Central Valley Weather Terminology
This Weather Glossary, features an alphabetical listing of over 1000 weather-related terms, phrases and abbreviations covering the environmental sciences. The terms with *asterisks before them are related to Storm Spotting and Chasing. The purpose of this glossary is to aid the general public in better understanding the numerous weather related terms and related NWS products.
Shortcut...just click on the first letter of the word you're looking for
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 adiatabtic lapse rate. The term absolute is used because this applies whether or not the air is dry or saturated. See instability.
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. It is also the zero point of the Kelvin Temperature Scale. The value is -273.16 degrees Kelvin, -273 degrees Celsius and -459.6 degreees Fahrenheit.
Acid Rain - Is caused by emissions of sulphur dioxide and nitrogen oxides. Although natural sources of sulphur oxides and nitrogen oxides do exist, more than 90% of the sulphur and 95% of the nitrogen emissions occurring in eastern North America are of human origin. These primary air pollutants arise from the use of coal in the production of electricity, from base-metal smelting, and from fuel combustion in vehicles. Once released into the atmosphere, they can be converted chemically into such secondary pollutants as nitric acid and sulfuric acid, both of which dissolve easily in water. The resulting acidic water droplets can be carried long distances by prevailing winds, returning to Earth as acid rain, snow, or fog.
Adiabatic Process - A thermodynamic change of state in a system in which there is no transfer of heat or mass across the boundaries of the system. In this process, compression will result in warming and expansion will result in cooling.
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. An advisory is for less serious conditions that cause significant inconvenience and, if caution is not exercised, could lead to situations that may threaten life and/or property.
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 gases in meteorology is water vapor (H2O).
Air Mass - An extensive body or large dome of air which has similar horizontal temperature and moisture characteristics. Often, a front separates two different air masses. Fronts are very narrow zones of transition. In other words, temperatures can change dramatically with short horizontal distances near fronts. Fronts are usually anywhere from 10 miles to hundreds of miles wide, while air masses can be thousands of miles wide.
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.Although such thunderstorms are not generally associated with fronts, they generally are less likely to be severe than other types of thunderstorms. However, that does not preclude them from having brief heavy downpours and they are still capable of producing brief heavy downbursts, of rain, and (in extreme cases) hail over 3/4 inch in diameter.
Air Pressure - or atmospheric pressure - is the force exerted on a surface by the weight of the air above it. The internationally recognized unit for measuring this pressure is the kilopascal.
Air Quality Standards - The maximum level which will be permitted for a given pollutant. Primary standards are to be sufficiently stringent to protect the public health. Secondary standards must protect the public welfare, including property and aesthetics.
Air Stagnation Advisory - This National Weather Service product is issued when major buildups of air pollution, smoke, dust, or industrial gases are expected near the ground for a period of time. This usually results from a stagnant high pressure system with weak winds being unable to bring in fresh air.
Albedo - The percent of reflectivity of wavelengths of an object's surface. This varies according to the texture, color, and expanse of the object's surface. Surfaces with high albedo include sand and snow, while low albedo rates include forests and freshly turned earth.
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.
Aleutian Low - A semi-permanent, subpolar area of low pressure located in the Gulf of Alaska near the Aleutian Islands. It is a generating area for storms and migratory lows often reach maximum instensity in this area. It is most active during the late fall to late spring. During the summer, it is weaker, retreating towards the North Pole and becoming almost nonexistent. During this time, the North Pacific High pressure system dominates.
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.
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 its layer effect, often with a wavy billowy appearance, 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 - (ACCAS) - A middle cloud, (bases generally 8 to 15 thousand feet), of which at least a fraction of their upper parts show vertical development that forms from altocumulus clouds. These clouds often are taller than they are wide, giving them a turret-shaped appearance. 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 and they appearance may precede the rapid development of thunderstorms.
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.
Anemometer - An instrument that measures the speed or force of the wind. Wind directions are always reported as the direction winds are coming from - a southerly wind pushes air from the south to the north.
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. See mercurial barometer.
Anomalous Propagation (AP) - 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. Often referred to as A.P.
Antarctic - Of or relating to the area around the geographic South Pole, from 90 degrees South to the Antarctic Circle at approximately 66 1/2 degrees South latitude, including the continent of Antarctica. Along the Antarctic Circle, the sun does not set on the day of the summer solstice (approximately December 21st) and does not rise on the day of the winter solstice (approximately June 21st).
