Weather Glossary - WCIV-TV | ABC News 4 - Charleston News, Sports, Weather

Weather Glossary

Air Mass
A large body of air having similar horizontal temperature and moisture characteristics.

Alberta Clipper
A small, fast-moving low-pressure system that forms in western Canada and travels southeastward into the United States. These storms, which generally bring little precipitation, generally precede an Arctic air mass.

Anticyclonic
Describes the movement of air around a high pressure, and rotation about the local vertical opposite the earth's rotation. This is clockwise in the Northern Hemisphere.

Anvil
A flat, elongated cloud formation at the top of a thunderstorm.

Back Door Cold Front
A front that moves east to west in direction rather than the normal west to east movement.

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.

Bow Echo
An accelerated portion of a squall line of thunderstorms, taking on bow configuration, created by strong downburst winds.

Cap
A layer of relatively warm air aloft (usually several thousand feet above the ground) which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability.

Ceiling
The height of the lowest layer of clouds, when the sky is broken or overcast.

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.

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.

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.

Dew Point
The temperature to which the air must be cooled for water vapor to condense.

Downburst
A severe localized downdraft from a thunderstorm.

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

Dry Line
A line that separates very warm, moist air to the east from hot, dry air to the west.

El Niño
A major warming of the equatorial waters in the Pacific Ocean. El Nino events usually occur every 3 to 7 years, and are characterized by shifts in "normal" weather patterns.

Flanking Line
A line of cumulus connected to and extending outward from the most active portion of a parent cumulonimbus, usually found on the southwest side of the storm. The cloud line has roughly a stair step appearance with the taller clouds adjacent to the parent cumulonimbus. It is most frequently associated with strong or severe thunderstorms.

Front
The transition zone between two distinct air masses. The basic frontal types are cold fronts, warm fronts, occluded fronts, and stationary fronts.

Gust
A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 mph.

Gust Front
The leading edge of the downdraft from a thunderstorm.

Gustnado
Gust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground.

Hook Echo
A radar pattern sometimes observed in the southwest quadrant of a tornadic thunderstorm. Appearing like a fishhook turned in toward the east, the hook echo is precipitation aloft around the periphery of a rotating column of air 2-10 miles in diameter.

Humidity
The amount of water vapor in the atmosphere. (See relative humidity).

Indian Summer
An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.

Inflow Notch
A radar signature characterized by an indentation in the reflectivity pattern on the inflow side of the storm. The indentation often is V-shaped, but this term should not be confused with V-notch. Supercell thunderstorms often exhibit inflow notches, usually in the right quadrant of a classic supercell, but sometimes in the eastern part of an HP storm or in the rear part of a storm (rear inflow notch).

Instability
A state of the atmosphere in which convection takes place spontaneously, leading to cloud formation and precipitation.

Jet Stream
Strong winds concentrated within a narrow band in the atmosphere. The jet stream often "steers" surface features such as fronts and low pressure systems.

Lake Effect
The effect of a lake (usually a large one) in modifying the weather near the shore and down wind. It is often refers to the enhanced rain or snow that falls downwind from the lake. This effect can also result in enhanced snowfall along the east coast of New England in winter.

La Niña
A cooling of the equatorial waters in the Pacific Ocean.

Landspout
A tornado that does not arise from organized storm-scale rotation and therefore is not associated with a wall cloud (visually) or a mesocyclone (on radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalents of waterspouts.

Low-level Jet
A region of relatively strong winds in the lower part of the atmosphere.

Macroburst
Large thunderstorm downbursts with a 2.5 mile diameter or greater outflow of damaging winds lasting 5 to 20 minutes.

Mesonet
A regional network of observing stations (usually surface stations) designed to diagnose mesoscale weather features and their associated processes.

Mesoscale Convective Complex
A large mesoscale convective system, generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs.

Microburst
A strong localized downdraft less than 2.5 miles in diameter from a thunderstorm. Peak gusts last from 2 to 5 minutes.

Multicell Cluster Thunderstorm
A thunderstorm consisting of two or more cells, of which most or all are often visible at a given time as distinct domes or towers in various stages of development.

Nor'easter
A low-pressure disturbance forming along the South Atlantic coast and moving northeast along the Middle Atlantic and New England coasts to the Atlantic Provinces of Canada. It usually causes strong northeast winds with rain or snow. Also called a Northeaster or Coastal Storm.

Outflow
Air that flows outward from a thunderstorm.

Pendant Echo
Radar signature generally similar to a hook echo, except that the hook shape is not as well defined.

Prevailing Westerlies
Winds in the middle latitudes (approximately 30 degrees to 60 degrees) that generally blow from west to east.

Pulse Storm
A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See overshooting top, cyclic storm.

Rain-Free Base

A horizontal, dark cumulonimbus base that has no visible precipitation beneath it. This structure usually marks the location of the thunderstorm updraft. Tornadoes most commonly develop (1) from wall clouds that are attached to the rain-free base, or (2) from the rain-free base itself. This is particularly true when the rain-free base is observed to the south or southwest of the precipitation shaft.

Red Flag Warning
    A term used by fire-weather forecasters to call attention to limited weather conditions of particular importance that may result in extreme burning conditions. It is issued when it is an on-going event or the fire weather forecaster has a high degree of confidence that Red Flag criteria will occur within 24 hours of issuance. Red Flag criteria occurs whenever a geographical area has been in a dry spell for a week or two, or for a shorter period , if before spring green-up or after fall color, and the National Fire Danger Rating System (NFDRS) is high to extreme and the following forecast weather parameters are forecasted to be met:
    1) a sustained wind average 15 mph or greater
    2) relative humidity less than or equal to 25 percent and
    3) a temperature of greater than 75 degrees F.
    In some states, dry lightning and unstable air are criteria. A Fire Weather Watch may be issued prior to the Red Flag Warning.

Reflectivity
Radar term referring to the ability of a radar target to return energy; used to estimate precipitation intensity and rainfall rates.

Relative Humidity
The amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. (Expressed as a percentage).

Return Flow

South winds on the back (west) side of an eastward-moving surface high pressure system. Return flow over the central and eastern United States typically results in a return of moist air from the Gulf of Mexico (or the Atlantic Ocean).

Roll Cloud
A relatively rare, low-level horizontal, tube-shaped accessory cloud completely detached from the cumulonimbus base. When present, it is located along the gust front and most frequently observed on the leading edge of a line of thunderstorms. The roll cloud will appear to be slowly "rolling" about its horizontal axis. Roll clouds are not and do not produce tornadoes.

Shelf Cloud
Long, wedge-shaped clouds associated with the gust front. Shelf clouds indicate the downdraft or outflow of a thunderstorm.

Shear (Wind Shear)
Variation in wind speed and/or direction over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.

Squall Line
A non-frontal band or line of thunderstorms.

Stationary Front
A transition zone between air masses, with neither advancing upon the other.

Straight-Line Winds
Thunderstorm winds most often found with the gust front. They originate from downdrafts and can cause damage which occurs in a "straight line", as opposed to tornadic wind damage which has circular characteristics.

Subtropical Jet
The branch of the jet stream that is found in the lower latitudes.

Supercell
A highly organized thunderstorm with a rotating updraft, known as a mesocyclone. It poses an inordinately high threat to life and property. Often produces large hail, strong winds, and tornadoes.

Sustained Winds
The wind speed obtained by averaging the observed values over a one minute period.

Tilted Storm
A thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.

Trade Winds
Persistent low-level tropical winds that blow from the subtropical high pressure centers towards the equatorial low.

Trough

An elongated area of low pressure at the surface or aloft.

Updraft
A small-scale current of rising air. This is often associated with cumulus and cumulonimbus clouds.

Upper Level System
A general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere.

Virga
Precipitation falling from the base of a cloud and evaporating before it reaches the ground.

Wall Cloud
An isolated lowering of a cloud that is attached to the rain-free base of a thunderstorm, generally to the rear of the visible precipitation area. Wall clouds indicate the updraft of or the inflow to a thunderstorm.

Wedge
A large tornado with a condensation funnel that is at least as wide (horizontally) at the ground as it is tall (vertically) from the ground to cloud base.

Wind Aloft
The wind speeds and wind directions at various levels in the atmosphere above the area of surface.

Zonal Flow
Large-scale atmospheric flow in which the east-west component (i.e., latitudinal) is dominant.

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