Monday, January 17, 2011

Weather terms we use in our forecast, and what they mean.

Here are some weather terms or slang we use here at The Weather Center...Most used in our forecast,some used in our pictures... More will be added throughout the week.
Accretion- Growth of precipitation particles by collision of ice crystals with supercooled liquid droplets which freeze on impact.
Accessory Clouds- Clouds that are dependent on a larger cloud system for development and continuance. Accessory clouds associated with the thunderstorm include roll, shelf, mammatus, and wall clouds.
Adiabatic- changes in temperature caused by the expansion (cooling) or compression (warming) of a body of air as it rises or descends in the atmosphere.
Adiabatic Process- The change of temperature of air without transferring heat. In an adiabatic process compression results in warming, and expansion results in cooling.
Advection- The horizontal transport of air, moisture or other atmospheric properties. Commonly used with temperatures, i.e., "warm air advection".
Advection Fog- a type of fog that results from the advection of moist air over a cold surface and the cooling of the air to its dew point that follows; this type of fog is most common in coastal regions..
AGL- above ground level.
Air Mass- A large body of air that has similar horizontal temperature and moisture characteristics.
Air-mass Thunderstorm- Generally, a thunderstorm not associated with a front or other type of synoptic-scale forcing mechanism. Air mass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon in response to insolation, and dissipate rather quickly after sunset.
Air Parcel- An imaginary small body of air that is used to explain the behavior of air. A parcel is large enough to contain a very great number of molecules, but small enough so that the properties assigned to it are approximately uniform throughout.
Air Pressure- (atmospheric pressure) air 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.
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.
Anomaly- The deviation of (usually) temperature or precipitation in a given region over a specified period from the normal value for the same region.
Anticyclone- A large body of air in which the atmospheric pressure is higher than the pressure in the surrounding air. The winds blow clockwise around an anticyclone in the Northern Hemisphere.
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 Cloud- The flat, spreading top of a Cb (cumulonimbus), often shaped like an anvil. Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and sometimes may spread upwind (see back-sheared anvil).
Anvil Crawler - 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.
Anvil Dome - A large overshooting top or penetrating top.
Arctic Air- a mass of very cold, dry air that usually originates over the Arctic Ocean north of Canada and Alaska.
Arctic High- a very cold high pressure that originates over the Arctic Ocean.
Arcus- A low, horizontal cloud formation associated with the leading edge of thunderstorm outflow (i.e., the gust front). Roll clouds and shelf clouds both are types of arcus clouds.
Atmospheric Stability- An indication of how easily a parcel of air is lifted. If the air is very stable it is difficult to make the parcel rise. If the air is very unstable the parcel may rise on its own once started.
Aurora Borealis- Also known as the northern lights - The luminous, radiant emission from the upper atmosphere over middle and high latitudes, and centered around the earth's magnetic poles. These silent fireworks are often seen on clear winter nights in a variety of shapes and colors.
AWIPS- Advanced Weather Information Processing System. New NWS computer system integrating graphics, satellite and radar imagery. The successor to AFOS.
CAA- Cold Air Advection
Cap (or Capping
Inversion)- 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.
 Convective Available Potential Energy. A measure of the amount of energy available for 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.
Celsius- a temperature scale in which zero is the freezing point of water and one hundred is the boiling point.
Chance- A 30, 40 or 50 percent chance of occurrence of measurable precipitation.
Clear- Sky condition of less than 1/10 cloud coverage. Clear Slot- A local region of clearing skies or reduced cloud cover
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.
Cloudy- the state of the sky when 7/10ths or more of the sky is covered by clouds.
Cold Advection- (CAA) Transport of cold air into a region by horizontal winds.
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.
Condensation- The process by which water vapor becomes a liquid; the opposite of evaporation, which is the conversion of liquid to vapor.
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.
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.
Cyclogenesis- Development or intensification of a low-pressure center Cyclone- An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere