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NOAA - Meteorology glossary
Category: Sciences > Meteorology
Date & country: 14/10/2013, US
Words: 671


Circulation
The pattern of the movement of air. General circulation is the flow of air of large, semi-permanent weather systems, while secondary circulation is the flow of air of more temporary weather systems.

Cirriform
High altitude ice clouds with a very thin wispy appearance.

Cirrocumulus
Cirrus clouds with vertical development.

Cirrostratus
Cirrus clouds with a flat sheetlike appearance.

Cirrus
High clouds, usually above 18,000 feet, composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands.

Clear
Sky condition of less than 1/10 cloud coverage.

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.

Climate
The prevalent long term weather conditions in a particular area. Climatic elements include precipitation, temperature, humidity, sunshine and wind

Climate change
This strictly refers to all forms of climatic inconsistency. But it is often used in a more restricted sense to imply a significant change. Within the media, climate change has been used synonymously with global warming. Scientists, however, use the term in a wider sense to include past climate changes also.

Climate Normals
Averages of temperatures, precipitation, snowfall, etc. made over standard 30 year periods. These normals span across 3 decades and are rederived every 10 years.

Climatology
the scientific study of climate.

Closed Low
A low pressure area with a distinct center of cyclonic circulation which can be completely encircled by one or more isobars or height contour lines. The term usually is used to distinguish a low pressure area aloft from a low-pressure trough. Closed lows aloft typically are partially or completely detached from the main westerly current, and thus move relatively slowly.

Cloud
A visible cluster of tiny water and/or ice particles in the atmosphere.

Cloud Base
For a given cloud or cloud layer, it is the lowest level in the atmosphere where cloud particles are visible.

Cloud Condensation Nuclei
small particles in the air on which water vapor condenses and forms cloud droplets.

Cloud Streets
Rows of cumulus or cumulus-type clouds aligned parallel to the low-level flow. Cloud streets sometimes can be seen from the ground, but are seen best on satellite photographs.

Cloud Tags
Ragged, detached cloud fragments; fractus or scud.

Cloudburst
A sudden, intense rainfall that is normally of short duration.

Cloudy
the state of the sky when 7/10ths or more of the sky is covered by clouds.

Coastal Flood Warning
Issued when there is widespread coastal flooding expected within 12 hours, more than just typical overwash.

Coastal Flooding
The inundation of land areas along the coast caused by sea water above normal tidal actions. This is often caused by prolonged strong onshore flow of wind and/or high astronomical tides.

Coastal Forecast
A forecast of wind, wave and weather conditions between the coastline and 25 miles offshore.

Coastal Waters
include the area from a line approximating the mean high water along the mainland or island as far out as 25 miles including the bays, harbors and sounds.

Cold Advection
(CAA) Transport of cold air into a region by horizontal winds.

Cold Air Damming
Cold air damming occurs when a cold dome of high pressure settles over northeastern New England. The clockwise circulation around the high pressure center brings northeasterly winds to the mid Atlantic region. The northeasterly winds bank cold air against the eastern slopes of the Appalachian Mountains. Warmer air from the west or southwest is lifted above the cold air as it moves instead of warming the surface.

Cold Front
A narrow transition zone separating advancing colder air from retreating warmer air. The air behind a cold front is cooler and typically drier than the air it is replacing.

Cold Low
a low pressure system with cold air mass from near the surface to all vertical levels (also called a cold core low).

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-air Funnel
A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.

Collar Cloud
Frequently used as a synonym for wall cloud, although it actually is a generally circular ring of cloud surrounding the upper portion of a wall cloud.

Combined Seas
The combined height of swell and wind waves.

Comma Cloud
A synoptic scale cloud pattern with a characteristic comma-like shape, often seen on satellite photographs associated with large and intense low-pressure systems.

Condensation
The process by which water vapor becomes a liquid; the opposite of evaporation, which is the conversion of liquid to vapor.

Condensation Nuclei
Small particles in the air around which water vapor condenses.

Conduction
The transfer of heat by molecular action between bodies that are in contact.

