density of the droplets often blocks sunlight, casting shadows on the earth's surface. With increasing vertical height, they are often associated with convection. Bases of these clouds are generally no more than 3,000 feet above the ground, but they can develop past the troposphere in both temperate and tropical latitudes. They are classified as low clouds and include all varieties of cumulus and cumulonimbus. The opposite in type are the horizontal development of stratiform clouds.

CUMULONIMBUS
A vertically developed cumulus cloud, often capped by an anvil-shaped cirriform cloud. Also called a thunderstorm cloud, it is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail, tornadoes or strong, gusty winds.

CUMULONIMBUS MAMMATUS
A portion of a cumulonimbus cloud that appears as a pouch or udder on the under surface of the cloud. Although they do not cause severe weather, they often accompany storms. They may slowly vary in size, since they are an area of negative buoyancy convection, and is associated with severe turbulence in the lower sections of the cloud.
Related terms: mammatocumulus and Dave's Dictionary

CUMULUS
One of the three basic cloud forms (the others are cirrus and stratus). It is also one of the two low cloud types. A cloud that develops in a vertical direction from the base (bottom) up. They have flat bases and dome- or cauliflower-shaped upper surfaces. The base of the cloud is often no more than 3,000 feet above the ground, but the top often varies in height. Small, separate cumulus are associated with fair weather (cumulus humilis). With additional heating from the earth's surface, they can grow vertically throughout the day. The top of such a cloud can easily reach 20,000 or more into the troposphere. Under certain atmospheric conditions, these clouds can develop into larger clouds, known as towering cumulus (cumulus congestus), and may produce a rain shower. Further development may create a cumulonimbus.
Related term: Dave's Dictionary

CUMULUS CONGESTUS
A strongly sprouting cumulus cloud with generally sharp outlines and often with great vertical development. It may occur as tower-like clouds with cauliflower tops. These clouds may produce abundant showers and may develop further into cumulonimbus.
Related term: towering cumulus

CUMULUS FRACTUS
Cumulus clouds that appear in irregular fragments, as if they had been shred or torn. Also appears in stratus clouds (called stratus fractus), but not in cirrus clouds.

CUMULUS HUMILIS
Cumulus clouds with little or no vertical development characterized by a generally flat appearance. Their growth is usually limited by a temperature inversion, which is marked by the unusually uniform height of the clouds. Also called fair-weather cumulus.

CUMULUS MEDIOCRIS
Cumulus clouds characterized by moderate vertical development with upper protuberances not very marked in appearance. This cloud does not produce precipitation, but could develop into towering cumulus or cumulonimbus which do.

CURRENT
A horizontal movement of water, such as the Gulf Stream off the east coast of North America, or air, such as the jet stream.

CUT-OFF HIGH
A warm high which has become displaced and is on the polar ward side of the jet stream. It occurs mostly during the spring and is most frequent over northeastern Siberia, Alaska, and Greenland. It is an example of a blocking high.

CUT-OFF LOW
A closed cold core low completely removed from the primary westerly flow. Cutoff lows may remain detached from the westerlies for days while exhibiting very little forward (eastward) progress. In some instances, a cutoff low may move to the west, or retrograde, opposite to the prevailing flow. It is important to note that a cutoff low is a closed low, but not all closed lows are cutoff lows.

CYCLOGENESIS
The process that creates a new low pressure system or cyclone, or intensifies a pre-existing one. It is also the first appearance of a trough.

CYCLONE
An area of closed pressure circulation with rotating and converging winds, the center of which is a relative pressure minimum. The circulation is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also called a low pressure system and the term used for a tropical cyclone in the Indian Ocean. Other phenomena with cyclonic flow may be referred to by this term, such as dust devils, tornadoes, and tropical and extra tropical systems. The opposite of an anticyclone or a high pressure system.

CYCLONIC FLOW
Winds that blow in and around a cyclone, that is counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere.

Index
D:

DAILY MEAN
The average temperature for a day computed by averaging either the hourly readings or, more commonly, the maximum and minimum temperatures.

DATA BUOYS
Buoys placed throughout the Gulf of Mexico and along the Atlantic and Pacific coasts of the United States that relay information on air and water temperature, wind speed, air pressure, and wave conditions via radio signals.

DAWN
The first appearance of light in the eastern sky before sunrise. It marks the beginning of morning twilight. The visual display is created by the scattering of light reaching the upper atmosphere prior to the sun's rise to the observer's horizon.
Related term: daybreak

DAY
Considered a basic unit of time as defined by the earth's motion. It represents the time needed for one complete revolution of the earth about its own axis. Also know as a sidereal day, it is approximately equal to 23 hours, 56 minutes, and 4.09 seconds.
Related term: night

DEBRIS CLOUD
Considered a rotating cloud of debris or dust that is on the ground or near the ground. The debris cloud appearing beneath a thunderstorm will most likely confirm the presence of a tornado.

DEEPENING
Used in describing the history of a low pressure system or an area of cyclonic circulation, it means a decrease in the central pressure of the system. Although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is increasing in cyclonic circulation and acquiring more energy. The opposite of filling.

DEGREE
A measure of temperature difference representing a single division on a temperature scale.
Related terms: Celsius, Fahrenheit, and Kelvin

DEGREE DAY
A measure of the departure of the mean daily temperature from a given standard. That is one degree day for each degree (Fahrenheit or Celsius) of departure above or below the standard during one day.
Related terms: cooling degree day and heating degree day

DENSE FOG ADVISORY
Advisory issued when fog reduces visibility to 1/8 mile or less, creating possible hazardous conditions.

DENSITY
The ratio of the mass of a substance to the volume it occupies. In oceanography, it is equivalent to specific gravity and represents the ratio of the weight of a given volume of sea water to that of an equal volume of distilled water at 4.0C or 39.2F.

DENSITY ALTITUDE
The altitude at which a given density is found in the standard atmosphere. Used in aviation, it is computed from the station pressure at takeoff and the virtual temperature at the particular altitude under consideration.

DEPRESSION
In meteorology, it is another name for an area of low pressure, a low, or trough. It also applies to a stage of tropical cyclone development and is known as a tropical depression to distinguish it from other synoptic features.

DERECHO
A line of intense, widespread, and fast-moving thunderstorms that moves across a great distance. They are characterized by damaging straight-line winds over hundreds of miles. Spanish for straight.

DEW
Condensation in the form of small water drops that forms on grass and other small objects near the ground when the temperature has fallen to the dew point, generally during the nighttime hours.

DEW POINT
The temperature to which air must be cooled at a constant pressure to become saturated.

DIABLO WINDS
Dry winds in the Diablo mountain range in central California that can exceed 60 miles per hour. Similar to the Santa Ana winds, they develop as the wind flows from high pressure over Nevada to lower pressure along the central California coast.

DIFFLUENCE
A rate at which wind flow spreads apart along an axis oriented normal to the flow in question. The opposite of confluence.

DIFFRACTION
The result of light waves interfering with other after passing through a narrow aperture, causing them to bend or spread.

DIRECTIONAL SHEAR
The shear created by a rapid change in wind direction with height.

DISCONTINUITY
Comparatively large contrast in meteorological elements over a relatively small distance or period of time. In oceanography, it is the abrupt change or jump of a variable at a line or surface.

DISTURBANCE
This has several applications. It can apply to a low or cyclone that is small in size and influence. It can also apply to an area that is exhibiting signs of cyclonic development. It may also apply to a stage of tropical cyclone development and is known as a tropical disturbance to distinguish it from other synoptic features.

DIURNAL
Pertaining to actions or events that occur during a twenty-four hour cycle or recurs every twenty-four hours. Meteorological elements that are measured diurnally include clouds, precipitation, pressure, relative humidity, temperature, and wind.

DIVERGENCE
Wind movement that results in a horizontal net outflow of air from a particular region. Divergence at lower levels is associated with a downward movement of air from aloft. Contrast with convergence.

DOG DAYS
The name given to the very hot summer weather that may persists for four to six weeks between mid-July through early September in the United States. In western Europe, this period may exist from the first week in July to mid-August and is often the period of the greatest frequency of thunder. Named for Sirius, the Dog Star, which lies in conjunction with the sun during this period, it was once believed to intensify the sun's heat during the summer months.

DOLDRUMS
Located between 30 degrees North and 30 degrees South latitudes in the vicinity of the equator, this area typically has calm or light and variable winds. Also a nautical term for the equatorial trough.
Related terms: Inter tropical Convergence Zone (ITCZ), Horse Latitudes and Dave's Dictionary

DOPPLER RADAR
Weather radar that measures direction and speed of a moving object, such as drops of precipitation, by determining whether atmospheric motion is horizontally toward or away from the radar. Using the Doppler effect, it measures the velocity of particles. Named for J. Christian Doppler, an Austrian physicist, who in 1842 explained why the whistle of an approaching train had a higher pitch than the same whistle when the train was going away.
Related terms: NEXRAD and Dave's Dictionary

DOWNBURST
A severe localized downdraft from a thunderstorm or shower. This outward burst of cool or colder air creates damaging winds at or near the surface. Sometimes the damage resembles tornadic damage.
Related term: microburst

DOWNDRAFT
A sudden descent of cool or cold air to the ground, usually with precipitation, and associated with a thunderstorm or shower.
Related term: updraft

DOWNPOUR
A heavy rain.
Related term: cloudburst

DOWNSLOPE EFFECT
The warming of an air flow as it descends a hill or mountain slope.
Related term: upslope

DRAINAGE WIND
A katabatic wind, it is caused by the cooling of air along the slopes of a mountain.
Related term: mountain breeze

DRIFTING SNOW
Snow particles blown from the ground by the wind to a height of less than six feet.

DRIFTS
Normally used when referring to snow or sand particles are deposited behind obstacles or irregularities of the surface or driven into piles by the wind.

DRIZZLE
Slowly falling precipitation in the form of tiny water droplets with diameters less than 0.02 inches or 0.5 millimeters. It falls from stratus clouds and is often associated with low visibility and fog. It is reported as "DZ" in an observation and on the METAR.

DROPSONDE
A radiosonde dropped with a parachute from an aircraft rather than lifted by a balloon to measure the atmosphere below.

DROUGHT
Abnormal dry weather for a specific area that is sufficiently prolonged for the lack of water to cause serious hydrological imbalance.

DRY ADIABAT
The line on a Skew T-Log P chart that depicts the lifting of dry air, or air that is unsaturated. As a parcel rises adiabatically, its pressure decreases and its temperature falls due to the expansion of the air parcel. When an air parcel is unsaturated and rises, then the temperature decreases at a rate of 1C per 100 meters (5.5F per 1,000 feet).
Related term: moist adiabat and adiabatic process

DRY BULB THERMOMETER
A thermometer used to measure the ambient temperature. The temperature recorded is considered identical to air temperature. One of the two thermometers that make up a psychrometer.

DRY LINE
The boundary between the dry desert air mass of the Southwest U.S. and the moist air mass from the Gulf of Mexico. It usually lies north-south across the central and southern High Plains states during spring and summer. The passage of a dry line results in a sharp decrease in humidity, clearing skies, and a wind shift from southeasterly or south to southwesterly or west. Its presence influences severe weather development in the Great Plains.

DRY SLOT
An area of dry, and usually cloud-free, air that wraps into the southern and eastern sections of a synoptic scale or mesoscale low pressure system. Best seen on a satellite picture, such as a water vapor image.

DUSK
The period of waning light from the time of sunset to dark.
Related terms: twilight and dawn

DUST
Small particles of earth or other matter suspended in the air. It is reported as "DU" in an observation and for wide spread dust on the METAR.

DUST BOWL
The term given to the area of the Great Plains including Texas, Oklahoma, Kansas, Colorado, and New Mexico that was most greatly affected during the Great Drought of the 1930's.
Related term: drought

DUST DEVIL
A small, rapidly rotating column of wind, made visible by the dust, dirt or debris it picks up. It usually occurs in arid or semi-arid areas and is most likely to develop on clear, dry, hot afternoons in response to surface heating.
Related term: whirlwind

DUSTSTORM
A severe weather condition characterized by strong winds and dust-filled air over a large area. Visibility is reduced to between 5/8ths and 5/16ths statute mile. It is reported as "DS" in an observation and on the METAR.

D-VALUE
The deviation of actual altitude along a constant pressure surface from the standard atmosphere altitude of that surface.

DYNAMICS
A branch of mechanics that deals with forces and their relations to patterns of motion. In meteorology, this relates especially to wind and water patterns.

Index
E:

EARTHLIGHT (EARTHSHINE)
The faint illumination of the dark part of the moon's disk produced by sunlight reflected onto the moon from the earth's surface and atmosphere.

EARTHQUAKE
A sudden, transient motion or trembling of the earth's crust, resulting from the waves in the earth caused by faulting of the rocks or by volcanic activity.

EASTERLIES
Usually applied to the broad patterns of persistent winds with an easterly component, such as the easterly trade winds.

EASTERLY WAVE
An inverted, migratory wave-like disturbance or trough in the tropical region that moves from east to west, generally creating only a shift in winds and rain. The low level convergence and associated convective weather occur on the eastern side of the wave axis. Normally, it moves slower than the atmospheric current in which it is embedded and is considered a weak trough of low pressure. It is often associated with possible tropical cyclone development and is also known as a tropical wave.

ECHO
The energy return of a radar signal after it has hit the target.
Related term: radar echo

ECLIPSE
The obscuring of one celestial body by another.
Related terms: lunar eclipse and solar eclipse

ECLIPTIC
The sun's apparent path across the sky that tracks a circle through the celestial sphere.

ECOLOGY
The study of the relationships between living organisms and their environment.

EDDY
A small disturbance of wind in a large wind flow, which can produce turbulent conditions. They can also be areas of warmer air north of the main westerlies or colder air south of the westerlies. In oceanic circulation, it is a circular movement of water usually formed where currents pass obstructions, between two adjacent currents flowing counter to each other, or along the edge of a permanent current.
Related terms: cut-off high and cut-off low

ELECTROMAGNETIC RADIATION
Also called radiation, it is waves of energy propagated though space or through a material media.

ELECTROMAGNETIC SPECTRUM
The band of electromagnetic radiation with components that are separated into their relative wave lengths. The portion of the spectrum that the human eye can detect is called visible light, between the longer infrared waves and the shorter ultraviolet waves. The various types of energy comprising the spectrum are (from longest to shortest) radio, infrared, visible, ultraviolet, x-rays, gamma rays, and cosmic rays.

ELEVATION
The measure of height with respect to a point on the earth's surface above mean sea level. Sometimes referred to as station elevation.

El Nio
The cyclical warming of East Pacific Ocean sea water temperatures off the western coast of South America that can result in significant changes in weather patterns in the United States and elsewhere. This occurs when warm equatorial waters move in and displace the colder waters of the Humbolt Current, cutting off the upwelling process.
Related terms: La Nio and Dave's Dictionary

ENVIRONMENT
The sum total of all the external conditions that effect an organism, community, material, or energy.

EQUATOR
The geographic circle at 0 degrees latitude on the earth's surface. It is equal distance from the North and South Poles and divides the Northern Hemisphere from the Southern.

EQUATORIAL TROUGH
The quasi-continuous area of low pressure between the subtropical high pressure areas in both the Northern and Southern Hemisphere.
Related terms: Intertropical Convergence Zone (ITCZ) and doldrums

EQUINOX
The point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. In the Northern Hemisphere, the vernal equinox falls on or about March 20 and the autumnal equinox on or about September 22.
Related term: Dave's Dictionary

EROSION
The movement of soil or rock from one area to another by the action of the sea, running water, moving ice, precipitation, or wind.

EVAPORATION
The physical process by which a liquid, such as water is transformed into a gaseous state, such as water vapor. It is the opposite physical process of condensation.

EVAPOTRANSPIRATION
The total amount of water that is transferred from the earth's surface to the atmosphere. It is made up of the evaporation of liquid or solid water plus the transpiration from plants.

EXOSPHERE
This region is considered the very outer limits of the earth's atmosphere. Its lower boundary is often called the critical level of escape, where gas atoms are so widely spaced that they rarely collide with one another and have individual orbits. It is estimated to be some 400 plus miles (640 kilometers) above the surface.
Related term: ionosphere

EXTRATROPICAL CYCLONE
Any cyclone that is no longer tropical in origin. Generally considered to be a migratory frontal cyclone found in the middle and high latitudes. An extratropical storm is a cyclone that no longer derives its energy source from the processes involved in sustaining a tropical cyclone, but thrives on baroclinic processes; i.e., the temperature contrast between warm and cold air masses. The term extratropical is typically used when a tropical cyclone moves away from the tropics and moves pole ward into cooler waters thus losing its tropical characteristics.
Related terms: extratropical low and extratropical storm.