Antarctic Ocean - Although not officially recognized as a separate ocean body, it is commonly applied to those portions of the Atlantic, Pacific, and Indian Oceans that reach the Antarctic continent on their southern extremes.
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. Also known as an area of high pressure, it is the opposite of an area of low pressure, or a cyclone.
*Anvil - The upper portion of a cumulonimbus cloud that becomes flat and spread-out, sometimes they can spread for hundreds of miles downstream from the parent cloud and sometimes may they may spread upwind (back-sheared anvil). 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.
*Anvil Crawler - [Slang], a lightning discharge occurring within the anvil of a thunderstorm, characterized by one or more channels that appear to crawl along the underside of the anvil. They typically appear during the weakening or dissipating stage of the parent thunderstorm, or during an active MCS.
AP - Anomalous Propagation - A Radar term for false (non-precipitation) echoes resulting from nonstandard propagation of the radar beam under certain atmospheric conditions. Same as Anomalous Propagation
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.
Arctic Air Mass - An air mass that develops around the Arctic and is characterized by being cold aloft and extending to great heights. The boundary of this air mass is often defined by the Arctic front, a semi-permanent, semi-continous feature.
Arid (Arid Climate) - A term used for an extemely dry climate. The degree to which a climate lacks effective, life-promoting moisture. It is considered the opposite of humid when speaking of climates.
Ashfall Advisory - An advisory issued for conditions associated with airborne ash plume resulting in ongoing deposition at the surface. Ashfall may originate directly from a volcanic eruption, or indirectly by wind suspending the ash.
ASOS - The 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 degrees below the horizon. See twilight.
Atmosphere - The gaseous air surrounding and bound to the earth, by gravity 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 four divisions of the atmosphere include the troposphere,earth's surface to an altitude of about 10 km; the stratosphere, - from 10 km to 50 km; the mesosphere,- from 50 km to 80 km; nd the thermosphere- beyond 80 km which is the ionosphere, and the exosphere. combimed
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). Also known as barometric pressure.
Aurora - It is created by the sporadic 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.
Aurora Borealis - also known as Northern Lights - The luminous, radiant emission from the upper atmosphere over middle and high latitudes, and centred around the earth's magnetic poles. These silent fireworks are often seen on clear winter nights in a variety of shapes and colours.
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.
Autumnal Equinox - Taking place in the Northern Hemispheric autumn, 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 September 21 and is considered the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere. It is the astronomical opposite of the vernal equinox.
AVN - AViatioN model - One of the operational forecast models run at NCEP. The AVN is run four times daily, at 0000, 0600, 1200, and 1800 GMT. As of fall 1996, forecast output was available operationally out to 72 hours only from the 0000 and 1200 runs. At 0600 and 1800, the model is run only out to 54 hours.
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. A comparable system is the North Pacific High.
Back-building Thunderstorm - A thunderstorm in which new development takes place on the upwind side (usually the west or southwest side), such that the storm seems to remain stationary or propagate in a backward direction.
Backing - A change in wind direction that shifts counterclockwise 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.
Backing Winds - Winds which shift in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (e.g. westerly at the surface but becoming more southerly aloft). The opposite of veering winds. In storm spotting, a backing wind usually refers to the turning of a south or southwest surface wind with time to a more east or southeasterly direction. Backing of the surface wind can increase the potential for tornado development by increasing the directional shear at low levels.
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.
*Barber Pole - [Slang], For a thunderstorm updraft with a visual appearance including cloud striations that are curved in a manner similar to the stripes of a barber pole. The structure typically is most pronounced on the leading edge of the updraft, while drier air from the rear flank downdraft often erodes the clouds on the trailing side of the updraft.
Baroclinic Zone - A region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems; barotropic systems, on the other hand, do not exhibit significant changes in intensity. Also, wind shear is characteristic of a baroclinic zone.
Barometer - An instrument used to measure atmospheric pressure. Two examples are the aneroid barometer and the mercurial barometer. The international standard of measurement is the kilopascal although millibars and inches of mercury are also commonly used.
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). Also known as atmospheric pressure.
Barotropy - The state of a fluid in which surfaces of constant density or temperature are coincident with surfaces of constant pressure. It is considered zero baroclinity. An example is a barotropic pattern.