Confluence
A pattern of wind flow in which air flows inward toward an axis oriented parallel to the general direction of flow. It is the opposite of difluence. Confluence is not the same as convergence. Winds often accelerate as they enter a confluent zone, resulting in speed divergence which offsets the (apparent) converging effect of the confluent flow.

Congestus (or Cumulus Congestus)
A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil shaped top of a Cb.

Continental Air Mass
A dry air mass originating over a large land area.

Contrail
A cloud-like stream formed in cold, clear air behind the engines of an airplane.

Convection
The transfer of heat within a the air by its movement. The term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere.

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.

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.

Convergence
An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region. Divergence is the opposite, where winds cause a horizontal net outflow of air from a specified region.

Cooling Degree Day
A form of degree day used to estimate the required energy for cooling. one cooling degree day occurs for each degree the daily mean temperature is above 65 degrees Fahrenheit.

Coordinated Universal Time (UTC)
The time in the zero degree meridian time zone.

Coriolis Force
An apparent force caused by the rotation of the Earth. In the Northern Hemisphere winds are deflected to the right, and in the Southern Hemisphere to the left. 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. The air flowing around a hurricane spins counter-clockwise in the northern hemisphere

Corona
a disk of light surrounding the sun or moon; this is a result of the diffraction of light by small water droplets.

CRS
Console Replacement System. This consists of a computer system and computer voice that is used to automate NOAA Weather Radio.

Cumulonimbus Cloud
A vertically developed cloud, often capped by an anvil shaped cloud. Also called a thunderstorm cloud, it is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail or gusty winds.

Cumulus Cloud
A cloud in the shape of individual detached domes, with a flat base and a bulging upper portion resembling cauliflower.

Cumulus Congestus
A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil shaped top of a Cb.

Cut Off Low
An upper level low pressure system that is no longer in the normal west to east upper air flow. Usually a cut-off low will lie to the South of the established upper air flow.

Cyclogenesis
Development or intensification of a low-pressure center

Cyclone
An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere. Also the term used for a hurricane in the Indian Ocean and in the Western Pacific Ocean.

Dart Leader
In lightning, the leader which, after the first stroke, initiates each succeeding stroke of a composite flash of lightning.

Decouple
The tendency for the surface wind to become much lighter than wind above it at night when the surface temperature cools.

Degree Day
a measure of the departure of the daily mean temperature from the normal daily temperature; heating and cooling Degree Days are the departure of the daily mean temperature from sixty-five degrees Fahrenheit.

Dendrite
hexagonal ice crystals with complex and often fernlike branches.

Dense Fog
a fog in which the visibility is less than one-quarter mile.

Dense Fog Advisory
Issued when fog is expected to reduce visibility to 1/4 mile or less over a widespread are for at least 3 hours.

Density Altitude
The pressure altitude corrected for temperature deviations from the standard atmosphere. It is used by pilots when setting aircraft performance.

Density Of Air
The mass of air divided by its volume. The air's density depends on its temperature, its pressure and how much water vapor is in the air.

Depression
a region of low atmospheric pressure that is usually accompanied by low clouds and precipitation.

Depth Hoar
Large (one to several millimeters in diameter), cohesionless, coarse, faceted snow crystals which result from the presence of strong temperature gradients within the snowpack

Derechoe
A widespread and usually fast-moving windstorm associated with convection. Derechoes include any family of downburst clusters produced by an extratropical MCS, and can produce damaging straight-line winds over areas hundreds of miles long and more than 100 miles across.

Dew
Moisture from water vapor in the air that has condensed on objects near the ground, whose temperatures have fallen below the dewpoint temperature.

Dew Point
The temperature to which the air must be cooled for water vapor to condense and form fog or clouds.

Diamond Dust
A fall of non-branched (snow crystals are branched) ice crystals in the form of needles, columns, or plates. (same as ice crystals)

Differential Motion
Cloud motion that appears to differ relative to other nearby cloud elements, e.g. clouds moving from left to right relative to other clouds in the foreground or background. Cloud rotation is one example of differential motion, but not all differential motion indicates rotation. For example, horizontal wind shear along a gust front may result in differential cloud motion without the presence of rotation.