EYE
The center of a tropical storm or hurricane, characterized by a roughly circular area of light winds and rain-free skies. An eye will usually develop when the maximum sustained wind speeds exceed 78 mph. It can range in size from as small as 5 miles to up to 60 miles, but the average size is 20 miles. In general, when the eye begins to shrink in size, the storm is intensifying.

EYE WALL
An organized band of convection surrounding the eye, or center, of a tropical cyclone. It contains cumulonimbus clouds, intense rainfall and very strong winds.

Index
F:

FAHRENHEIT TEMPERATURE SCALE
A temperature scale where water at sea level has a freezing point of +32F and a boiling point of +212F. More commonly used in areas that observe the English system of measurement. Created in 1714 by Gabriel Daniel Fahrenheit (1696-1736), a German physicist, who also invented the alcohol and mercury thermometers.

FAIR
This is a subjective description. Considered as pleasant weather conditions with regard to the time of year and the physical location.

FATHOM
The common unit of depth in the ocean for countries using the English system of measurement. It is six feet or 1.83 meters. It can also be used in expressing horizontal distance, since 120 fathoms is equal to one cable or nearly on tenth of a nautical mile.

FEEDER BANDS
In tropical parlance, the lines or bands of thunderstorms that spiral into and around the center of a tropical system. Also known as outer convective bands, a typical hurricane may have three or more of these bands. They occur in advance of the main rain shield and are usually 40 to 80 miles apart. In thunderstorm development, they are the lines or bands of low level clouds that move or feed into the updraft region of a thunderstorm.

FETCH
An area of the water surface over which waves are generated by a wind having a constant direction and speed. Also, it is the name given to the length of the fetch area, measured in the direction of the wind from which the seas are generated. One of the ingredients for lake effect snow is the fetch of the water over which cold air can gain moisture.
Related term: Dave's Dictionary

FEW
The amount of sky cover for a cloud layer between 1/8th and 2/8ths, based on the summation layer amount for that layer.

FILLING
Used in describing the history of a low pressure system or an area of cyclonic circulation, it means an increase in the central pressure of the system. Although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is decreasing in cyclonic circulation and losing its characteristics. The opposite of deepening.

FIREWHIRL
A tornado-like rotating column of fire and smoke created by intense heat from a forest fire or volcanic eruption.

FIRST GUST
Another name for the initial wind surge observed at the surface as the result of down drafts forming the leading edge or gust front of a thunderstorm.
Related term: gust front

FLANKING LINE
A line of attached cumulus or towering cumulus clouds of descending height, appearing as stair steps (usually on the southwest side) of the most active part of a super cell.

FLASH FLOOD
A flood that rises and falls quite rapidly with little or no advance warning, usually as the result of intense rainfall over a relatively small area. Flash floods can be caused by situations such as a sudden excessive rainfall, the failure of a dam, or the thaw of an ice jam.

FLOOD
High water flow or an overflow of rivers or streams from their natural or artificial banks, inundating adjacent low lying areas.

FLOOD PLAIN
Level land that may be submerged by flood waters.

FLOOD STAGE
The level of a river or stream where overflow onto surrounding areas can occur.

FOEHN
A warm dry wind on the lee side of a mountain range, whose temperature is increased as the wind descends down the slope. It is created when air flows downhill from a high elevation, raising the temperature by adiabatic compression. Classified as a katabatic wind.
Related terms: chinook wind and Santa Ana wind

FOG
A visible aggregate of minute water droplets suspended in the atmosphere at or near the surface of the earth, reducing horizontal visibility to less than 5/8 statute miles. It is created when the temperature and the dew point of the air have become the same, or nearly the same, and sufficient condensation nuclei are present. It is reported as "FG" in an observation and on the METAR.
Related terms: advection fog, freezing fog, frontal fog, ice fog, radiation fog, sea fog, and Dave's Dictionary

FOG BANK
A fairly well-defined mass of fog observed in the distance. Most commonly seen at sea, over a lake, or along coastal areas.

FOGBOW
A whitish semicircular arc seen opposite the sun in fog. The outer margin has a reddish tinge, its inner margin has a bluish tinge, and the middle of the band is white. An additional bow with reversed colors sometimes appears inside the first.

FORECAST
A statement of expected future occurrences. Weather forecasting includes the use of objective models based on certain atmospheric parameters, along with the skill and experience of a meteorologist.
Related term: prediction

FRACTUS
The elements of cumulus and stratus clouds that appear in irregular fragments, as if they had been shred or torn. Never appears in cirrus clouds. Also known as scud.
Related terms: cumulus fractus and stratus fractus

FREEZING DRIZZLE
Drizzle, falling as a liquid, but freezing on impact with the colder ground or other exposed surfaces. It is reported as "FZDZ" in an observation and on the METAR.

FREEZING FOG
Used to describe the phenomena when fog is present and the air temperature is below 0C. It is reported as "FZFG" in an observation and on the METAR.

FREEZING POINT/FREEZE
The process of changing a liquid to a solid. The temperature at which a liquid solidifies under any given set of conditions. Pure water under atmospheric pressure freezes at 0C or 32F. It is the opposite of fusion. In oceanography, the freezing point of water is depressed with increasing salinity.

FREEZING PRECIPITATION
Precipitation that is liquid, but freezes upon impact with a solid surface, such as the ground or other exposed surfaces.
Related terms: freezing rain and freezing drizzle

FREEZING RAIN
Rain that falls as liquid and freezes upon impact to form a coating of glaze on the colder ground or other exposed surfaces. It is reported as "FZRA" in an observation and on the METAR.

FRESH WATER
Water found rivers, lakes, and rain, that is distinguished from salt water by its appreciable lack of salinity.

FRICTION
In meteorology, it is the turbulent resistance of the earth on the atmosphere. Considered as the resistance of fluids (air and water) to the relative motion of a solid body. The amount is dependent on the size and shape of the body.

FRICTION LAYER
The thin layer of atmosphere adjacent to the earth's surface. Surface friction is effective in slowing down wind up to approximately 1,500 to 3,000 feet above the ground. Above this level, air tends to flow parallel to the isobars. Wind distribution within this layer is determined by vertical temperature gradient and the physical contours of the underlying surface features.
Related terms: surface boundary layer and boundary layer

FRONT
The transition zone or interface between two air masses of different densities, which usually means different temperatures. For example, the area of convergence between warm, moist air and cool, dry air.
Related terms: cold front and warm front

FRONTAL PASSAGE
It is the passage of a front over a specific point on the surface. It is reflected by the change in dew point and temperature, the shift in wind direction, and the change in atmospheric pressure. Accompanying a passage may be precipitation and clouds. May be referred to as "fropa."

FRONTOGENESIS
The birth or creation of a front. This occurs when two adjacent air masses exhibiting different densities and temperatures are brought together by prevailing winds, creating a front. It could happen when either air mass, or both, move over a surface which strengthens their original properties. However, it occurs most often along the eastern coasts of North America and Asia, when the air mass moving out over the ocean has a weak or no distinct boundary. The opposite of frontolysis.

FRONTOLYSIS
The destruction or dying of a front where the transition zone is losing its contrasting properties. The opposite of frontogenesis.

FROST
The covering of ice crystals that forms by direct sublimation on exposed surfaces whose temperature is below freezing.

FROZEN PRECIPITATION
Precipitation that reaches the ground in a frozen state. Examples include snow, snow pellets, snow grains, ice crystals, ice pellets, and hail.

FUJITA-PEARSON SCALE
A scale that classifies the severity of wind damage intensity based on the degree of destruction as it relates to the wind speed as well as path length and path width of the event. It is normally used to identify the most intense damage exhibited by a tornado. Developed by T. Theodore Fujita and Allen Pearson.
Related term: Fujita-Pearson Scale

FUNNEL CLOUD
A violent, rotating column of air visibly extending from the base of a towering cumulus or cumulonimbus toward the ground, but not in contact with it. It is reported as "FC" in an observation and on the METAR.

FUSION
The change of state from a solid to a liquid at the same temperature. The heat of fusion is the number of gram calories of heat necessary to change one gram of a substance from the solid to the liquid state. It is the opposite of freezing.

Index
G:

GALE
On the Beaufort Wind Scale, a wind with speeds from 28 to 55 knots (32 to 63 miles per hour). For marine interests, it can be categorized as a moderate gale (28 to 33 knots), a fresh gale (34 to 40 knots), a strong gale (41 to 47 knots), or a whole gale (48 to 55 knots). In 1964, the World Meteorological Organization defined the categories as near gale (28 to 33 knots), gale (34 to 40 knots), strong gale (41 to 47 knots), and storm (48 to 55 knots).

GALE WARNING
A warning for marine interests for impending winds from 28 to 47 knots (32 to 54 miles per hour).

GEOPHYSICS
The study of the physics or nature of the Earth and its environment. It deals with the composition and physical phenomena of the earth and its liquid and gaseous envelopes. Areas of studies include the atmospheric sciences and meteorology, geology, seismology, and vulcanology, and oceanography and related marine sciences, such as hydrology. By extension, it often includes astronomy and the related astro-sciences.

GEOSPHERE
Considered the solid portions of the earth, including the hydrosphere and the lithosphere, as opposed to the atmosphere, which lies above it. At their conjunction is the biosphere.

GEOSTATIONARY SATELLITE
An orbiting weather satellite that maintains the same position over the equator during the earth's rotation. Also known as GOES, an acronym for Geostationary Operational Environmental Satellite.
Related terms: polar-orbiting satellite and Dave's Dictionary

GEOSTROPHIC WIND
A steady horizontal motion of air along straight, parallel isobars or contours in an unchanging pressure or contour field. It is assumed that there is no friction, that the flow is straight with no curvature and there is no divergence or convergence with no vertical acceleration.

GLACIER WINDS
Air flow that descends from glaciers, occasionally at a high rate of speed. Caused by the temperature difference between the air in contact with the glacier and the air at the same altitude, it reaches maximum intensity in the early afternoon.
Related term: katabatic wind

GLAZE
A smooth clear icy coating of super cooled water droplets that spread out and freeze onto objects on contact. A storm that produces the accretion of glaze is called an ice storm.
Related term: clear ice

GRADIENT WIND
A steady horizontal air motion along curved parallel isobars or contours in an unchanging pressure or contour field, assuming there is no friction and no divergence or convergence.

GRAUPEL
A form of frozen precipitation consisting of snowflakes or ice crystals and super cooled water droplets frozen together.
Related term: snow pellets

GRAVITATION
The mutual attraction between two masses of matter. The rotation of the earth and the atmosphere modifies this attraction to produce the field of gravity.

GRAVITY
The force of attraction of the earth on an object. The direction is downward relative to the earth, and it decreases with elevation or altitude away from the earth's surface.

GREEN FLASH
A brilliant green coloration of the upper edge of the sun, occasionally seen as the sun's apparent disk is about to set below a clear horizon.

GREENHOUSE EFFECT
The overall warming of the earth's lower atmosphere primarily due to carbon dioxide and water vapor which permit the sun's rays to heat the earth, but then restrict some heat-energy from escaping back into space.

GREENWICH MEAN TIME (GMT)
The name of the twenty-four hour time scale which is used throughout the scientific and military communities. Standard Time begins at Greenwich, England, home of the Royal Observatory which first utilized this method of world time. This is also the Prime Meridian of Longitude. The globe is divided into twenty-four (24) time zones of 15 degrees of arc, or one hour in time apart. To the east of this meridian, time zones are number from 1 to 12 and prefixed with a minus (-), indicting the number of hours to be subtracted to obtain Greenwich Time (GMT). To the west, the time zones are also numbered 1 through 12, but are prefixed with a plus (+), indicating the number of hours to be added to obtain GMT.
Related terms: Universal Time Coordinate (UTC) and Zulu (Z)

GROUND CLUTTER
A pattern of radar echoes reflecting off fixed ground targets such as buildings or hills near the radar. This may hide or confuse the proper return echo signifying actual precipitation.

GROUND FOG
Fog created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its initial dew point. Primarily takes place at night or early morning.
Related term: radiation fog

GROWING SEASON
Considered the period of the year during which the temperature of cultivated vegetation remains sufficiently high enough to allow plant growth. Usually considered the time period between the last killing frost in the spring and the first killing frost of the autumn. The frost-free growing season is between the first and last occurrence of 32°F temperatures in spring and autumn.

GULF STREAM
The warm, well-defined, swift, relatively narrow ocean current which exists off the east coast of the United States, beginning near Cape Hatteras. The term also applies to the oceanic system of currents that dominate the western and northern Atlantic Ocean: the Florida current, which flows through the Florida Straits between the Florida Keys and Cuba and northwards; the Gulf Stream, which begins around Cape Hatteras and flows northeasterly off the continental slope into the North Atlantic; and the North Atlantic current, which begins around the Grand Banks off Newfoundland and continues east-northeastwards towards the British Isles.

GULLY WASHER
A heavy rain shower that occurs suddenly, possibly creating a flash flood.

GUST
A sudden significant increase in or rapid fluctuations of wind speed. Peak wind must reach at least 16 knots (18 miles per hour) and the variation between peaks and lulls is at least 10 knots (11.5 miles per hour). The duration is usually less twenty seconds.

GUST FRONT
The leading edge of the cool, gusty surface winds produced by thunderstorm down drafts. Sometimes confused with an outflow boundary.
Related term: first gust

GUSTNADO
A weak, and usually short-lived, tornado that forms along the gust front of a thunderstorm, appearing as a temporary dust whirl or debris cloud.

Index
H:

HABOOB
Sudanese name for dust storm or sandstorm with strong winds that carry small particles of dirt or sand into the air, particularly severe in areas of drought.

HAIL
Precipitation that originates in convective clouds, such as cumulonimbus, in the form of balls or irregular pieces of ice, which comes in different shapes and sizes. Hail is considered to have a diameter of 5 millimeter or more; smaller bits of ice are classified as ice pellets, snow pellets, or graupel. Individual lumps are called hailstones. It is reported as "GR" in an observation and on the METAR. Small hail and/or snow pellets is reported as "GS" in an observation and on the METAR.

HALO
The ring of light that seems to encircle the sun or moon when veiled by cirrus clouds. To produce this phenomena, the ice crystals must be in a heterogeneous arrangement to refract the sunlight. The most commonly observed is a halo that forms at a 22 radius, although another one at 46 radius may also be seen.

HAZE
A suspension of fine dust and/or smoke particles in the air. Invisible to the naked eye, the particles reduce visibility by being sufficiently numerous to give the air an opalescent appearance. It is reported as "HZ" in an observation and on the METAR.

HEAT
A form of energy transferred between two systems by virtue of a difference in temperature. The first law of thermodynamics demonstrated that the heat absorbed by a system may be used by the system to do work or to raise its internal energy.

HEAT BALANCE
The equilibrium which exists on the average between the radiation received by the earth and atmosphere from the sun and that emitted by the earth and atmosphere. The balance between heat loss (long wave radiation from the earth back into the atmosphere) and heat gain (incoming solar radiation).

HEAT EXHAUSTION
The effect of excessive heat, particularly when combined with high humidity, on a human being. Signs of heat exhaustion include a general weakness, heavy sweating and clammy skin, dizziness and/or fainting, and muscle cramps.

HEAT INDEX
The combination of air temperature and humidity that gives a description of how the temperature feels. This is not the actual air temperature.
Related term: Heat Index Chart

HEATING DEGREE DAY
One heating degree day is given for each degree that the daily mean temperature is below 65F. It is used as an indication of fuel consumption.
Related terms: degree day or cooling degree day

HEAT LIGHTNING
Lightning that appears as a glowing flash on the horizon. It is actually lightning occurring in distant thunderstorms, just over the horizon and too far away for thunder to be heard.

HEAT STROKE
Introduced to the body by overexposure to high temperatures, particularly when accompanied by high humidity. The signs of heat stroke include when an individual's body temperature is greater than 105F, the skin is hot and dry, there is a rapid and irregular pulse, perspiration has stopped, and one has lost consciousness. Seek immediate medical aid. May be called a sun-stroke when caused by direct exposure to the sun.

HEAT WAVE
A period of abnormally and uncomfortably hot weather. It could last from several days to several weeks. The Weather Channel uses the following criteria for a heat wave: a minimum of ten states must have 90F plus temperatures and the temperatures must be at least five degrees above normal in parts of that area for at least two days or more.