Barotropic System - A weather system in which temperature and pressure surfaces are coincident,i.e., temperature is uniform (no temperature gradient) on a constant pressure surface. Barotropic systems are characterized by a lack of wind shear, and thus are generally unfavorable areas for severe thunderstorm development. See baroclinic zone.
Usually, in operational meteorology, references to barotropic systems refer to equivalent barotropic systems - systems in which temperature gradients exist, but are parallel to height gradients on a constant pressure surface. In such systems, height contours and isotherms are parallel everywhere, and winds do not change direction with height.
As a rule, a true equivalent barotropic system can never be achieved in the real atmosphere. While some systems (such as closed lows or cutoff lows) may reach a state that is close to equivalent barotropic, the term barotropic system usually is used in a relative sense to describe systems that are really only close to being equivalent barotropic, i.e., isotherms and height contours are nearly parallel everywhere and directional wind shear is weak.
Base Velocity - Base Velocity images provides a picture of the basic wind field from the ½° elevation scan. It is useful for determining areas of strong wind from downbursts or detecting the speed of cold fronts. However, since the radar only measures radial velocity, the strength of the wind will always be less than what is actually occurring unless the wind is moving directly toward or away from the radar. Also, the surface winds are only for areas near the radar. As distance increases from the radar, the reported value will be for increasing heights above the earth's surface.
*Bear's Cage - [Slang], For a region of storm-scale rotation, in a thunderstorm, which is wrapped in heavy precipitation. This area often coincides with a radar hook echo and/or mesocyclone, especially one associated with an HP storm. The term reflects the danger involved in observing such an area visually, which must be done at close range in low visibility.
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 , measured in knots, a descriptive term, and the visible effects upon land objects and/or sea surfaces. (see Beaufort table listed below) The scale was devised by Sir Francis Beaufort (1777-1857), hydrographer to the British Royal Navy.
Beaufort Wind Scale:
*Beaver('s) Tail - [Slang], For a particular type of inflow band with a relatively broad, flat appearance suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly parallel to the pseudo-warm front, i.e., usually east to west or southeast to northwest. As with any inflow band, cloud elements move toward the updraft, i.e., toward the west or northwest. Its size and shape change as the strength of the inflow changes.
Spotters should note the distinction between a beaver tail and a tail cloud. A "true" tail cloud typically is attached to the wall cloud and has a cloud base at about the same level as the wall cloud itself. A beaver tail, on the other hand, is not attached to the wall cloud and has a cloud base at about the same height as the updraft base (which by definition is higher than the wall cloud). Unlike the beaver tail, the tail cloud forms from air that is flowing from the storm's main precipitation cascade region (or outflow region). Thus, it can be oriented at a large angle to the pseudo-warm front.
Bellot Winds - Refers to the winds in the Canadian Arctic that blow through the narrow Bellot Strait between Somerset Island and the Boothia Peninsula, connecting the Gulf of Boothia and Franklin Strait.
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 in the eastern part of the North Atlantic. Then it may be referred to as the Azores High.
Bernoulli's Theorem - A statement of the conservation of energy for a steady, nonviscous, incompressible level flow. It is an inverse relationship in which pressures are least where velocities are greatest. An example is the Santa Ana winds. 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.
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 degrees Fahrenheit, winds exceeding 45 mph, and visibility reduced by snow to near zero. True blizzard conditions are most common on the prairies of Canada and the United States. Blizzards are a rare occurrence on the west coast and in Atlantic Canada.
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. Two examples are a cut-off high and an 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 winter winds from the northwest or north and temperatures that may drop 20 to 30 degrees Fahrenheit in a few minutes. Also known as a Texas Norther.
Boiling Point - The temperature at which a liquid changes to a vaporous state. The temperature at which the equilibrium vapor pressure between a liquid and its vapor is equal to the external pressure on the liquid. The boiling point of pure water at standard pressure is 100 degrees Celsius or 212 degrees Fahrenheit.
Boundary Layer - In general, a layer of air adjacent to a bounding surface. It is 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. May also be called the surface boundary layer but since the effects of friction die out gradually with height, so the "top" of this layer cannot be defined exactly.
There is a thin layer immediately above the earth's surface known as the surface boundary layer (or simply the surface layer). This layer is only a part of the planetary boundary layer, and represents the layer within which friction effects are more or less constant throughout (as opposed to decreasing with height, as they do above it). It maay also be called the friction layer. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface.
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. Damaging straight-line winds often occur near the "crest" or center of a bow echo. Areas of circulation also can develop at either end of a bow echo, which sometimes can lead to tornado formation - especially in the left (usually northern) end, where the circulation exhibits cyclonic rotation.
Boyle's Law - States that when the temperature is held constant, the volume of a gas is inversely proportional to its pressure. Therefore, if the pressure increases, the volume decreases and visa versa. For example, if the volume if halved, then the pressure is doubled. If the temperature is held constant, it becomes an isothermal process. Discovered by Robert Boyle (1627-1691), an Irish physicist and chemist and co-founder of the Royal Society.
Bubble High - A small high that may be created by precipitation and vertical instability associated with thunderstorm activity. A product of downdrafts, it is relatively cold and often has the characteristics of a different air mass. Unstable, overrunning air may form squall lines on its leading edge. Also known as a meso high.
Bust - [Slang], For an inaccurate forecast or a unsuccessful storm chase; ~ usually a situation in which significant weather is expected, ie, thunderstorms or severe weather are expected, but do not occur.
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 updrafts and is found in the inflow region of a thunderstorm. and it cannot be seen visually. Also known as a vault.
Cap (or Capping Inversion) - Composed of a layer of warmer, dryer air, aloft (usually several thousand feet above the ground), which may suppress or delay the development of thunderstorms. As an air parcel rises into this area, it becomes cooler than the surrounding air and is too cool to rise further. Also referred to as a lid. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability. However if the cap is removed or weakened, then explosive thunderstorm development can occur. The cap is an important ingredient in most severe thunderstorm episodes, as it serves to separate warm, moist air below and cooler, drier air above. With the cap in place, air below it can continue to warm and/or moisten, thus increasing the amount of potential instability. Or, air above it can cool, which also increases potential instability. But without a cap, either process (warming/moistening at low levels or cooling aloft) results in a faster release of available instability - often before instability levels become large enough to support severe weather development.
Cape - Acroymn for Convective Available Potential Energy. A measure of the amount of energy available or needed to create convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1,000 joules per kilogram (j/kg), and in extreme cases may exceed 5,000 j/kg. However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent. CAPE is represented on a sounding by the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is warmer than the former. (This area often is called positive area.)
Cb - Is the abbreviation for the Cumulonimbus cloud, characterized by strong vertical development in the form of mountains or huge towers topped at least partially by a smooth, flat, often fibrous anvil shaped top. Also known as a "thunderhead."
CC - Cloud-to-Cloud lightning. Lightning that occurs between two clouds
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 Sphere - The apparent sphere of infinite radius having the earth as it center. All heavenly bodies (planets, stars, etc.) appear on the "inner surface" of this sphere and the sun moves along the ecliptic.
*Cell - Convection in the form of a single updraft, downdraft, or updraft/downdraft couplet, typically seen as a vertical dome or tower as in a cumulus or towering cumulus cloud. A typical thunderstorm consists of several cells (see multi-cellular thunderstorm). The term "cell" also is used to describe the radar echo returned by an individual shower or thunderstorm. Such usage, although common, is technically incorrect.
CELSIUS Temperature Scale - A temperature scale where water at sea level has a freezing point of 0 degrees C (Celsius) and a boiling point of +100 degrees C. More commonly used in areas that observe the metric system of measurement. Created by Anders Celsius in 1742. Same as Centigrade. In 1948, the Ninth General Conference on Weights and Measures replaced "degree centigrade" with "degree Celsius."
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. This force on the earth and in the atmosphere due to the rotation about the earth's axis is incorporated with the field of gravitation to form gravity.
*CG - Cloud-to-Ground lightning flash. Lightning that occurs between a thunderhead and the ground
Charles' Law - States that when the pressure is held constant, the volume of a gas varies directly with the temperature. Therefore, if the pressure remains constant, the volume of a gas will increase with the increase of temperature. It was developed by Jacques Charles and is also known as the Charles-Guy-Lussac Law.
Chaser Convergence - Any unplanned gathering of storm chasers, often near a storm of interest. These are seen regularly during the spring convective season.
Chemosphere - A vaguely defined region of the upper atmosphere in which photochemical 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 downslope wind in the Rocky Mountains that may occur after an intense cold spell, when the temperature could rise by 20 to 40 degrees Fahrenheit in a matter of minutes. Also known as the Snow Eater. Chinooks occur when a mountain range is exposed to a strong winds blowing at right angles, or near right angles to the direction of the mountain ridge. Moist air is forced up the mountains bringing both cloud and precipitation to the windward side. The descending air then becomes warmer and drier as it is forced down the leeward side of the mountains.
CIN - Convective INhibition. A measure of the amount of energy needed in order to initiate convection. Values of CIN typically reflect the strength of the cap. They are obtained on a sounding by computing the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is cooler than the former. (This area sometimes is called negative area.)
Circulation - The flow or motion of a fluid in or through a given area or volume. In meteorology, it is the pattern of air as it moves, generally observed as a large flow characteristic of a relatively permanent pressure system in the atmosphere. Also viewed as smaller patterns in semi-permanent pressure systems. 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 polarward. 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 sheetlike 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. Cirrus clouds typically have a fibrous or hairlike appearance, and often are semi-transparent. 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.
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 horizonal 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 because of wind shear. Often referred to as CAT.
Clear Ice - A glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled 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. Often synonymous with glaze.
*Clear Slot - A local region of clearing skies or reduced cloud cover, indicating an intrusion of drier air; often seen as a bright area with higher cloud bases on the west or southwest side of a wall cloud. A clear slot is believed to be a visual indication of a rear flank downdraft.
Climate - The prevalent or characteristic meteorological conditions, and their extremes, of any place or region. That is determined by 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, diagnois, and projection of short term climate fluctuations on a regional and global basis.
Climate Predicition 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.
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 low pressure system that is completely encircled by an isobar. or height contour lines. They often move slowly, as they displaced south of the main westerlies. Although, strictly speaking, all lows are closed, this term distinguishes a low from a trough on a surface chart, and on upper level charts, it accentuates that the circulation is "closed." A form of cut-off low.
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 in rising currents of air. They may also form from the evaporation of fog. Condensation nuclei, such as in smoke or dust particles, form a surface upon which water vapor can condense.
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, or form(rarely) a small, relatively weak tornado but in general are less violent than most other types of tornadoes.
Cold Front - The leading edge of an advancing cold air mass that is underrunning 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. See 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 horizontally, is thermally barotropic. Both warmth and in circulation increase with height and usually there is a warm low aloft to support it. 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 Pool - A region of relatively cold air, represented on a weather map analysis as a relative minimum in temperature surrounded by closed isotherms. Cold pools aloft represent regions of relatively low stability, while surface-based cold pools are regions of relatively stable air.
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.
*Collar Cloud - A generally circular ring of cloud that may be observed on rare occasions surrounding the upper part of a wall cloud. This term sometimes is used (incorrectly) as a synonym for wall cloud.
Comma Cloud - A feature seen on satellite photographs with a distinctive comma-shape. This is indicative of a synoptic cloud pattern associated with large, well-developed and intense low pressure systems.
Condensation - The physical process by which water vapor undergoes a change in state from a gas to a liquid. The following conditions must be met: the in air must be cooled to its dew point, and there must be enough water vapor added to bring the air to the point of saturation, raising the relative humidity to 100%. It is the opposite physical process of evaporation.
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.
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.
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.
Contrail - Acroymn for CONdensation 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 vapor trail.
Convection - The mass motion within a fluid, resulting in the transport and mixing of the properties of that fluid. This could be the transport of heat and/or moisture. In meteorology, the term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere. The terms "convection" and "thunderstorms" often are used interchangeably, although thunderstorms are only one form of convection. Cbs, towering cumulusand ACCAS clouds all are visible forms of convection. However, convection is not always made visible by clouds. Convection which occurs without cloud formation is called dry convection, while the visible convection processes referred to above are forms of moist convection. It is often used to imply only upward vertical motion and then it is the opposite of subsidence.
Convective Outlook - A forecast containing the area(s) of expected thunderstorm occurrence and expected severity over the contiguous United States, issued several times daily by the SPC. The terms approaching, slight risk, moderate risk, and high risk are used to describe severe thunderstorm potential. Local versions sometimes are prepared by local NWS offices.
Convective Temperature - The approximate temperature that the air near the ground must warm to in order for surface-based convection to develop, based on analysis of a sounding. Calculation of the convective temperature involves many assumptions, such that thunderstorms sometimes develop well before or well after the convective temperature is reached (or may not develop at all). However, in some cases the convective temperature is a useful parameter for forecasting the onset of convection.