Dirty ridge
Most of the time, upper-level ridges bring fairly clear weather as the storms are steered around the ridge. Sometimes, however, strong storms undercut the ridge and create precipitation. Ridges that experience this undercutting by storms are known as dirty ridges because of the unusual precipitation.

Disturbance
a disruption of the atmosphere that usually refers to a low pressure area, cool air and inclement weather.

Diurnal
Daily; related to actions which are completed in the course of a calendar day, and which typically recur every calendar day (e.g., diurnal temperature rises during the day, and falls at night).

Divergence
The expansion or spreading out of a vector field; usually said of horizontal winds. It is the opposite of convergence.

Doldrums
the regions on either side of the equator where air pressure is low and winds are light.

Doppler Radar
A type of weather radar that determines whether atmospheric motion is toward or away from the radar. It determines the intensity of rainfall and uses the Doppler effect to measure the velocity of droplets in the atmosphere.

Downburst
A strong downdraft resulting in an outward burst of damaging winds on or near the ground. Downburst winds can produce damage similar to a strong tornado.

Downdraft
A column of generally cool air that rapidly sinks to the ground, usually accompanied by precipitation as in a shower or thunderstorm. .

Downslope wind
Air that descends an elevated plain and consequently warms and dries. Occurs when prevailing wind direction is from the same direction as the elevated terrain and often produces fair weather conditions.

Downstream
In the same direction as a stream or other flow, or toward the direction in which the flow is moving.

Drifting snow
Uneven distribution of snowfall caused by strong surface winds. Drifting snow does not reduce visibility.

Drizzle
Small, slowly falling water droplets, with diameters between .2 and .5 millimeters.

Drought
Abnormally dry weather in a region over an extended period sufficient to cause a serious hydrological (water cycle) imbalance in the affected area. This can cause such problems as crop damage and water-supply shortage.

Dry Adiabat
A line of constant potential temperature on a thermodynamic chart.

Dry Line
A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains.

Dry Punch
A surge of drier air; normally a synoptic-scale or mesoscale process. A dry punch at the surface results in a dry line bulge.

Dry Slot
A zone of dry (and relatively cloud-free) air which wraps east

Dry-line Bulge
A bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels

Dryline
A boundary which separates warm, dry air from warm, moist air. The differences in the two air masses may be significant. The dry line is usually a boundary of instability along which thunderstorms form.

Dust Devil
A small, rapidly rotating wind that is made visible by the dust, dirt or debris it picks up. Also called a whirlwind. Dust devils usually develop during hot, sunny days over dry and dusty or sandy areas.

Dust Storm
An area where high surface winds have picked up loose dust, reducing visibility to less than one-half mile.

Dust Whirl
A rotating column of air rendered visible by dust.

Dynamics
Generally, any forces that produce motion or affect change. In operational meteorology, dynamics usually refer specifically to those forces that produce vertical motion in the atmosphere.

Easterly Wave
A wavelike disturbance in the tropical easterly winds that usually moves from east to west. Such waves can grow into tropical depressions.

ECMF
European Center for Meteorology Forecast model.

Eddy
A small volume of air that behaves differently from the predominant flow of the layer in which it exists, seemingly having a life of its own. An example of such would be a tornado, which has its own distinct rotation, but is different than the large-scale flow of air surrounding the thunderstorm in which the tornado is born.

Enhanced greenhouse effect
The natural greenhouse effect has been enhanced by man's emissions of greenhouse gases. Increased concentrations of carbon dioxide, methane and nitrous oxide trap more infra-red radiation, so heating up the atmosphere.

Enhanced Wording
An option used by the SPC in tornado and severe thunderstorm watches when the potential for strong/violent tornadoes, or unusually widespread damaging straight-line winds, is high.

ENSO
El Nino-Southern Oscillation.

Entrance Region
The region upstream from a wind speed maximum in a jet stream (jet max), in which air is approaching (entering) the region of maximum winds, and therefore is accelerating. This acceleration results in a vertical circulation that creates divergence in the upper-level winds in the right half of the entrance region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the right rear quadrant (or right entrance region) of the jet max. Severe weather potential sometimes increases in this area as a result.

Equilibrium Level (or EL)
On a sounding, the level above the level of free convection (LFC) at which the temperature of a rising air parcel again equals the temperature of the environment.