HELICITY
A property of a moving fluid, such as air, representing the potential for helical flow (flow that follows a corkscrew pattern). Computed from the vertical wind profile of the lower atmosphere and measured relative to the motion as a storm, it is used to forecast the formation of meso cyclones.

HIGH CLOUDS
A term used to signify cirriform clouds that are composed of ice crystals and generally have bases above 20,000 feet. The main types of high clouds are cirrus,cirrocumulus, and cirrostratus. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.

HIGH LATITUDES
The latitude belt roughly between 60 and 90 North and South.
Related term: polar region

HIGH PRESSURE SYSTEM
An area of relative pressure maximum that has diverging winds and a rotation opposite to the earth's rotation. This is clockwise the in Northern Hemisphere and counterclockwise in the Southern Hemisphere. It is the opposite of an area of low pressure or a cyclone.
Related term: anticyclone

HOARFROST
Another name for frost. A deposit of hoarfrost occurs when air with a dew point below freezing is brought to saturation by cooling.

HOOK ECHO
A radar reflectivity pattern observed in a thunderstorm, appearing like a fish hook and indicating favorable conditions for tornadic development. However, hook echoes and tornadoes do not always accompany each other.

HORIZON
One of several lines or planes used as reference for observation and measurement relative to a given location on the surface of the earth. The geographic horizon, also called the apparent horizon, is the distant line along which earth and sky appear to meet. This is the usual concept of horizon and is used in weather observing. The local horizon is the actual lower boundary of the observed sky or the upper outline of terrestrial objects including nearby natural obstructions, such as mountains.

HORSE LATITUDES
Located between 30 North and 30 South in the vicinity of the equator, this area typically has calm or light and variable winds.
Related terms: equatorial trough, the Intertropical Convergence Zone (ITCZ), and doldrums

HUDSON BAY LOW
An area of low pressure over or near the Hudson Bay area of Canada that often introduces cold air to the north central and northeast United States.

HUMBOLDT CURRENT
Also known as the Peru Current, this ocean current flows northward along the western side of South America, offshore Chile and Peru. There is considerable upwelling of the colder subsurface waters due to the prevailing southerly winds. Dominant weather in this area includes coastal fog and low clouds. The presence or lack of this current is a vital part of the meteorological-oceanographic pattern known as El Nio.

HUMIDITY
The amount of water vapor in the air. It is often confused with relative humidity or dew point.
Related terms: absolute humidity, relative humidity, and specific humidity

HURRICANE
The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern North Pacific Ocean. This same tropical cyclone is known as a typhoon in the western Pacific and a cyclone in the Indian Ocean.
Related term: Dave's Dictionary

HURRICANE FACTS
Hurricane Facts: 1961 was the last time two Category 5 hurricanes formed in the same year. 1960 was the last time two Category 5 hurricanes formed and eventually made U.S. land fall. 2005 has now matched that with Katrina and now Rita.

HURRICANE WARNING
A formal advisory issued by forecasters at the National Hurricane Center when they have determined that hurricane conditions are expected in a coastal area or group of islands within a 24 hour period. A warning is used to inform the public and marine interests of the storm's location, intensity, and movement.

HURRICANE WATCH
A formal advisory issued by forecasters at the National Hurricane Center when they have determined that hurricane conditions are a potential threat to a coastal area or group of islands within a 24 to 36 hour period. A watch is used to inform the public and marine interests of the storm's location, intensity, and movement.

HYDROMETEOR
Any any form of atmospheric water vapor, including those blown by the wind off the earth's surface. Liquid or solid water formation that is suspended in the air includes clouds, fog, ice fog, and mist. Drizzle and rain are example of liquid precipitation, while freezing drizzle and freezing rain are examples of freezing precipitation. Solid or frozen precipitation includes ice pellets, hail, snow, snow pellets, snow grains, and ice crystals. Water vapor that evaporates before reaching the ground is virga. Examples of liquid or solid water particles that are lifted off the earth's surface by the wind includes drifting and blowing snow and blowing spray. Dew, frost, rime, and glaze are examples of liquid or solid water deposits on exposed objects.

HYDROLOGIC CYCLE
Often called the water cycle, it is the vertical and horizontal transport of water in all its states between the earth, the atmosphere, and the seas.

HYDROLOGY
The study of the waters of the earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of water in streams, lakes, and on or below the land surface.

HYDROSPHERE
Considered as the water portion of the earth's surface. Part of the geosphere.

HYGROGRAPH
An instrument that records the hygrometer's measure of water vapor.

HYGROMETER
An instrument that measures the water vapor content of the atmosphere.
Related term: psychrometer

HYPOTHERMIA
This situation occurs when the core temperature of one's body falls below normal. It is the failure of the body to maintain adequate production of heat under conditions of extreme cold.

Index
J:

JET STREAK
A region of accelerated wind speed along the axis of a jet stream.

JET STREAM
An area of strong winds that are concentrated in a relatively narrow band in the upper troposphere of the middle latitudes and subtropical regions of the Northern and Southern Hemispheres. Flowing in a semi-continuous band around the globe from west to east, it is caused by the changes in air temperature where the cold polar air moving towards the equator meets the warmer equatorial air moving polar ward. It is marked by a concentration of isotherms and strong vertical shear.
Related terms: arctic jet, low level jet, polar jet, and subtropical jet

Index
K:

KATABATIC WIND
A wind that is created by air flowing downhill. When this air is warm, it may be called a foehn wind, and regionally it may be known as a Chinook or Santa Ana. When this air is cold or cool, it is called a drainage wind, and regionally it may be known as a mountain breeze or glacier wind. The opposite of an anabatic wind.

KATAFRONT
A front where the warm air descends the frontal surface, except in the low layers of the atmosphere.

KELVIN TEMPERATURE SCALE
A temperature scale with the freezing point of +273K (Kelvin) and the boiling point of +373 K. It is used primarily for scientific purposes. Also known as the Absolute Temperature Scale. Proposed in 1848 by William T. Kelvin, 1st Baron of Largs (1824-1907), Irish-born Scottish physicist and mathematician.

K INDEX
The measure of thunderstorm potential based on the vertical temperature lapse rate, the moisture content of the lower atmosphere and the vertical extent of the moist layer.

KNOT
A nautical unit of speed equal to the velocity at which one nautical mile is traveled in one hour. Used primarily by marine interests and in weather observations. A knot is equivalent to 1.151 statute miles per hour or 1.852 kilometers per hour.
Related term: Dave's Dictionary

Index
L:

LAKE EFFECT SNOW
Snow showers that are created when cold dry air passes over a large warmer lake, such as one of the Great Lakes, and picks up moisture and heat.

LAND BREEZE
A diurnal coastal breeze that blows offshore, from the land to the sea. It is caused by the temperature difference when the sea surface is warmer than the adjacent land. Predominate during the night, it reaches its maximum about dawn. It blows in the opposite direction of a sea breeze.

LANDFALL
The point at which a tropical cyclone's eye first crosses a land mass.

LANDSPOUT
A small, weak tornado, which is not formed by a storm-scale rotation. It is generally weaker than a super cell tornado and is not associated with a wall cloud or mesocyclone. It may be observed beneath cumulonimbus or towering cumulus clouds and is the land equivalent of a waterspout.

LAPSE RATE
The change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height and is a sign of instability.
Related term: absolute instability

LATENT HEAT
The energy released or absorbed during a change of state.
Related terms: condensation and sublimation

LATITUDE
The location north or south in reference to the equator, which is designated at zero (0) degrees. Parallel lines that circle the globe both north and south of the equator. The poles are at 90° North and South latitude.

LEE/LEESIDE/LEEWARD
The side of an object or obstacle, such as a ship's sail, a mountain, or a hill, furthest away from the wind, and therefore, protected from the direct force of the wind. The opposite of windward.

LENTICULAR CLOUD
A cloud species which has elements resembling smooth lenses or almonds and more or less isolated. These clouds are caused by a wave wind pattern created by the mountains. They are also indicative of down-stream turbulence on the leeward side of a barrier.
Related term: Dave's Dictionary

LEVEL OF FREE CONVECTION (LFC)
The level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely. This occurs most readily in a conditionally unstable atmosphere.

LIFTED INDEX (LI)
A measure of atmospheric instability that is obtained by computing the temperature that the air near the ground would have if it were lifted to a higher level and comparing it to the actual temperature at that altitude. Positive values indicate more stable air and negative values indicate instability.

LIFTING CONDENSATION LEVEL (LCL)
The height at which a parcel of moist air becomes saturated when it is lifted dry adiabatically.

LIGHTNING
A sudden and visible discharge of electricity produced in response to the build up of electrical potential between cloud and ground, between clouds, within a single cloud, or between a cloud and surrounding air.
Related terms: ball lightning and heat lightning

LIGHT WAVES
That part of the electromagnetic spectrum that contains visible light. The colors, from longest wave length to shortest, are red, orange, yellow, green, blue, indigo, and violet (ROY G. BIV).
Related term: visible light

LINE ECHO WAVE PATTERN (LEWP)
A wave-shaped bulge in a line of thunderstorms. It may often be seen as a "S"-shaped radar echo signature and is often associated with severe weather.

LITHOMETEOR
Atmospheric phenomena which affect the state of the atmosphere. They constitute dry particles that hang suspended in the atmosphere, such as dust, smoke, sand, and haze.

LITHOSPHERE
The solid, outer portion of the earth's crust coupled to the rigid upper mantle. Part of the geosphere.

LONGITUDE
The location east or west in reference to the Prime Meridian, which is designated as zero (0) degrees longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's North and South Poles. Time zones are correlated to longitude.
Related term: Greenwich Mean Time

LONG WAVE TROUGH
A wave in the prevailing westerly flow aloft which is characterized by a large length and amplitude. A long wave moves slowly and is persistent. Its position and intensity govern weather patterns over a period of days or weeks.

LOW CLOUDS
A term used to signify clouds with bases below 6,000 feet and are of a stratiform or a cumuliform variety. Stratiform clouds include stratus and stratocumulus. Cumuliform clouds include cumulus and cumulonimbus. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.

LOW LATITUDES
The latitude belt between 30 and 0 degrees North and South of the equator. Also referred to as the tropical or torrid region.

LOW LEVEL JET (LLJ)
Strong winds that are concentrated in relatively narrow bands in the lower part of the atmosphere. It is often amplified at night. The southerly wind over the US Plains states during spring and summer is a notable example.
Related term: jet stream

LOW PRESSURE SYSTEM
An area of a relative pressure minimum that has converging winds and rotates in the same direction as the earth. This is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also known as an cyclone, it is the opposite of an area of high pressure, or a anticyclone.
Related terms: closed low, cold low, and cut-off low

LUNAR ECLIPSE
An eclipse of the moon occurs when the earth is in a direct line between the sun and the moon. The moon does not have any light of its own, instead, it reflects the sun's light. During a lunar eclipse, the moon is in the earth's shadow. It will often look dim and sometimes copper or orange in color.

Index
M:

MACKEREL SKY
The name given to cirrocumulus clouds with small vertical extent and composed of ice crystals. The rippled effect gives the appearance of fish scales.

MACROBURST
A large down burst with an outflow diameter of 2.5 miles (4 kilometers) or larger and damaging winds.

MACROSCALE
The meteorological scale covering an area ranging from the size of a continent to the entire globe.

MAGNETIC POLES
Either of the two points on the earth's surface where the magnetic meridians converge. They are not aligned with the geographical poles, but shift and do not lie exactly opposite of the other.

MAMMATOCUMULUS
An obsolete term for cumulonimbus mammatus, it is a portion of a cumulonimbus cloud that appears as a pouch or udder on the under surface of the cloud. Although they do not cause severe weather, they often accompany storms.

MARE'S TAIL
The name given to thin, wispy cirrus clouds composed of ice crystals that appear as veil patches or strands, often resembling a horse's tail.

MARGINAL VISUAL FLIGHT RULES (MVFR)
Refers to the general weather conditions pilots can expect at the surface. MVFR means Minimum or Marginal Visual Flight Rules. MVFR criteria means a ceiling between 1,000 and 3,000 feet and/or 3 to 5 miles visibility.
Related terms: IFR and VFR

MARITIME AIR MASS
An air mass influenced by the sea. It is a secondary characteristic of an air mass classification, signified by the small "m" before the primary characteristic, which is based on source region. For example, mP is an air mass that is maritime polar in nature. Also known as a marine air mass.

MAXIMUM
The greatest value attained by a function, for example, temperature, pressure, or wind speed. The opposite of minimum.

MEAN SEA LEVEL
The average height of the sea surface water level. For the United States, it is computed by averaging the levels of all tide stages over a nineteen year period, determined from hourly height readings measured from a fix, predetermined reference level. It is used as a basis for determining elevations, as the reference for all altitudes in upper air measurements, and as the level above which altitude is measured by a pressure altimeter for aviation. Often referred to as MSL.
Related term: sea level

MEAN TEMPERATURE
The average of temperature readings taken over a specified amount of time. Often the average of the maximum and minimum temperatures.

MEASURABLE RAIN
"Measurable rain" refers to a rainfall total of 0.01 inches or greater. When you hear the terms "isolated" showers or "few" showers (10-20%), "scattered" showers (30-50%), or "numerous" showers(60-70%), in the forecast, this refers to the percent of the forecast area covered by measurable rain. For instance, "scattered showers" means that the forecast area WILL receive rain, and approximately 30-50 percent of the area will experience showers.

MEASURED CEILING
A ceiling classification applied when the ceiling value has been determined by an instrument, such as a ceilometer or ceiling light, or by the known heights of unobscured portions of objects, other than natural landmarks, near the runway.
Related term: variable ceiling

MELTING LEVEL
The altitude at which ice crystals and snow flakes melt as they descend through the atmosphere.
Related term: bright band

MELTING POINT
The temperature at which a solid substance undergoes fusion, changing from a solid to a liquid state. Contrast with freezing point.

MERCURIAL BAROMETER
An instrument used for measuring the change in atmospheric pressure. It uses a long glass tube, open at one end and closed at the other. After first filling the open end with mercury, it is then temporarily sealed and placed into a cistern of mercury. A nearly perfect vacuum is established at the closed end after the mercury descends. The height of the column of mercury in the tube is a measurement of air pressure. As atmospheric pressure increases, the mercury is forced from the cistern up the tube; when the atmospheric pressure decreases, the mercury flows back into the cistern. Measurement is taken in inches of mercury. Although mercurial barometers are very accurate, practicality has led observers to use aneroid barometers. First used by Evangelista Torricelli (1608-1647), an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.

MERIDIONAL FLOW
Atmospheric circulation in which the north and south, or meridional, component of motion is unusually pronounced. This weakens the zonal flow.

MESOCYCLONE
A area of rotation of storm size that may often be found on the southwest part of a super cell. Its circulation can be larger than the tornado that may develop within it, but not necessarily. Originally a radar term for a rotation signature that met certain criteria, it is best seen on Doppler radar.

MESOHIGH
A small, concentrated area of high pressure that may be created by the cold outflow and rain-cooled air from thunderstorms. It often forms a pseudo cold front or squall line on its leading edges.
Related terms: bubble high

MESOLOW
A small scale low pressure center, ranging from the size of an individual thunderstorm to many tens of miles.

MESOSCALE
The scale of meteorological phenomena that range in size from several kilometers to around 100 kilometers. This includes MCCs, MCSs, and squall lines. Smaller phenomena are classified as microscale while larger are classified as synoptic-scale.

MESOSCALE CONVECTIVE COMPLEX (MCC)
A large mesoscale convective system (MCS) which is about the size of the state of Ohio or Iowa and lasts at least 6 hours. Generally forming during the afternoon and evening, the complex normally reaches its peak intensity at night when heavy rainfall and flooding become the primary threat. Severe weather may occur at anytime.

MESOSCALE CONVECTIVE SYSTEM (MCS)
A large organized convective weather system comprised of a number of individual thunderstorms. It normally persists for several hours and may be rounded or linear in shape. This term is often used to describe a cluster of thunderstorms that does not meet the criteria of a mesoscale convective complex (MCC).

MESOSPHERE
The layer of the atmosphere located between the stratosphere and the ionosphere, where temperatures drop rapidly with increasing height. It extends between 31 and 50 miles (17 to 80 kilometers) above the earth's surface.

METAR
Acronym for METeorological Aerodrome Report. It is the primary observation code used in the United States to satisfy requirements for reporting surface meteorological data. Minimum reporting requirements includes wind, visibility, runway visual range, present weather, sky condition, temperature, dew point, and altimeter setting.

METEOROLOGY/METEOROLOGIST
The science and study of the atmosphere and atmospheric phenomena. Various areas of meteorology include agricultural, applied, astrometerology, aviation, dynamic, hydrometeorology, operational, and synoptic, to name a few. A scientist who studies the atmosphere and atmospheric phenomena.
Related term: Dave's Dictionary

MICROBAROGRAPH
A instrument designed to continuously record a barometer's reading of very small changes in atmospheric pressure.

MICROBURST
A severe localized wind blasting down from a thunderstorm. It covers an area less than 2.5 miles (4 kilometers) in diameter and is of short duration, usually less than 5 minutes.
Related term: down burst

MICROSCALE
The smallest scale of meteorological phenomena that range in size from a few centimeters to a few kilometers. Larger phenomena are classified as mesoscale. It also refers to small scale meteorological phenomena with life spans of less than a few minutes that affect very small areas and are strongly influenced by local conditions of temperature and terrain.

MIDDLE CLOUDS
A term used to signify clouds with bases between 6,000 and 18,000 feet. At the higher altitudes, they may also have some ice crystals, but they are composed mainly of water droplets. Altocumulus, altostratus, and nimbostratus are the main types of middle clouds. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.

MIDDLE LATITUDES
The latitude belt roughly between 35 and 65 degrees North and South. May be referred to as the temperate region.

MILLIBAR (MB)
The standard unit of measurement for atmospheric pressure used by the National Weather Service. One millibar is equivalent to 100 newtons per square meter. Standard surface pressure is 1,013.2 millibars.

MINIMUM
The least value attained by a function, for example, temperature, pressure, or wind speed. The opposite of maximum.

MIST
A collection of microscopic water droplets suspended in the atmosphere. It does not reduce visibility as much as fog and is often confused with drizzle.

MIXED LAYER
It is the upper portion of the boundary layer in which air is thoroughly mixed by convection. In oceanography, it is the layer of the water that is mixed through wave action or thermohaline convection.

MIXED PRECIPITATION
Any of the following combinations of freezing and frozen precipitation: snow and sleet, snow and freezing rain, or sleet alone. Rain may also be present.

MOIST ADIABAT
The line on a Skew T-Log P chart that depicts the change in temperature of saturated air as it rises and undergoes cooling due to adiabatic expansion. As saturated air rises, the temperature changes at a rate of 0.55°C per 100 meters (2-3°F per 1,000 feet).
Related term: dry adiabat

MOISTURE
Refers to the water vapor content in the atmosphere, or the total water, liquid, solid or vapor, in a given volume of air.

MONSOON
The seasonal shift of winds created by the great annual temperature variation that occurs over large land areas in contrast with associated ocean surfaces. The monsoon is associated primarily with the moisture and copious rains that arrive with the southwest flow across southern India. The name is derived from the word mausim, Arabic for season. This pattern is most evident on the southern and eastern sides of Asia, although it does occur elsewherehe southwestern United States.
Related term: Dave's Dictionary

MOON PHASE MISCONCEPTOIN
The most common incorrect reason given for the cause of the Moon's phases is that we are seeing the shadow of the Earth on the Moon! But this cannot be correct: when the Moon passes through the shadow of the Earth, we get a lunar eclipse. Anyone who has seen a lunar eclipse, though, might remember that the Moon actually passes through the Earth's shadow only rarely, so that can't be why the Moon has phases. The real reason for the Moon's phases depends on two things: the Moon is round, and the angle it makes with the Earth and Sun changes over its orbit.

MOUNTAIN BREEZE
A katabatic wind, it is formed at night by the radiational cooling along mountainsides. As the slopes become colder than the surrounding atmosphere, the lower levels of air cool and drain to the lowest point of the terrain. It may reach several hundred feet in depth, and extreme cases, attain speeds of 50 knots or greater. It blows in the opposite direction of a valley breeze.

MOUNTAIN WAVE
A wave in the atmosphere caused by a barrier, such as a mountain. Sometimes it is marked by lenticular clouds to the lee side of mountain barriers. May be called a standing wave or a lee wave.

MUD SLIDE
Fast moving soil, rocks and water that flow down mountain slopes and canyons during a heavy a downpour of rain.

MUGGY
A subjective term for warm and excessively humid weather.

MULTICELL STORM
A thunderstorm made up of two or more single-cell storms.

MULTIPLE VORTEX TORNADO
A tornado which has two or more condensation funnels or debris clouds, often rotating around a common center.

Index
N:

NADIR
The point on any given observer's celestial sphere diametrically opposite of one's zenith.

NATIONAL CENTER FOR ATMOSPHERIC RESEARCH (NCAR)
A division of the University Corporation for Atmospheric Research, the Center plans, organizes, and conducts atmospheric and related research programs in collaboration with universities. For further information, contact NCAR, located in Boulder, Colorado.

NATIONAL CENTERS FOR ENVIRONMENTAL PREDICTION (NCEP)
As part of the National Weather Service, the centers provide timely, accurate, and continually improving worldwide forecast guidance products. Some of the centers include the Aviation Weather Center, the Climate Prediction Center, the Storm Prediction Center, and the Tropical Prediction Center. Formerly known as NMC.
For further information, contact the NCEP, with central offices located in Silver Spring, Maryland.

NATIONAL CLIMATIC DATA CENTER (NCDC)
The agency that archives climatic data from the National Oceanic and Atmospheric Administration as well as other climatological organizations.
For further information, contact the NCDC, located in Asheville, North Carolina.

NATIONAL HURRICANE CENTER (NHC)
A branch of the Tropical Prediction Center, it is the office of the National Weather Service that is responsible for tracking and forecasting tropical cyclones over the North Atlantic, Caribbean, Gulf of Mexico, and the Eastern Pacific.
For further information, contact the NHC, located in Miami, Florida.

NATIONAL METEOROLOGICAL CENTER (NMC)
Now incorporated into the National Centers for Environmental Prediction, it was the division of the National Weather Service that produced, processed, handled, and distributed meteorological and oceanographic information to users throughout the Northern Hemisphere, specifically U.S. governmental organizations.

NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION (NOAA)
A branch of the U.S. Department of Commerce, it is the parent organization of the National Weather Service. It promotes global environmental stewardship, emphasizing atmospheric and marine resources.
For further information, contact NOAA, located in Silver Spring, Maryland.

NATIONAL SEVERE STORMS FORECAST CENTER (NSSFC)
As of October 1995, the responsibilities of this Center were divided into two branches, the Storm Prediction Center and the Aviation Weather Center.

NATIONAL SEVERE STORMS LABORATORY (NSSL)
A branch of the National Oceanic and Atmospheric Administration, it provides accurate and timely forecasts and warnings of hazardous weather events, especially flash floods, hail, lightning, tornadoes, and other severe wind storms.
For further information, contact the NSSL, headquartered in Norman, Oklahoma.

NATIONAL WEATHER ASSOCIATION (NWA)
An organization whose membership promotes excellence in operational meteorology and related activities, recognizing the professional as well as the volunteer. For further information, contact the NWA.
Related term: American Meteorological Society

NATIONAL WEATHER SERVICE (NWS)
A primary branch of the National Oceanic and Atmospheric Administration, it is responsible for all aspects of observing and forecasting atmospheric conditions and their consequences, including severe weather and flood warnings. For further information, contact the NWS.

NAUTICAL MILE
A unit of length used in marine navigation that is equal to a minute of arc of a great circle on a sphere. One international nautical mile is equivalent to 1,852 meters or 1.151 statue miles.
Related Term: sea mile

NAUTICAL TWILIGHT
The time after civil twilight, when the brighter stars used for celestial navigation have appeared and the horizon may still be seen. It ends when the center of the sun is 12 degrees below the horizon, and it is too difficult to perceive the horizon, preventing accurate sighting of stars.
Related term: twilight

NEAP TIDE
A tide of decreased range, which occurs about every two weeks when the moon is at one quarter or three-quarters full.
Related term: spring tide

NEGATIVE VORTICITY ADVECTION
The advection of lower values of vorticity into an area.
Related term: positive vorticity advection

NEPHELOCOCCYGIA
A term applied when people find familiar objects within the shape of a cloud.

NEWHALL WINDS
The local name for winds blowing downward from desert uplands through the Newhall Pass southward into the San Fernando Valley, north of Los Angeles.

NEWTON
The unit of force giving a mass of about one kilogram (2.205 pounds) an acceleration of about one meter (1 yard) per second per second.

NEXRAD
Acronym for NEXt Generation Weather RADar. A network of advanced Doppler radars implemented in the United States between 1992 and 1996, it detects the location and intensity of precipitation out to a range of 143 miles from the radar site. NEXRAD Doppler radar is highly sensitive and can detect precipitation from very light rain and snow up to the strongest thunderstorms with accuracy and detail. However, sometimes the radar's extreme sensitivity will cause ground clutter and other non-precipitation echoes to be displayed in the vicinity of the radar site.

NIGHT
The period of the day between dusk and dawn.

NIMBOSTRATUS
This cloud exhibits a combination of rain or snow, and sometimes the base of the cloud cannot be seen because of the heaviness of precipitation. They are generally associated with fall and winter conditions, but can occur during any season.

NITROGEN (N2)
A colorless, tasteless, odorless gas that is the most abundant constituent of dry air. It comprises 78.09%.

NOCTILUCENT CLOUDS
Rarely seen clouds of tiny ice particles that form approximately 75 to 90 kilometers above the earth's surface. They have been seen only during twilight (dusk and dawn) during the summer months in the higher latitudes. They may appear bright against a dark night sky, with a blue-silver color or orange-red.

NOCTURNAL THUNDERSTORMS
Thunderstorms which develop after sunset. They are often associated with the strengthening of the low level jet and are most common over the Plains states. They also occur over warm water and may be associated with the seaward extent of the overnight land breeze.

NOR'EASTER
A cyclonic storm occurring off the east coast of North America. These winter weather events are notorious for producing heavy snow, rain, and tremendous waves that crash onto Atlantic beaches, often causing beach erosion and structural damage. Wind gusts associated with these storms can exceed hurricane force in intensity. A nor'easter gets its name from the continuously strong northeasterly winds blowing in from the ocean ahead of the storm and over the coastal areas.
Related term: Dave's Dictionary

NORMAL
The recognized standard value of a meteorological element as it has been averaged in a given location over a fixed number of years. Normals are concerned with the distribution of data within limits of common occurrence. The parameters may include temperatures (high, low, and deviation), pressure, precipitation (rain, snow, etc.), winds (speed and direction), thunderstorms, amount of clouds, percent relative humidity, etc.

NORTH PACIFIC HIGH
A semi-permanent, subtropical area of high pressure in the North Pacific Ocean. It is strongest in the Northern Hemispheric summer and is displaced towards the equator during the winter when the Aleutian Low becomes more dominate.
Related terms: Azores High and the Bermuda High

NOWCAST
A short-term weather forecast for expected conditions in the next few hours.

NUMERICAL FORECASTING
The use of numerical models, such as the fundamental equations of hydrodynamics subjected to observed initial conditions, to forecast the weather. These models are run on high-speed computers at the National Centers for Environmental Prediction.

Index
O:

OBSCURATION
Any phenomena in the atmosphere, excluding precipitation, that reduces horizontal visibility. According to the National Weather Service, some of the obstructions to visibility include blowing and widespread dust, fog (including freezing fog and patchy fog), haze, mist, sand and blowing sand, smoke, blowing spray, and volcanic ash. It is reported as "X" in an observation and on the METAR.
Related term: partial obscuration

OBSERVATION
In meteorology, the evaluation of one or more meteorological elements, such as temperature, pressure, or wind, that describe the state of the atmosphere, either at the earth's surface or aloft. An observer is one who records the evaluations of the meteorological elements.

OCCLUDED FRONT
Also known as an occlusion, it is a complex front formed when a cold front overtakes a warm front. It develops when three thermally different air masses conflict. The type of frontal boundary they create depends on the manner in which they meet.
Related terms: cold front and warm front

OCEAN
The intercommunicating body of salt water occupying the depressions of the earth's surface, or one of its major primary subdivisions, bounded by the continents, or the equator, and other imaginary lines. A sea is subdivision of an ocean.

OCEANOGRAPHY
The study of the ocean, embracing and integrating all knowledge pertaining to the ocean's physical boundaries, the chemistry and physics of sea water, and marine biology.

OMEGA BLOCK
A warm high aloft which has become displaced and is on the polar ward side of the jet stream. It frequently occurs in the late winter and early spring in the Northern Hemisphere. The name comes from its resemblance to the Greek letter, Omega, when analyzed on upper air charts.
Related term: blocking high

OPAQUE
A condition where a material, such as a cloud, blocks the passage of radiant energy, especially light. Opaque sky cover refers to the amount of sky cover that completely hides all that might be above it.

OROGRAPHIC LIFTING
Where the flow of air is forced up and over barriers such as highlands or mountains. Moist air being forced aloft begins to cool, consequently condensation forms, and rain or snow begins to fall. By the time the air reaches the leeward side of the barrier, it sinks and warms, resulting in decreasing relative humidity, cessation of precipitation, and the dissipation of clouds. May be called an orographic uplift.

OUTFLOW
Also referred to as an outflow boundary, it is the outward flow of air from a system, such as a thunderstorm. It is the result of cold down drafts and its passage includes a wind shift and temperature drop.
Related terms: bubble high and meso high

OVERCAST
The amount of sky cover for a cloud layer that is 8/8ths, based on the summation layer amount for that layer.

OVERRUNNING
This occurs when a relatively warm air mass is forced above a cooler air mass of greater density. Weather generally associated with this event includes cloudiness, cool temperatures, and steady precipitation.

OXYGEN (O2)
A colorless, tasteless, odorless gas that is the second most abundant constituent of dry air, comprising 20.946%.

OZONE (O3)
A nearly colorless gas and a form of oxygen (O2). It is composed of an oxygen molecule made up of three oxygen atoms instead of two.

OZONE LAYER
An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, around 9.5 to 12.5 miles (15 to 20 kilometers) above the earth's surface.

Index
P:

PALMER DROUGHT INDEX
A long-term meteorological drought severity index produced by the NOAA/USDA (Department of Agriculture) Joint Agricultural Weather Facility. The index depicts prolonged times, as in months or years, of abnormal dryness or wetness. It responds slowly, changing little from week to week, and reflects long-term moisture runoff, recharge, and deep percolation, as well as evapotranspiration.

PALOUSER
A strong, dangerous, katabatic wind that descends from the mountains into the Palouse River valley in northern Idaho and eastern Washington. May be called a Cow-Killer.

PARCEL
A volume of air small enough to contain uniform distribution of its meteorological properties and large enough to remain relatively self-contained and respond to all meteorological processes.

PARHELION
The scientific name for sun dogs. Either of two colored luminous spots that appear at roughly 22 degrees on both sides of the sun at the same elevation. They are caused by the refraction of sunlight passing through ice crystals. They are most commonly seen during winter in the middle latitudes and are exclusively associated with cirriform clouds. They are also known as mock suns.
Related term: Dave's Dictionary

PARTIAL OBSCURATION
Denotes that 1/8th or more of the sky, but not all of the sky, is hidden by any surface-based phenomena in the atmosphere, excluding precipitation. It often reduces horizontal visibility but not the vertical. It is reported as "X" in an observation and on the METAR.
Related term: obscuration

PARTLY CLOUDY
The state of the weather when the clouds are conspicuously present, but do not completely dull the sky or the day at any moment. The National Weather Service does not have an amount of sky cover for this condition.
Related terms: clear, few, scattered, broken, and overcast

PASCAL
The unit of pressure produced when one newton acts on about one square meter.

PASCAL'S LAW
When an external pressure is applied to any confined fluid at rest, the pressure is increased at every point in the fluid by the amount of external pressure applied. It means that the pressure of the atmosphere is exerted not only downward on the surface of an object, but also in all directions against a surface which is exposed to the atmosphere. Formulated by Blaise Pascal (1623-1662), a French mathematician, theologian, and physicist.

PEAK GUST
The highest instantaneous wind speed observed or recorded.

PERIGEE
The point nearest the earth on the moon's orbit. This term can be applied to any other body orbiting the earth, such as satellites. It is the opposite of apogee.

PERIHELION
The point of the earth's orbit that is nearest to the sun. Although the position is part of a 21,000 year cycle, currently it occurs around January, when the earth is about 3 million miles closer to the sun than at aphelion. This term can be applied to any other celestial body in orbit around the sun. It is the opposite of aphelion.

