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WeatherChannel - Weather Terms Category: Meteorology and astronomy > Weather Glossary Date & country: 30/09/2008, USA Words: 751

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 wat...

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. The...

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 ...

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 occu...

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 sk...

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,...

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. 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.

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...

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. Related term: Dave's Dictionary

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: downburst

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. Related term: Dave's Dictionary

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 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 beh...

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 We...

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
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.

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 an...

coromell
The prevailing evening land breeze which takes place from November to May in the vicinity of La Paz, at the southern tip of Baja California, Mexico.

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 term...

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 ...

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 tropos...

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...

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 va...

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 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 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 l...

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 c...

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

dalton's law
States that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the gases. Formulated by John Dalton, an English physicist.

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 o...

degree
A measure of temperature difference representing a single division on a temperature scale. Related terms: Celsius, Fahrentheit, 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 4.0°C or 39.2°F.

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 conjunctio...

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: Intertropical 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 ...

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.