Convergence - A contraction of a vector field; the opposite of divergence. Wind movement that results in a horizontal net inflow of air into a particular region. That is more air is entering a given area than is leaving at that level. To compensate for the resulting "excess," a vertical motion may result: upward forcing if convergence is at low levels, or downward forcing (subsidence) if convergence is at high levels. Upward forcing from low-level convergence increases the potential for thunderstorm development (when other factors, such as instability, are favorable).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. Refer to degree day or heating degree day.
*Core Punch - [Slang], For a penetration by a vehicle into the heavy precipitation core of a thunderstorm. Core punching is not a recommended procedure for storm spotting or chasing.
Coriolis Effect - A force per unit mass that arises solely from the earth's in rotation, acting as a deflecting force. It is dependent on the latitude and the speed of the moving air mass. In synoptic scale weather systems (hurricanes and large mid-latitude storms), the Coriolis force causes the air to rotate around a low pressure center in a cyclonic direction. 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. If the earth did not rotate, the air would flow directly in towards the low pressure center, but on a spinning earth, the Coriolis force results in the are arcing in towards the low pressure center. The coriolis force is of much too small a magnitude to have any relevance to the direction of rotation in a sink or toilet.
"3D" Correlated Shear - 3D Correlated Shear indicates that rotation was found at multiple elevation angles in a storm, however those features DO NOT meet the size and symmetry criteria for a mesocyclone, and finally Mesocyclone itself, which is the most severe of the 3 entries, indicating rotation was detected at multiple levels in the storm and the stringent wind and wind shear criteria were met.
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, and often audible, electric discharge that is intermediate 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 corona discharge or 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 cumulus clouds come between the sun and the observer. A similar effect occurs when the sun shines though a break in a layer of clouds.
Crystalization - 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.
*Cumuliform Anvil - A thunderstorm anvil with visual characteristics resembling cumulus-type clouds (rather than the more typical fibrous appearance associated with cirrus). A cumuliform anvil arises from rapid spreading of a thunderstorm updraft, and thus implies a very strong updraft. See anvil rollover, knuckles, mushroom.
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 detach cloud that develops in a vertical direction from the base (bottom) up. They have flat bases and dome or cauliflower-shaped upper surfaces, with sharp outlines, showing vertical development in the form of domes, mounds, or towers. 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. Also dee Cb, towering cumulus.
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. Also known as towering cumulus.
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.
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. Located under the anvil area of most storms, it may slowly vary in size, since it is an area of negative buoyance convection, and is associated with severe turbulence in the lower sections of the cloud. Formerly called mammatocumulus.
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.
Cut-Off High - A warm high which has become displaced and is on the polarward 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 low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current. It is a cold low which has become displaced and is on the equatorward side of the jet stream. It frequently occurs during the spring and is often located over the southwestern United States and along the northwestern coast of Africa. Cutoff lows may remain nearly stationary for days, or on occasion may move westward opposite to the prevailing flow aloft (i.e., retrogression).
"Cutoff low" and "closed low" often are used interchangeably to describe low pressure centers aloft. However, not all closed lows are completely removed from the influence of the basic westerlies. Therefore, the recommended usage of the terms is to reserve the use of "cutoff low" only to those closed lows which clearly are detached completely from the westerlies.
*Cyclic Storm - A thunderstorm that undergoes cycles of intensification and weakening (pulses) while maintaining its individuality. Cyclic supercells are capable of producing multiple tornadoes (i.e., a tornado family) and or several bursts of severe weather. A storm which undergoes only one cycle (pulse), and then dissipates, is known as a pulse storm.
Cyclogenesis - Development or intensification of a low-pressure center (cyclone). This process can essentially intenisifly am existing low or create a 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 extratropical systems. The opposite of an anticyclone or a high pressure system.
*Cyclonic Circulation (or Cyclonic Rotation) - Circulation (or rotation) which is in the same sense as the Earth's rotation, i.e., counterclockwise (in the Northern Hemisphere) as would be seen from above. Nearly all storms and strong or violent tornadoes exhibit cyclonic rotation, but some smaller vortices, such as gustnadoes, sometimes rotate anticyclonically (clockwise). Compare with anticyclonic rotation.