PHOTOMETER
Any of a number of atmospheric phenomena which appear as luminous patterns in the sky. They do not directly cause adverse weather. They include halos, coronas. rainbows, and fogbows.

PHOTOSPHERE
The intensely bright portion of the sun visible to the unaided eye; the "surface" of the sun. Reaching temperatures estimated at about 11,000°F, it is the portion of the sun's atmosphere which emits continuous electromagnetic radiation.

PILOT BALLOON
A small balloon whose ascent is used to determine the direction and speed of low level atmospheric winds. Also known as a pibal.

PILOT REPORT
A report of in-flight weather by an aircraft pilot or crew member. Often referred to as a PIREP.

PLAN POSITION INDICATOR
Also known as a PPI Scope, it is a radar indicator scope displaying range and azimuth of targets in polar coordinates.

PLOW/PLOUGH WIND
The spreading downdraft and strong straight-line winds preceding a thunderstorm. So named in the American Midwest because of its ability to flatten tall grasses as it passes.
Related term: first gust

POLES/POLAR
The poles are the geographic point at 90 degrees latitude North and South on the earth's surface. They are equal distance from the equator. The polar region is considered to be that area between 60° and 90° latitude, both North and South.

POLAR AIR MASS
An air mass that forms over a high latitude region. Continental polar air (cP) is formed over cold surface regions and is typically very stable with low moisture. Maritime polar air (mP), produced over warmer waters, is less stable with high moisture.

POLAR FRONT
A semi-continuous, semi-permanent boundary between polar air masses and tropical air masses. An integral part of an early meteorological theory known as the Polar Front Theory.

POLAR JET
Marked by a concentration of isotherms and strong vertical shear, this jet is the boundary between the polar air and the subtropical air. It often divides into two branches, the north and the south, and marks the high speed core of the prevailing westerlies. It is associated with the location and motion of the high and low pressure areas of the middle latitudes, and therefore, is variable in position, elevation, and wind speed. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer, and its core winds increase during the winter and become less strong in the summer.

POLAR-ORBITING SATELLITE
A satellite whose orbit passes over both of the earth's between poles.
Related term: geostationary satellite

POLLUTANT
Particles, gases, or liquid aerosols in the atmosphere which have an undesirable effect on humans or their surroundings. Something unfavorable to health and life that has been added to the environment.

POSITIVE VORTICITY ADVECTION
The advection of higher values of vorticity into an area. It is also known as cyclonic vorticity.
related term: negative vorticity advection

POUNDS PER SQUARE INCH (PSI)
A unit for measuring pressure. One PSI equals the pressure resulting from a force of one pound force acting over an area of one square inch.

PRECIPITATION
Any and all forms of water, liquid or solid, that falls from clouds and reaches the ground. This includes drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow, snow pellets, and snow grains. The amount of fall is usually expressed in inches of liquid water depth of the substance that has fallen at a given point over a specified time period.
Related term: Dave's Dictionary

PRE-FRONTAL SQUALL LINE
A line of thunderstorms that precedes an advancing cold front.
Related term: squall line

PRE-FRONTAL TROUGH
An elongated area of relatively low pressure preceding a cold front that is usually associated with a shift in wind direction.
Related term: trough

PRESSURE
The force per unit area exerted by the weight of the atmosphere above a point on or above the earth's surface.
Related terms: atmospheric pressure and barometric pressure

PRESSURE ALTIMETER
An aneroid barometer calibrated to indicate altitude in feet instead of units of pressure. It is read accurately only in a standard atmosphere and when the correct altimeter setting is used.

PRESSURE ALTITUDE
The altitude in standard atmosphere at which a given pressure will be observed. It is the indicated altitude of a pressure altimeter at an altitude setting of 29.92 inches of mercury, and is therefore the indicated altitude above the 29.92 constant pressure surface.

PRESSURE CHANGE
The net difference between the barometric pressure at the beginning and ending of a specified interval of time, usually the three hour period preceding an observation.

PRESSURE CHARACTERISTIC
The pattern of the pressure change during the specified period of time, usually the three hour period preceding an observation. This is recorded in three categories: falling, rising, or steady.

PRESSURE GRADIENT
The amount of pressure change that occurs over a fixed distance at a fixed altitude.

PRESSURE JUMP
A sudden increase in the observed atmospheric pressure or station pressure.

PRESSURE TENDENCY
The pressure characteristic and amount of pressure change during a specified time period, usually the three hour period preceding the observation.

PREVAILING WIND
A wind that blows from one direction more frequently than any other during a given period, such as a day, month, season, or year.

PREVAILING VISIBILITY
It is considered representative of visibility conditions at the observation station. It is the greatest distance that can be seen throughout at least half the horizon circle, but not necessarily continuous.

PROFILER
A type of Doppler radar that typically measures both wind speed and direction from the surface to 55,000 feet in the atmosphere.

PROGNOSTIC CHART
A chart of forecast predictions that may include pressure, fronts. precipitation, temperature, and other meteorological elements. Also known as a prog.

PSYCHROMETER
An instrument used to measure water vapor content of the atmosphere. It consists of two thermometers, a wet bulb and dry bulb. May also be referred to as a sling psychrometer.

PULSE
A very short duration of time. In regard to a radar, it is a brief burst of a electromagnetic radiation emitted by the radar.

Index
Q:

QUANTITATIVE PRECIPITATION FORECAST (QPF)
A forecast of rainfall, snowfall or liquid equivalent of snowfall.

QUASI-STATIONARY FRONT
A front which is nearly stationary or moves very little since the last synoptic position. Also known as a stationary front.

Index
R:

RADAR
Acronym for Radio Detection And Ranging. An electronic instrument used to detect distant objects and measure their range by how they scatter or reflect radio energy. Precipitation and clouds are detected by measuring the strength of the electromagnetic signal reflected back.
Related terms: Doppler radar and NEXRAD

RADARSONDE OBSERVATION
An upper air observation used to determine winds and other meteorological data, by tracking the range, elevation, and azimuth of a radar target carried aloft. A type of rawinsonde.

RADIAL VELOCITY
A type of velocity that expresses motion toward or away from a given location. In Doppler radar, it is the component of motion that is parallel to the radar beam.

RADIATION
The process by which energy is propagated through any medium by virtue of the wave motion of that medium. Electromagnetic radiation, which emits heat and light, is one form. Sound waves are another.

RADIATIONAL COOLING
The cooling of the earth's surface and the adjacent air. Although it occurs primarily at night, it happens when the earth's surface suffers a net loss of heat due to outgoing radiation.
Related terms: terrestrial radiation

RADIATION FOG
Fog that is created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its dew point. Formation is best when there is a shallow surface layer of relatively moist air beneath a drier layer, clear skies, and light surface winds. This primarily occurs during the night or early morning.
Related term: ground fog

RADIOSONDE
An instrument attached to a weather balloon used to measure pressure, temperature, humidity, and winds aloft. Observations are made when the radiosonde is aloft and emits radio signals as it ascends. May be referred to as a RAOB, an acronym for Radiosonde Observation.

RAIN
Precipitation in the form of liquid water droplets greater than 0.5 mm. If widely scattered, the drop size may be smaller. It is reported as "R" in an observation and on the METAR. The intensity of rain is based on rate of fall. "Very light" (R--) means that the scattered drops do not completely wet a surface. "Light" (R-) means it is greater than a trace and up to 0.10 inch an hour. "Moderate" (R) means the rate of fall is between 0.11 to 0.30 inch per hour. "Heavy" (R+) means over 0.30 inch per hour.

RAIN CHANCE
Terms such as "slight chance" of rain (10-20%), "chance" of rain (30-50%) or rain "likely" (60-70%) are used when there is uncertainty of receiving measurable precipitation anywhere in the forecast area. For instance, if there is only a 30-50 percent chance that rain will fall anywhere in the Metro area, then the forecast will call for a "chance" of rain.

RAINBOW
A luminous arc featuring all colors of the visible light spectrum (red, orange, yellow, green, blue, indigo, and violet). It is created by refraction, total reflection, and the dispersion of light. It is visible when the sun is shining through air containing water spray or raindrops, which occurs during or immediately after a rain shower. The bow is always observed in the opposite side of the sky from the sun.

RAINFALL
The amount of precipitation of any type, primarily liquid. It is usually the amount that is measured by a rain gauge.
Related terms: rain and quantitative precipitation

RAIN FOREST
A forest which grows in a region of heavy annual precipitation. There are two major types, tropical and temperate.

RAIN GAUGE
An instrument used to measure the amount of rain that has fallen. Measurement is done in hundredths of inches (0.01").

RAIN SHADOW
Also referred to as a precipitation shadow, it is the region on the lee side of a mountain or similar barrier where the precipitation is less than on the windward side. For example, the relatively dry Washoe Valley of western Nevada is in the rain shadow of the Sierra Nevada.

RANGE RESOLUTION
The ability of radar to distinguish between targets on the same azimuth but at different ranges.

RAWINSONDE
An upper air observation that evaluates the winds, temperature, relative humidity, and pressure aloft by means of a balloon-attached radiosonde that is tracked by a radar or radio direction-finder. It is a radiosonde observation combined with a winds-aloft observation, called a rawin.

RECONNAISSANCE (RECCO) CODE
An aircraft weather reconnaissance code that has come to refer primarily to in-flight tropical weather observations, but actually signifies any detailed weather observation or investigation from an aircraft in flight.

REFLECTIVITY
A measure of the process by which a surface can turn back a portion of incident radiation into the medium through which the radiation approached. It also refers to the degree by which precipitation is able to reflect a radar beam.
Related term: albedo

REFRACTION
The bending of light or radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature.

RELATIVE HUMIDITY
A type of humidity that considers the ratio of the actual vapor pressure of the air to the saturation vapor pressure. It is usually expressed in percentage.

RELATIVE VORTICITY
The sum of the rotation of an air parcel about the axis of the pressure system and the rotation of the parcel about its own axis.
Related term: vorticity

RESOLUTION
In relation to radar, it is the ability to read two distinct targets separately. The clearer the resolution, the nearer the two objects can be to each other and still be distinguishable.

RETROGRESSION
In meteorology, it is the movement of a weather system in a direction opposite to the direction of the basic flow in which it is embedded. Often used in reference to a long wave trough or other macroscale feature. For example, a long wave trough that may move slightly westward when the "normal" movement and flow is eastward.

RIDGE
An elongated area of high atmospheric pressure that is associated with an area of maximum anticyclonic circulation. The opposite of a trough.

RIME
The rapid freezing of super cooled water droplets as they touch an exposed object, forming a white opaque granular deposit of ice. It is one of the results of an ice storm, and when formed on aircraft it is called rime icing.
Related term: glaze

RIP CURRENT
It is formed by a strong surface water movement, or current, of a short duration that flows seaward from the shore. The return flow is piled up onshore by the incoming waves and wind. It is localized, of narrow width, and its position relative to the beach can change as the wave condition changes. Therefore, the higher the waves, the stronger the current.

ROCKETSONDE
A type of radiosonde that is shot into the atmosphere by a rocket, allowing it to collect data during its parachute descent from a higher position in the atmosphere than a balloon could reach.

ROLL CLOUD
A relatively rare, low-level, horizontal, tube-shaped cloud. Although they are associated with a thunderstorm, they are completely detached from the base of the cumulonimbus cloud.

ROSSBY WAVES
The movement of ridges and troughs in the upper wind patterns, primarily the jet stream, circling the earth. Named for Carl-Gustaf Rossby, a U.S. Weather Bureau (NWS) employee, who first theorized about the existence of the jet stream in 1939.

ROTATION
The spinning of a body, such as the earth, about its axis.

ROTOR CLOUD
An altocumulus cloud formation that can be found in the lee of a mountain or similar barrier. The air rotates around a horizontal axis, creating turbulence. Altocumulus lenticularis is an example.

RUNWAY VISUAL RANGE (RVR)
It is the maximum distance at which the runway, or the specified lights or markers delineating it, can be seen from a position above a specified point on its center line. This value is normally determined by visibility sensors located alongside and higher than the center line of the runway. RVR is calculated from visibility, ambient light level, and runway light intensity.

Index
S:

SAFFIR-SIMPSON DAMAGE-POTENTIAL SCALE
Developed in the early 1970s by Herbert Saffir, a consulting engineer, and Robert Simpson, then Director of the National Hurricane Center, it is a measure of hurricane intensity on a scale of 1 to 5. The scale categorizes potential damage based on barometric pressure, wind speeds, and surge.
Related term: Saffir Simpson Scale

ST. ELMO'S FIRE
A luminous, and often audible, electric discharge that is sporadic in nature. It occurs from objects, especially pointed ones, when the electrical field strength near their surfaces attains a value near 1000 volts per centimeter. It often occurs during stormy weather and might be seen on a ship's mast or yardarm, aircraft, lightning rods, and steeples. Also known as corposant or corona discharge.

SALINITY
A measure of the quantity of dissolved salts in sea water. The total amount of dissolved solids in sea water in parts per thousand by weight.

SALT WATER
The water of the ocean, distinguished from fresh water by its appreciable salinity.

SAND
Loose particles of hard, broken rock or minerals. In observing, sand is reported when particles of sand are raised to sufficient height that reduces visibility. It is reported as "SA" in an observation and on the METAR.

SANDSTORM
A strong wind carrying sand particles through the air. They are low level occurrences, usually only ten feet in height to not more than fifty feet above the surface. Due to the frequent winds created by surface heating, they are most predominate during the day and die out in the night. Visibility is reduced to between 5/8ths and 6/16ths statute mile, and if less than 5/16ths, then the storm is considered a heavy sandstorm. It is reported as "SS" in an observation and on the METAR.

SANTA ANA WINDS
The hot, dry winds, generally from the east, that funnel through the Santa Ana river valley south of the San Gabriel and San Bernadino Mountains in southern California, including the Los Angeles basin. Classified as katabatic, it occurs most often during the winter and it is an example of a foehn wind.

SARGASSO SEA
An area of the North Atlantic Ocean between Bermuda and the Azores. It is in the middle of the North Atlantic oceanic gyre, with converging surface waters. Consequently, it has less biological features than any other region of the ocean because the lack of mixing with more nutrient-rich waters.

SATELLITE
Any object that orbits a celestial body, such as a moon. However, the term is often used in reference to the manufactured objects that orbit the earth, either in a geostationary or a polar manner. Some of the information that is gathered by weather satellites, such as GOES9, includes upper air temperatures and humidity, recording the temperatures of cloud tops, land, and ocean, monitoring the movement of clouds to determine upper level wind speeds, tracing the movement of water vapor, monitoring the sun and solar activity, and relaying data from weather instruments around the world.

SATELLITE IMAGES
Images taken by a weather satellite that reveal information, such as the flow of water vapor, the movement of frontal system, and the development of a tropical system. Looping individual images aids meteorologists in forecasting. One way a picture can be taken is as a visible shot, that is best during times of visible light (daylight). Another way is as an IR (infrared) shot, that reveals cloud temperatures and can be used day or night.

SATURATE
To treat or charge something to the point where no more can be absorbed, dissolved, or retained. In meteorology, it is used when discussing the amount of water vapor in a volume of air.

SATURATION POINT
The point when the water vapor in the atmosphere is at its maximum level for the existing temperature.

SCATTERED
The amount of sky cover for a cloud layer between 3/8ths and 4/8ths, based on the summation layer amount for that layer.

SCATTERING
The process by which small particles suspended in the air diffuse a portion of the incident radiation in all directions. This is a primary reason for colors, such as blue skies, rainbows, and orange sunsets. When working with radars, this often refers to the more or less random changes in direction of radio energy.

SCUD
Low fragments of clouds, usually stratus fractus, that are unattached and below a layer of higher clouds, either nimbostratus or cumulonimbus. They are often along and behind cold fronts and gust fronts, being associated with cool moist air, such as an outflow from a thunderstorm. When observed from a distance, they are sometimes mistaken for tornadoes.

SEA BREEZE
A diurnal coastal breeze that blows onshore, from the sea to the land. It is caused by the temperature difference when the surface of the land is warmer than the adjacent body of water. Predominate during the day, it reaches its maximum early to mid afternoon. It blows in the opposite direction of a land breeze.

SEA BREEZE FRONT
A coastal phenomena, it is restricted to large bodies of water and their immediate coast lines. This is usually the landward extent of the sea breeze. Due to the imbalance of heating between land and water, a region of maximum upward motion or convergence occurs by mid-afternoon in the summer some 10 to 15 miles inland. Air mass thunderstorms or a line of towering cumulus clouds with showers can form along the front. At the beach, there are blue skies and a light breeze. This often occurs along the coast of the Gulf of Mexico and Florida's east coast.

SEA FOG
A type of advection fog which forms in warm moist air cooled to saturation as the air moves across cold water.
Related term: Arctic Sea Smoke

SEA ICE
Ice that is formed by the freezing of sea water. It forms first as small crystals, thickens into sludge, and coagulates into sheet ice, pancake ice, or ice floes of various shapes and sizes.

SEA LEVEL
The height or level of the sea surface at any time. It is used as a reference for elevations above and below.
Related term: mean sea level

SEA LEVEL PRESSURE
The atmospheric pressure at mean sea level, usually determined from the observed station pressure.

SEA MILE
A unit of length distinguished from a nautical mile. One sea mile is equivalent to 1,000 fathoms (6,000 feet). SEASON
A division of the year according to some regularly recurring phenomena, usually astronomical or climatic. For example, in the Northern Hemisphere, winter is said to begin on the winter solstice and end on the vernal equinox when spring begins, covering the months of December, January, and February. In the tropics, there is the dry and the rainy season, depending on the amount of precipitation.

SEA SPRAY
Sometimes called salt spray, it is the drops of sea water (salt water) blown from the top of a wave.
Related terms: blowing spray and condensation nuclei

SEA SURFACE TEMPERATURE (SST)
The temperature of the water's surface. It is measured using buoy and ship data, infrared satellite imagery, and coastal observations.

SEMI-PERMANENT PRESSURE SYSTEMS
A relatively stable, stationary pressure-and-wind system where the pressure is predominately high or low with the changing season. They are not of a transitory nature, like migratory lows that develop from temperature and density differences.
Related terms: Icelandic Low, Aleutian Low, North Pacific High, Siberian High, and Bermuda High

SEVERE WEATHER
Generally, any destructive weather event, but usually applies to localized storms, such as blizzards, intense thunderstorms, or tornadoes.

SEVERE THUNDERSTORM
A thunderstorm with winds measuring 50 knots (58 mph) or greater, 3/4 inch hail or larger, or tornadoes. Severe thunderstorms may also produce torrential rain and frequent lightning.
Related term: supercell

SHEAR
It is the rate of change over a short duration. In wind shear, it can refer to the frequent change in wind speed within a short distance. It can occur vertically or horizontally. Directional shear is a frequent change in direction within a short distance, which can also occur vertically or horizontally. When used in reference to Doppler radar, it describes the change in radial velocity over short distances horizontally.

SHEAR LINE
A line of maximum horizontal wind shear. A narrow zone across which there is an abrupt change in the horizontal wind component parallel to it.

SHORT WAVE
A progressive wave of smaller amplitude, wave length, and duration than a long wave. It moves in the same direction as the basic current in which it is embedded and may induce upward vertical motion ahead of it. They are more numerous than long waves and often disappear with height in the atmosphere.

SHOWALTER STABILITY INDEX
A measure of the local static stability of the atmosphere. It is determined by lifting an air parcel to 500 millibars and then comparing its temperature to that of the environment. If the parcel is colder than its new environment, then the atmosphere is more stable. If the parcel is warmer than its new environment, then the atmosphere is unstable and the potential for thunderstorm development and severe weather increases.

SHOWER
Precipitation from a convective cloud that is characterized by its sudden beginning and ending, changes in intensity, and rapid changes in the appearance of the sky. It occurs in the form of rain (SHRA), snow (SHSN), or ice (SHPE). It is reported as "SH" in an observation and on the METAR.

SIBERIAN EXPRESS
A fierce, cold flow of air that originates in Siberia, then moves into Alaska and northern Canada before moving southward into the United States.

SIBERIAN HIGH
The semi-permanent high pressure area that forms over Siberia during the winter. The average central pressure exceeds 1030 millibars from late November to early March. It is characterized by clear, dry weather. Over southern Asia, the predominate surface wind is northeasterly, just the opposite of the predominate summer winds which bring the monsoon.

SIDEREAL TIME
The measure of time as defined by the diurnal motion of the vernal equinox. A sidereal day is equivalent to one complete rotation of the earth relative to the equinox, which is 23 hours, 56 minutes, and 4.091 seconds. A sidereal year is the interval required for the earth to make one absolute revolution around the sun, which is 365 days, 6 hours, 9 minutes, and 9.5 seconds. Compare with the solar day.

SKEW T-LOG P DIAGRAM
A thermodynamic diagram, using the temperature and the logarithm of pressure as coordinates. It is used to evaluate and forecast air parcel properties. Some values that can be determined are the Convective Condensation Level (CCL), the Lifting Condensation Level (LCL), and the Level of Free Convection (LFC).

SKY
The vault-like apparent surface against which all aerial objects are seen from the earth.

SKY COVER
The amount of the celestial dome that is hidden by clouds and/or obscurations.

SLEET
Also known as ice pellets, it is winter precipitation in the form of small bits or pellets of ice that rebound after striking the ground or any other hard surface. It is reported as "PE" in an observation and on the METAR.

SLUSH
Snow or ice on the ground that has been reduced to a softy watery mixture by rain and/or warm temperatures.

SMALL CRAFT ADVISORY
An advisory issued for marine interests, especially for operators of small boats or other vessels. Conditions include wind speeds between 20 knots (23 mph) and 34 knots (39 mph).

SMOKE
Small particles produced by combustion that are suspended in the air. A transition to haze may occur when the smoke particles have traveled great distance (25 to 100 miles or more), and when the larger particles have settled out. The remaining particles become widely scattered through the atmosphere. It is reported as "FU" in an observation and on the METAR.

SNOW
Frozen precipitation in the form of white or translucent ice crystals in complex branched hexagonal form. It most often falls from stratiform clouds, but can fall as snow showers from cumuliform ones. It usually appears clustered into snowflakes. It is reported as "SN" in an observation and on the METAR.

SNOW ADVISORY
A statement or advisory issued when snow is expected to create hazardous travel conditions. It warns of less severe weather conditions than a winter storm

SNOW BANNER
A plume of snow blown off a mountain crest, resembling smoke blowing from a volcano.

SNOW BLINDNESS
Temporary blindness or impaired vision that results from bright sunlight reflected off the snow surface. The medical term is niphablepsia.

SNOWBURN
A burn of the skin, like a sunburn, but caused by the sun's rays reflected off the snow surface.

SNOW COVER
The areal extent of ground covered by the snow. It is usually expressed as a percent of the total area of a given region.

SNOW CREEP
A continuous, extremely slow, downhill movement of a layer of snow.

SNOW CRUST
The crisp, almost icy, surface on fallen snow, usually formed by the slight melting and refreezing of the surface snow.

SNOW DEPTH
The actual depth of snow on the ground at any instant during a storm, or after any single snowstorm or series of storms.

SNOW DEVIL
A small, rotating wind that picks up loose snow instead of dirt (like a dust devil) or water (like a waterspout). Formed mechanically by the convergence of local air currents. May be called a snowspout.

SNOW EATER
Any warm downslope wind, or foehn, that blows over snowy terrain and melts the snow.
Related term: Chinook and Dave's Dictionary

SNOWFALL
The rate at which snow falls, usually expressed in inches of snow depth over a six hour period.

SNOWFLAKES
An ice crystal or an aggregate of ice crystals which fall from clouds.

SNOW FLURRY/FLURRIES
Light showers of snow, generally very brief without any measurable accumulation. May be reported as "SHSN--" in an observation and on the METAR.

SNOW GARLAND
Snow appearing as a beautiful long thick rope draped on trees, fences and other objects. Formed by the surface tension of thin films of water bonding individual snow crystals.

SNOW GRAINS
Frozen precipitation in the form of very small, white, opaque grains of ice. The solid equivalent of drizzle. It is reported as "SG" in an observation and on the METAR.

SNOW LEVEL
The elevation in mountainous terrain where the precipitation changes from rain to snow, depending on the temperature structure of the associated air mass.

SNOW LINE
The lowest elevation area of a perennial snow field on high terrain, such as a mountain range.

SNOWPACK
The amount of annual accumulation of snow at higher elevations.

SNOW PELLETS
Frozen precipitation in the form of white, round or conical opaque grains of ice. Their diameter ranges from 0.08 to 0.2 inch (2 to 5 mm). They are easily crushed and generally break up after rebounding from a hard surface, unlike hail. Sometimes it is called small or soft hail. It is reported as "GS" in an observation and on the METAR.

SNOW ROLLER
The product of moist, cohesive snow that when initiated by wind rolls across the landscape, gathering snow until it can no longer move. It is shaped like a rolled sleeping bag, some reaching four feet across and seven feet in diameter.

SNOW SHOWER
Frozen precipitation in the form of snow, characterized by its sudden beginning and ending. It is reported as "SHSN" in an observation and on the METAR.

SNOW SQUALL
A heavy snow shower accompanied by sudden strong winds, or a squall.

SOLAR DAY
The complete rotation of the earth in relation to the sun. Although it varies, an average has determined a mean solar day of 24 hours. It is universally used for civil purposes.
Related term: sidereal day

SOLAR ECLIPSE
An eclipse of the sun occurs when the moon is in a direct line between the sun and the earth, casting some of the earth's surface in its shadow. The moon's disk shaped outline appears to cover the sun's brighter surface, or photosphere. That part of the earth that is directly in the moon's shadow will see a total eclipse of the sun, while the areas around it will see a partial eclipse.

SOLSTICE
The point at which the sun is the furthest on the ecliptic from the celestial equator. The point at which sun is at maximum distance from the equator and days and nights are most unequal in duration. The Tropic of Cancer and the Tropic of Capricorn are those parallels of latitude which lies directly beneath a solstice. In the Northern Hemisphere, the winter solstice falls on or about December 21 and the summer solstice on or about June 21.
Related term:Dave's Dictionary

SOUNDING
A plot of the atmosphere, using data rom upper air or radiosonde observations. Usually confined to a vertical profile of the temperatures, dew points, and winds above a fixed location.

SOUTHERN OSCILLATION
A periodic reversal of the pressure pattern across the tropical Pacific Ocean during El Nio events. It is represents the distribution of temperature and pressure over an oceanic area.

SPECIFIC HUMIDITY
The ratio of the density of the water vapor to the density of the air, a mix of dry air and water vapor. It is expressed in grams per gram or in grams per kilograms. The specific humidity of an air parcel remains constant unless water vapor is added to or taken from the parcel.

SPRING
The season of the year which occurs as the sun approaches the summer solstice, and characterized by increasing temperatures in the mid-latitudes. Customarily, this refers to the months of March, April, and May in the North Hemisphere, and the months of September, October, and November in the Southern Hemisphere. Astronomically, this is the period between the vernal equinox and the summer solstice.

SPRING TIDE
A tide of increased range, which occurs about every two weeks when the moon is new or full.
Related term: neap tide

SQUALL
A sudden onset of strong winds with speeds increasing to at least 16 knots (18 miles per hour) and sustained at 22 or more knots (25 miles per hour) for at least one minute. The intensity and duration is longer than that of a gust. It is reported as "SQ"s in an observation and on the METAR.

SQUALL LINE
A narrow band or line of active thunderstorms that is not associated with a cold front. It may form from an outflow boundary or the leading edge of a mesohigh.

STABLE/STABILITY
Occurs when a rising air parcel becomes denser than the surrounding air. It will then return to its original position. When the density of the air parcel remains the same as the surrounding air after being lifted, it is also considered stable, since it does not have the tendency to rise or sink further. Contrast with unstable air and instability.

STAGNATION AREA
An area that has a combination of stable stratification, weak horizontal wind speed, and little, if any, significant precipitation. It is usually associated with an area of high pressure.
Related terms: Tule fog

STANDARD ATMOSPHERE
A standard atmosphere has been defined by the International Civil Aeronautical Organization (ICAO). It assumes a mean sea level temperature of 15C a standard sea level pressure of 1,013.25 millibars or 29.92 inches of mercury, and a temperature lapse rate of 0.65C per 100 meters up to 11 kilometers in the atmosphere.

STANDARD SURFACE PRESSURE
The measurement of one atmosphere of pressure under standard conditions. It is equivalent to 1,013.25 millibars, 29.92 inches of mercury, 760 millimeters of mercury, 14.7 pounds per square inch, or 1.033 grams per square centimeter.

STANDING CLOUD
Any type of isolated cloud, generally formed over peaks or ridges of mountainous areas, that appears stationary or standing over the terrain.
Related term: altocumulus lenticularis

STANDING WAVE
An atmospheric wave that is stationary with respect to the medium in which it is embedded.
Related term: mountain wave

STATIONARY FRONT
A front which is nearly stationary or moves very little since the last synoptic position. May be known as a quasi-stationary front.

STATION ELEVATION
The vertical distance above mean sea level that is the reference level for all current measurements of atmospheric pressure at that station.

STATION PRESSURE
The atmospheric pressure with respect to the station elevation.

STEAM FOG
A type of advection fog that is produced by evaporation when cool air passes over a warm wet surface and the fog rises, giving the appearance of steam. Also called sea smoke when it occurs over the ocean.
Related term: Arctic Sea Smoke

STORM
An individual low pressure disturbance, complete with winds, clouds, and precipitation. The name is associated with destructive or unpleasant weather. Storm-scale refers to disturbances the size of individual thunderstorms.
Related terms: thunderstorms, tornadoes, and tropical cyclones

STORM PREDICTION CENTER (SPC)
A branch of the National Centers for Environmental Prediction, the Center monitors and forecasts severe and non-severe thunderstorms, tornadoes, and other hazardous weather phenomena across the United States. Formerly known as the Severe Local Storms (SELS) unit of the National Severe Storms Forecast Center. For further information, contact the SPC, located in Norman, Oklahoma.

STORM TRACKS
The path or tracks generally followed by a cyclonic disturbance.

STORM WINDS
On the Beaufort Wind Scale, a wind with speeds from 56 to 63 knots (64 to 72 miles per hour).

STRAIGHT-LINE WINDS
Any surface wind that is not associated with rotation. An example is the first gust from a thunderstorm, as opposed to tornadic winds.

STRATIFORM
Clouds composed of water droplets that exhibit no or have very little vertical development. The density of the droplets often blocks sunlight, casting shadows on the earth's surface. Bases of these clouds are generally no more than 6,000 feet above the ground. They are classified as low clouds, and include all varieties of stratus and stratocumulus. The opposite in type are the vertical development of cumuliform clouds.

STRATOCUMULUS
A low cloud composed of layers or patches of cloud elements. It can form from cumulus clouds becoming more stratiformed and often appears as regularly arranged elements that may be tessellated, rounded, or roll-shaped with relatively flat tops and bases. It is light or dark gray in color, depending on the size of the water droplets and the amount of sunlight that is passing through them.

STRATOPAUSE
The boundary zone or transition layer between the stratosphere and the mesosphere. Characterized by a decrease in temperature with increasing altitude.

STRATOSPHERE
The layer of the atmosphere located between the troposphere and the mesosphere, characterized by a slight temperature increase and absence of clouds. It extends between 11 and 31 miles (17 to 50 kilometers) above the earth's surface. It is the location of the earth's ozone layer.

STRATUS
One of the three basic cloud forms (the others are cirrus and cumulus. It is also one of the two low cloud types. It is a sheetlike cloud that does not exhibit individual elements, and is, perhaps, the most common of all low clouds. Thick and gray, it is seen in low, uniform layers and rarely extends higher than 5,000 feet above the earth's surface. A veil of stratus may give the sky a hazy appearance. Fog may form from a stratus cloud that touches the ground. Although it can produce drizzle or snow, it rarely produces heavy precipitation. Clouds producing heavy precipitation may exist above a layer of stratus.
Related term: Dave's Dictionary

STRATUS FRACTUS
Stratus clouds that appear in irregular fragments, as if they had been shred or torn. Also appears in cumulus clouds (called cumulus fractus), but not in cirrus clouds.

SUBLIMATION
The process of a solid (ice) changing directly into a gas (water vapor), or water vapor changing directly into ice, at the same temperature, without ever going through the liquid state (water). The opposite of crystallization.

SUBPOLAR
The region bordering the polar region, between 50 and 70 North and South latitude. This is generally an area of semi-permanent low pressure that exists and where the Aleutian and Icelandic Lows may be found. However, a dome of high pressure may form over the cold continental surfaces during the winter, for example, the North American High and the Siberian High.

SUBREFRACTION
Less than normal bending of light or a radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature.
Related term: superrefraction

SUBSIDENCE
A sinking or downward motion of air, often seen in anticyclones. It is most prevalent when there is colder, denser air aloft. It is often used to imply the opposite of atmospheric convection.

SUBTROPICAL
The region between the tropical and temperate regions, an area between 35 and 40 North and South latitude. This is generally an area of semi-permanent high pressure that exists and is where the Azores and North Pacific Highs may be found.

SUBTROPICAL AIR
An air mass that forms over the subtropical region. The air is typically warm with a high moisture content due to the low evaporative process.

SUBTROPICAL JET
Marked by a concentration of isotherms and vertical shear, this jet is the boundary between the subtropical air and the tropical air. It is found approximately between 25 and 35 North latitude and usually above an altitude of 40,000 feet. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer.

SUMMATION LAYER AMOUNT
The amount of sky cover for each layer is given in eighths of sky cover attributable to clouds or obscurations. The summation amount for any given layer is equal to the sum of the sky cover for the layer being evaluated plus the sky cover for all lower layers, including partial obscuration. A summation amount for a layer can not exceed 8/8ths.

SUMMER
Astronomically, this is the period between the summer solstice and the autumnal equinox. It is characterized as having the warmest temperatures of the year, except in some tropical regions. Customarily, this refers to the months of June, July, and August in the North Hemisphere, and the months of December, January, and February in the Southern Hemisphere.

SUN DOG
Either of two colored luminous spots that appear at roughly 22 on both sides of the sun at the same elevation. They are caused by the refraction of sunlight passing through ice crystals. They are most commonly seen during winter in the middle latitudes and are exclusively associated with cirriform clouds. The scientific name for sun dogs is parhelion and they are also known as mock suns.

SUN PILLAR
Horizontal ice crystals in the form of plates, which occur in clouds and ice fog near the earth's surface, reflect sunlight into vertical sun pillars for a spectacular display.

SUNRISE
The daily appearance of the sun on the eastern horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun appears on the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunrise is calculated for mean sea level.
related term: sunset

SUNSET
The daily disappearance of the sun below the western horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun just disappears below the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunset is calculated for mean sea level.
Related terms: sunrise

SURGE
The increase in sea water height from the level that would normally occur were there no storm. Although the most dramatic surges are associated with hurricanes, even smaller low pressure systems can cause a slight increase in the sea level if the wind and fetch is just right. It is estimated by subtracting the normal astronomic tide from the observed storm tide.

SUPERCELL
A severe thunderstorm characterized by a rotating, long-lived, intense updraft. Although not very common, they produce a relatively large amount of severe weather, in particular, extremely large hail, damaging straight-line winds, and practically all violent tornadoes.

SUPERCOOLING
The reduction of the temperature of any liquid below the melting point of that substance's solid phase. Cooling a substance beyond its nominal freezing point. Supercooled water is water that remains in a liquid state when it is at a temperature that is well below freezing. The smaller and purer the water droplets, the more likely they can become supercooled.

SUPERREFRACTION
Greater than normal bending of light or radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature.
Related term: subrefraction

SURFACE BOUNDARY LAYER
The lowest layer of the earth's atmosphere, usually up to 3,300 feet, or one kilometer, from the earth's surface, where the wind is influenced by the friction of the earth's surface and the objects on it.
Related terms: boundary layer and friction layer

SWELL
Ocean waves that have traveled out of their generating area. Swell characteristically exhibits a more regular and longer period and has flatter wave crests than waves within their fetch.

SYNOPTIC CHART
Any map or chart that depicts meteorological or atmospheric conditions over a large area at any given time.

SYNOPTIC SCALE
The size of migratory high and low pressure systems in the lower troposphere that cover a horizontal area of several hundred miles or more.
Related terms: macroscale, mesoscale, and storms

SYZYGY
The points in the moon's orbit about the earth at which the moon is new or full.

Index
T:

TELECONNECTIONS
Information used by forecasters to determine what the weather might be elsewhere when compared with past weather conditions at the same degree of longitude.

TEMPERATE CLIMATE
Climates with distinct winter and summer seasons, typical of regions found between the Tropics of Cancer and Capricorn and the Arctic and Antarctic Circles. Considered the climate of the middle latitudes.

TEMPERATURE
The measure of molecular motion or the degree of heat of a substance. It is measured on an arbitrary scale from absolute zero, where the molecules theoretically stop moving. It is also the degree of hotness or coldness. In surface observations, it refers primarily to the free air or ambient temperature close to the surface of the earth.

TERMINAL DOPPLER WEATHER RADAR (TDWR)
Doppler radar installed at major airports throughout the United States to detect microbursts.

TERRESTRIAL RADIATION
Long wave radiation that is emitted by the earth back into the atmosphere. Most of it is absorbed by the water vapor in the atmosphere, while less than ten percent is radiated directly into space.

TEXAS NORTHER
Local name in the south-central Great Plains for strong winter winds blowing north or northwest following a sharp cold front with dropping temperatures. Marked by a dark, blue-black sky.
Related term: Blue Norther

THAW
A warm spell of weather when ice and snow melt. To free something from the binding action of ice by warming it to a temperature above the melting point of ice.

THEODOLITE
An optical instrument used to track the motion of a pilot balloon, or pibal, by measuring the elevation and azimuth angles.

THERMAL LOW
Also known as heat low, it is an area of low pressure due to the high temperatures caused by intensive heating at the surface. It tends to remain stationary over its source area, with weak cyclonic circulation. There are no fronts associated with it. An example is the low that develops over southwestern United States and northwestern Mexico during the summer months.

THERMOCLINE
A vertical negative temperature gradient in some layer of a body of water which is appreciably greater than the gradients above and below it. In the ocean, this may be seasonal, due to the heating of the surface water in the summer, or permanent.

THERMODYNAMICS
Study of the processes that involve the transformation of heat into mechanical work, of mechanical work into heat, or the flow of heat from a hotter body to a colder body.

THERMOGRAPH
Essentially, a self-recording thermometer. A thermometer that continuously records the temperature on a chart.

THERMOHALINE
In oceanography, it pertains to when both temperature and salinity act together. An example is thermohaline circulation which is vertical circulation induced by surface cooling, which causes convective overturning and consequent mixing.

THERMOMETER
An instrument used for measuring temperature. The different scales used in meteorology are Celsius, Fahrenheit, and Kelvin or Absolute.

THERMOSPHERE
A thermal classification, it is the layer of the atmosphere located between the mesosphere and outer space. It is a region of steadily increasing temperature with altitude, and includes all of the exosphere and most, if not all, of the ionosphere.

THICKNESS
The thickness of a layer in the atmosphere is proportional to the mean temperature of that whole layer. The layer most often used in meteorology is between 1000 and 500 millibars. There can be different temperature profiles in the lowest layer of the atmosphere with the same 1000-500 millibar thickness value, depending on what is happening above that lowest layer. For example, if the lower levels are warming but higher levels are cooling, the overall mean temperature, the thickness, could remain the same. Likewise, on a sunny day, the amount of incoming solar radiation, affects the temperature right at the earth's surface, without necessarily having much effect on the thickness of the whole layer.

THUNDER
The sound emitted by rapidly expanding gases along the channel of a lightning discharge. Over three-quarters of lightning's electrical discharge is used in heating the gases in the atmosphere in and immediately around the visible channel. Temperatures can rise to over 10,000 °C in microseconds, resulting in a violent pressure wave, composed of compression and rarefaction. The rumble of thunder is created as one's ear catches other parts of the discharge, the part of the lightning flash nearest registering first, then the parts further away.

THUNDER SNOW
A wintertime thunderstorm from which falls snow instead of rain. Violent updrafts and at or below freezing temperatures throughout the atmosphere, from surface to high aloft, discourage the melting of snow and ice into rain. Intense snowfall rates often occur during these situations.

THUNDERSTORM
Produced by a cumulonimbus cloud, it is a microscale event of relatively short duration characterized by thunder, lightning, gusty surface winds, turbulence, hail, icing, precipitation, moderate to extreme up and downdrafts, and under the most severe conditions, tornadoes.

TIDE
The periodic rising and falling of the earth's oceans and atmosphere. It is the result of the tide-producing forces of the moon and the sun acting on the rotating earth. This propagates a wave through the atmosphere and along the surface of the earth's waters.

TILT
The inclination to the vertical of a significant feature of the pressure pattern or of the field of moisture or temperature. For example, midlatitide troughs tend to display a westward tilt with altitude through the troposphere.

TIROS
A series of Television InfraRed Observation Satellites that demonstrated the feasibility and capability of observing the cloud cover and weather patterns of earth from space. An experimental program, it was the first spaceborne system that allowed meteorologists to acquire information that was immediately put to use in an operational setting. The first U.S. weather satellite, TIROS I, was launched on April 1, 1960, and TIROS X, the last of the series, was launched on July 2, 1965.

TORNADO
A violently rotating column of air in contact with and extending between a convective cloud and the surface of the earth. It is the most destructive of all storm-scale atmospheric phenomena. They can occur anywhere in the world given the right conditions, but are most frequent in the United States in an area bounded by the Rockies on the west and the Appalachians in the east.

TORNADO ALLEY
A geographic corridor in the United States which stretches north from Texas to Nebraska and Iowa. In terms of sheer numbers, this section of the United States receives more tornadoes than any other.

TOWERING CUMULUS
Another name for cumulus congestus, it is a rapidly growing cumulus or an individual dome-shaped clouds whose height exceeds its width. Its distinctive cauliflower top often mean showers below, but lacking the characteristic anvil of a cumulonimbus, it is not a thunderstorm.

TRACE
Generally, an unmeasurable or insignificant quantity. A precipitation amount of less than 0.005 inch.

TRADE WINDS
Two belts of prevailing winds that blow easterly from the subtropical high pressure centers towards the equatorial trough. Primarily lower level winds, they are characterized by their great consistency of direction. In the Northern Hemisphere, the trades blow from the northeast, and in the Southern Hemisphere, the trades blow from the southeast.

TRAJECTORY
The curve that a body, such as a celestial object, describes in space. This applies to air parcel movement also.

TRANSLUCENT
Not transparent, but clear enough to allow light to pass through.

TRANSMISSOMETER
An electronic instrument system which provides a continuous record of the atmospheric transmission between two fixed points. By showing the transmissivity of light through the atmosphere, the horizontal visibility may be determined.

TRANSPARENT
A condition where a material is clear enough not to block the passage of radiant energy, especially light.

TRANSPIRATION
The process by which water in plants is transferred as water vapor to the atmosphere.
Related terms: evapotranspiration

TRIPLE POINT
The point at which any three atmospheric boundaries meet. It is most often used to refer to the point of occlusion of an extratropical cyclone where the cold, warm, and occluded fronts meet. Cyclogenesis may occur at a triple point. It is also the condition of temperature and pressure under which the gaseous, liquid, and solid forms of a substance can exist in equilibrium.

TROPICS/TROPICAL
The region of the earth located between the Tropic of Cancer, at 23.5 degrees North latitude, and the Tropic of Capricorn, at 23.5 degrees South latitude. It encompasses the equatorial region, an area of high temperatures and considerable precipitation during part of the year.

TROPICAL AIR MASS
An air mass that forms in the tropics or subtropics over the low latitudes. Maritime tropical air is produced over oceans and is warm and humid, while continental tropical air is formed over arid regions and is very hot and dry.

TROPICAL CYCLONE
A warm core low pressure system which develops over tropical, and sometimes subtropical, waters, and has an organized circulation. Depending on sustained surface winds, the system is classified as a tropical disturbance, a tropical depression, a tropical storm, or a hurricane or typhoon.

TROPICAL DEPRESSION
A tropical cyclone in which the maximum sustained surface winds are 38 miles per hour (33 knots) or less. Characteristically having one or more closed isobars, it may form slowly from a tropical disturbance or an easterly wave which has continued to organize.

TROPICAL DISTURBANCE
An area of organized convection, originating in the tropics and occasionally the subtropics, that maintains its identity for 24 hours or more. It is often the first developmental stage of any subsequent tropical depression, tropical storm, or hurricane.

TROPICAL PREDICTION CENTER (TPC)
A division of the National Centers for Environmental Prediction, the Center issues watches, warnings, forecasts, and analyses of hazardous weather conditions in the tropics for both domestic and international communities. The National Hurricane Center is a branch. For further information, contact the TPC, located in Miami, Florida.

TROPICAL STORM
A tropical cyclone in which the maximum sustained surface winds are from 39 miles per hour (34 knots) to 73 miles per hour (63 knots). At this point, the system is given a name to identify and track it.

TROPICAL WAVE
Another name for an easterly wave, it is an area of relatively low pressure moving westward through the trade wind easterlies. Generally, it is associated with extensive cloudiness and showers, and may be associated with possible tropical cyclone development.

TROPIC OF CANCER
The most northern point on the earth where the sun is directly overhead, located at approximately 23.5 degrees North latitude.

TROPIC OF CAPRICORN
The most southern point on the earth where the sun is directly overhead, located at approximately 23.5 degrees South latitude.

TROPOPAUSE
The boundary zone or transition layer between the troposphere and the stratosphere. This is characterized by little or no increase or decrease in temperature or change in lapse rate with increasing altitude.

TROPOSPHERE
The lowest layer of the atmosphere located between the earth's surface to approximately 11 miles (17 kilometers) into the atmosphere. Characterized by clouds and weather, temperature generally decreases with increasing altitude.

TROUGH
An elongated area of low atmospheric pressure that is associated with an area of minimum cyclonic circulation. The opposite of a ridge.

TSUNAMI
An ocean wave with a long period that is formed by an underwater earthquake or landslide, or volcanic eruption. It may travel unnoticed across the ocean for thousands of miles from its point of origin and builds up to great heights over shallower water. Also known as a seismic sea wave, and incorrectly, as a tidal wave.

TULE FOG
Ground fog in the central valley of California and the leading cause of weather-related casualties in that state. It forms at night and in the early morning when the ground cools, lowering the air temperature near the ground to or below its initial dew point.
Related term: radiation fog

TURBULENCE
The irregular and instantaneous motions of air which is made up of a number of small of eddies that travel in the general air current. Atmospheric turbulence is caused by random fluctuations in the wind flow. It can be caused by thermal or convective currents, differences in terrain and wind speed, along a frontal zone, or variation in temperature and pressure.

TWILIGHT
Often called dusk, it is the evening period of waning light from the time of sunset to dark. The time of increasing light in the morning is called dawn. Twilight ends in the evening or begins in the morning at a specific time and can be categorized into three areas of decreasing light. Civil twilight is the time in the evening when car headlights need to be turned on to be seen by other drivers. Nautical twilight is when the bright stars used by navigators have appeared and the horizon may still be seen. Astronomical twilight is when the sunlight is still shining on the higher levels of the atmosphere, yet it is dark enough for astronomical work to begin. During dawn, the reverse order occurs until full daylight.

TWISTER
A slang term used in the United States for a tornado.

TYPHOON
The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the western North Pacific Ocean. This same tropical cyclone is known as a hurricane in the eastern North Pacific and North Atlantic Ocean, and as a cyclone in the Indian Ocean.

Index
U:

ULTRAVIOLET
Electromagnetic radiation that has a wavelength shorter than visible light and longer than x-rays. Although it accounts for only 4 to 5 percent of the total energy of insolation, it is responsible for many complex photochemical reactions, such as fluorescence and the formation of ozone.

UNDERCAST
In aviation, it is an opaque cloud layer viewed from an observation point above the layer. From the ground, it would be considered an overcast.

UNITED STATES WEATHER BUREAU
The official name of the National Weather Service prior to 1970.

UNIVERSAL TIME COORDINATE
One of several names for the twenty-four hour time which is used throughout the scientific and military communities. Related terms: Zulu (Z) and Greenwich Mean Time (GMT)

UNIVERSITY CORPORATION FOR ATMOSPHERIC RESEARCH (UCAR)
A non-profit university membership consortium which carries out programs to benefit atmospheric, oceanic, and related sciences around the globe. Among other activities, UCAR operates the National Center for Atmospheric Research (NCAR) with National Science Foundation sponsorship.
For further information, contact UCAR, located in Boulder, Colorado.

UNSTABLE/ INSTABILITY
Occurs when a rising air parcel becomes less dense than the surrounding air. Since its temperature will not cool as rapidly as the surrounding environment, it will continue to rise on its own.
Related terms: instability and stable air

UPDRAFT
A small scale current of air with vertical motion. If there is enough moisture, then it may condense, forming a cumulus cloud, the first step towards thunderstorm development.
Related term: downdraft

UPPER AIR/UPPER LEVEL
The portion of the atmosphere which is above the lower troposphere. It is generally applied to the levels above 850 millibars. Therefore, upper level lows and highs, troughs, winds, observations, and charts all apply to atmospheric phenomena above the surface.

UPSLOPE EFFECT
The cooling of an air flow as it ascends a hill or mountain slope. If there is enough moisture and the air is stable, stratiform clouds and precipitation may form. If the air is unstable, there might be an increased chance of thunderstorm development.
Related term: downslope effect

UPSLOPE FOG
Fog that forms when warm, moist surface air is forced up a slope by the wind. It is adiabatically cooled to below its initial dew point, which means the air cools by expansion as it rises. It forms best where there is a gradual slope, and it can become quite deep, requiring considerable time to dissipate.
Related term: Cheyenne Fog

UPWELLING
The process by which water rises from a lower to a higher depth, usually as a result of divergence and offshore currents. It influences climate by bringing colder, more nutrient-rich water to the surface. A vital factor of the El Nio event.

UPWIND
The direction from which the wind is blowing. Also the windward side of an object. The opposite of the downwind or leeward side.

Index
V:

VALLEY BREEZE
An anabatic wind, it is formed during the day by the heating of the valley floor. As the ground becomes warmer than the surrounding atmosphere, the lower levels of air heat and rise, flowing up the mountainsides. It blows in the opposite direction of a mountain breeze.

VAPOR PRESSURE
The pressure exerted by the molecules of a given vapor. In meteorology, it is considered as the part of total atmospheric pressure due to the water vapor content. It is independent of other gases or vapors.

VAPOR TRAIL
A cloudlike streamer or trail often seen behind aircraft flying in clear, cold, humid air. A vapor trail is created when the water vapor from the engine exhaust gases are added to the atmosphere. Also called a contrail, for condensation trail.

VARIABLE CEILING
Occurs when the height of a ceiling layer increases and decreases rapidly, The ascribed height is the average of all the varying values.

VEERING
A clockwise shift in the wind direction in the Northern Hemisphere at a certain location. In the Southern Hemisphere, it is counterclockwise. This can either happen horizontally or vertically (with height). For example, the wind shifts from the north to the northeast to the east. It is the opposite of backing.

VERNAL EQUINOX
Taking place in the Northern Hemispheric spring, it is the point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. It falls on or about March 20 and is considered the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It is the astronomical opposite of the autumnal equinox.

VERTICAL TEMPERATURE PROFILE
A series of temperature measurements taken at various levels in the atmosphere that show the thermal structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.

VERTICAL VISIBILITY
The distance an observer can see vertically into an undefined ceiling, or the height corresponding to the top of a ceiling light projector beam, or the height at which a ceiling balloon disappears during the presence of an indefinite ceiling.

VERTICAL WIND PROFILE
A series of wind direction and wind speed measurements taken at various levels in the atmosphere that show the wind structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.

VIRGA
Streaks or wisps of precipitation, such as water or ice particles, that fall from clouds but evaporate before reaching the ground. From a distance, the event sometimes may be mistaken for a funnel cloud or tornado. Typically, it may fall from altocumulus, altostratus, or high based cumuonimbus.

VISIBILITY
A measure of the opacity of the atmosphere, and therefore, the greatest distance one can see prominent objects with normal eyesight. The National Weather Service has various terms for visibility. Surface visibility is the prevailing visibility determined from the usual point of observation. Prevailing visibility is considered representative of visibility conditions at the station. Sector visibility is the visibility in a specified direction that represents at least a 45 degree arc of the horizon circle. Tower visibility is the prevailing visibility determined from the airport traffic control tower (ATCT) at stations that also report surface visibility.

VISIBLE LIGHT
The portion of the electromagnetic spectrum that can be detected by the human eye. It travels at the same speed as all other radiation, that is at 186,000 mile per second. It has a wave length longer than ultraviolet light and shorter than x-rays.
Related term: light waves

VISUAL FLIGHT RULES (VFR)
Refers to the general weather conditions pilots can expect at the surface. VFR criteria means a ceiling greater than 3,000 feet and greater than 5 miles.
Related terms: MVFR and IFR

VORTEX
Any circular or rotary flow in the atmosphere that possesses vorticity.

VORTICITY
The measurement of the rotation of a small air parcel. It has vorticity when the parcel spins as it moves along its path. Although the axis of the rotation can extend in any direction, meteorologists are primarily concerned with the rotational motion about an axis that is perpendicular to the earth's surface. If it does not spin, it is said to have zero vorticity. In the Northern Hemisphere, the vorticity is positive when the parcel has a counterclockwise, or cyclonic, rotation. It is negative when the parcel has clockwise, or anticyclonic, rotation.
Related term: relative vorticity

VORTICITY MAXIMUM
A center of vorticity, or the maximum of the vorticity field of a fluid.

Index
P:

WALKER CIRCULATION
A deep east-west overturning in the atmosphere normally confined to within about 20 degrees latitude of the equator extending from low-levels to near the tropopause. One side of the Walker Circulation is associated with rising motion, clouds, and rain while the opposite side with sinking motion, generally fair weather and little or no rain. This circulation is large in spatial extend normally extending anywhere from way around the world or completely around the world (thus either 1 or 2 upward and 1 or 2 downward branches to the circulation). The rising and sinking branches are climatologically anchored to specific geographical locations, but these locations are seasonally dependent. In winter the rising branch is centered near the "maritime continent" of greater Indonesia while in summer it is centered near southern India. The sinking branches are generally found in the eastern equatorial Pacific in winter and summer. Sub-branches of rising motion are generally tied to equatorial areas of South America and Africa in summer.

WALL CLOUD
An abrupt lowering of a cloud from its parent cloud base, a cumulonimbus or supercell, with no visible precipitation underneath. Forming in the area of a thunderstorm updraft, or inflow area, it exhibits rapid upward movement and cyclonic rotation. It often develops before strong or violent tornadoes.

WARM
To have or give out heat to a moderate or adequate degree. A subjective term for temperatures between cold and hot. In meteorology, an air parcel that is warm is only so in relation to another parcel.

WARM ADVECTION
The horizontal movement of warmer air into a location.
Related term: cold advection

WARM FRONT
The leading edge of an advancing warm air mass that is replacing a retreating relatively colder air mass. Generally, with the passage of a warm front, the temperature and humidity increase, the pressure rises, and although the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere), it is not as pronounced as with a cold frontal passage. Precipitation, in the form of rain, snow, or drizzle, is generally found ahead of the surface front, as well as convective showers and thunderstorms. Fog is common in the cold air ahead of the front. Although clearing usually occurs after passage, some conditions may produced fog in the warm air.
Related terms: occluded front and cold front

WARM HIGH
A high pressure system that has its warmest temperatures at or near the center of circulation. Contrast with a cold high.
Related terms: cut-off high and omega block

WARM LOW
A low pressure system that has its warmest temperatures at or near the center of circulation. Also referred to as a warm core low.
Related term: cold low

WARNING
A forecast issued when severe weather has developed, is already occurring and reported, or is detected on radar. Warnings state a particular hazard or imminent danger, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, heavy snows, etc.

WASATCH WINDS
Strong winds blowing easterly out of the Wasatch Mountains in Utah, sometimes reaching speeds greater than 75 miles per hour.

WATCH
A forecast issued well in advance of a severe weather event to alert the public of the possibility of a particular hazard, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, or heavy snows.

WATER
Refers to the chemical compound, H2O, as well as its liquid form. At atmospheric temperatures and pressures, it can exist in all three phases: solid (ice), liquid (water), and gaseous (water vapor). It is a vital, life-sustaining part of life on earth.

WATER CYCLE
The vertical and horizontal transport of water in all its states between the earth, the atmosphere, and the seas.
Related term: hydrologic cycle

WATERSPOUT
A small, weak tornado, which is not formed by a storm-scale rotation. It is generally weaker than a supercell tornado and is not associated with a wall cloud or mesocyclone. It may be observed beneath cumulonimbus or towering cumulus clouds and is the water equivalent of a landspout.

WATER VAPOR (H2O)
Water in gaseous form. It is one of the most import constituents of the atmosphere. Due to its molecular content, air containing water vapor is lighter than dry air. This contributes to the reason why moist air has a tendency to rise.

WAVE(S)
In general, any pattern with some roughly identifiable periodicity in time and/or space. It is also considered as a disturbance that moves through or over the surface of the medium with speed dependent on the properties of the medium. In meteorology, this applies to atmospheric waves, such as long waves and short waves. In oceanography, this applies to waves generated by mechanical means, such as currents, turbidity, and the wind.
Related terms: Rossby waves and cyclonic waves

WAVE CYCLONE
A cyclone which forms and moves along a front. The circulation around the cyclone's center produces a wavelike deformation on the front. May also be call a migratory cyclone or low.

WAVE LENGTH
The least distance between particles moving in the same phase of oscillation of a wave. In oceanography, it is the horizontal distance between the highest parts of two successive wave crests above the still water level, separated by a trough that is below the still water level, and it is measured in meters.

WEATHER
The state of the atmosphere at a specific time and with respect to its effect on life and human activities. It is the short term variations of the atmosphere, as opposed to the long term, or climatic, changes. It is often referred to in terms of brightness, cloudiness, humidity, precipitation, temperature, visibility, and wind.

WEATHERING
The decay and breakup of rocks on the earth's surface by natural chemical and mechanical processes. The mechanical action includes large changes of temperature, extreme temperatures, frost, or the impact of wind borne sand or water. Chemical action includes the chemical reactions between atmospheric constituents in a moist environments or in rain water. Biological agents are mainly fungi which attack organic material.

WEATHER STATION
Weather Station - A weather station is a facility with instruments and equipment to make weather observations by monitoring atmospheric conditions to study the weather. This weather station has a thermometer for measuring temperature; barometer for measuring changes in air pressure; hygrometer for measuring humidity; anemometer for measuring wind speed and wind direction; and rain gauge for measuring precipitation.

WEATHER SURVEILLANCE RADAR (WSR-88D)
The newest generation of Doppler radars, the 1988 Doppler weather radar. The radar units, with help from a set of computers, show very detailed images of precipitation and other phenomena, including air motions within a storm.

WEATHER VANE
Originally used as a wind vane, it is an instrument that indicates the wind direction. The name developed based on observations on what kind of weather occurred with certain wind directions. Creative designs often adorn the tops of barns and houses.

WEDGE
Primarily refers to an elongated area of shallow high pressure at the earth's surface. It is generally associated with cold air east of the Rockies or Appalachians. It is another name for a ridge, ridge line, or ridge axis. Contrast with a trough. Wedge is also a slang term for a large, wide tornado with a wedge-like shape.

WESTERLIES
Usually applied to the broad patterns of persistent winds with a westerly component. It is the dominant persistent atmospheric motion, centered over the midlatitudes of each hemisphere. Near the earth's surface, the westerlies extend from approximately 35 to 65 degrees latitude, while in the upper levels they extend further polarward and equatorward.

WEST VIRGINIA HIGH
An area of stagnant high pressure located over West Virginia during Indian Summer.

WET BULB DEPRESSION
Dependent on the temperature and the humidity of the air, it is the difference between the dry bulb and the wet bulb readings.

WET BULB THERMOMETER
A thermometer used to measure the lowest temperature in the ambient atmosphere in its natural state by evaporating water from a wet muslin-covered bulb of a thermometer. The wet bulb temperature is used to compute dew point and relative humidity. One of the two thermometers that make up a psychrometer.

WHIRLWIND
A small-scale, rapidly rotating column of wind, formed thermally and most likely to develop on clear, dry, hot afternoons. Often called a dust devil when visible by the dust, dirt or debris it picks up. Also slang for a landspout or a tornado.

WHITEOUT
When visibility is near zero due to blizzard conditions or occurs on sunless days when clouds and surface snow seem to blend, erasing the horizon and creating a completely white vista.

WIND
Air that flows in relation to the earth's surface, generally horizontally. There are four areas of wind that are measured: direction, speed, character (gusts and squalls), and shifts. Surface winds are measured by wind vanes and anemometers, while upper level winds are detected through pilot balloons, rawin, or aircraft reports.

WIND CHILL INDEX
The calculation of temperature that takes into consideration the effects of wind and temperature on the human body. Describes the average loss of body heat and how the temperature feels. This is not the actual air temperature.
Related term: Wind Chill Index

WIND DIRECTION
The direction from which the wind is blowing. For example, an easterly wind is blowing from the east, not toward the east. It is reported with reference to true north, or 360 degrees on the compass, and expressed to the nearest 10 degrees, or to one of the 16 points of the compass (N, NE, WNW, etc.).

WIND SHEAR
The rate of wind speed or direction change with distance. Vertical wind shear is the rate of change of the wind with respect to altitude. Horizontal wind shear is the rate of change on a horizontal plane.

WIND SHIFT
The term applied to a change in wind direction of 45 degrees or more, which takes place in less than 15 minutes. It may the result of a frontal passage, from katabatic winds, sea breezes, or thunderstorms, and in some instances, the change may be gradual or abrupt.

WIND SPEED
The rate of the motion of the air on a unit of time. It can be measured in a number of ways. In observing, it is measured in knots, or nautical miles per hour. The unit most often used in the United States is miles per hour.

WIND VANE
An instrument that indicates the wind direction. The end of the vane which offers the greatest resistance to the motion of the air moves to the downwind position.
Related term: weather vane

WINDWARD
The direction from which the wind is blowing. Also the upwind side of an object. The opposite of the downwind or leeward side.

WIND WAVE
An ocean or lake wave resulting from the action of wind on the water's surface. After it leaves its fetch area, it is considered a swell.

WINTER
Astronomically, this is the period between the winter solstice and the vernal equinox. It is characterized as having the coldest temperatures of the year, when the sun is primarily over the opposite hemisphere. Customarily, this refers to the months of December, January, and February in the North Hemisphere, and the months of June, July, and August in the Southern Hemisphere.

WINTER STORM
Any one of several storm systems that develop during the late fall to early spring and deposit wintry precipitation, such as snow, freezing rain, or ice.
Related terms: blizzard, ice storm, and nor'easter

WORLD METEOROLOGICAL ORGANIZATION (WMO)
From weather prediction to air pollution research, climate change related activities, ozone layer depletion studies and tropical storm forecasting, the World Meteorological Organization coordinates global scientific activity to allow increasingly prompt and accurate weather information and other services for public, private and commercial use, including international airline and shipping industries. Established by the United Nations in 1951, it is composed of 184 members.
For more information, contact the WOE, located in Geneva, Switzerland.

Index
X:

X-RAYS
The portion of the electromagnetic spectrum that has a very short wave length. It has a wave length longer than gamma rays, yet shorter than visible light. X-rays can penetrate various thicknesses of all solids, and when absorbed by a gas, can result in ionization.

Index
Y:

YEAR
The interval required for the earth to complete one revolution around the sun. A sidereal year, which is the time it take for the earth to make one absolute revolution around the sun, is 365 days, 6 hours, 9 minutes, and 9.5 seconds. The calendar year begins at 12 o'clock midnight local time on the night of December 31st-January 1st. Currently, the Gregorian calendar of 365 days is used, with 366 days every four years, a leap year. The tropical year, also called the mean solar year, is dependent on the seasons. It is the interval between two consecutive returns of the sun to the vernal equinox. In 1900, that took 365 days, 5 hours, 48 minutes, and 46 seconds, and it is decreasing at the rate of 0.53 second per century.

YELLOW SNOW
Snow that is given golden, or yellow, appearance by the presence of pine or cypress pollen in it.

Index
Z:

ZENITH
The point which is elevated 90 degrees from all points on a given observer's astronomical horizon. The point on any given observer's celestial sphere that lies directly above him. The opposite of nadir.

ZODIAC
The position of the sun during the course of the year as it appears to move though successive constellations. Also, the band where the ecliptic runs centrally through the celestial sphere and contains the sun, the moon, and all the planets except Venus and Pluto.

ZONAL FLOW
The flow of air along a latitudinal component of existing flow, normally from west to east. Related term: meridional flow

ZONAL INDEX
The measure of the strength of the westerly winds of the middle latitudes. It is expressed as the horizontal pressure difference between 35 degrees and 55 degrees latitude, or as the corresponding geostrophic wind.

ZULU TIME
One of several names for the twenty-four hour time which is used throughout the scientific and military communities. Related terms: Universal Time Coordinate (UTC) and Greenwich Mean Time (GMT)

Index