Glossary

Periods in the history of the earth characterized by a growth of the ice caps towards the equator and a general lowering of global surface temperatures, especially in temperate mid- latitudes. The most recent ice age ended about 10,000 years ago. Ice advances in this period are known to have altered the whole pattern of global atmospheric circulation.

A barely visible crystalline form of ice that has the shape of needles, columns or plates. Ice crystals are so small that they seem to be suspended in air. Ice crystals occur at very low temperatures (around zero degrees F and colder) in a stable atmosphere.

An accumulation of broken river ice caught in a narrow channel that frequently produces local floods during a spring break- up.

Liquid rain falling and freezing on contact with cold objects creating ice build- ups of 1/4th inch or more that can cause severe damage.

The ceiling classification applied when the reported ceiling value represents the vertical visibility upward into surface- based obscuration.

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

Bands of low clouds, arranged parallel to the low- level winds and moving into or toward a thunderstorm.

Local jets of air near the ground flowing inward toward the base of a tornado.

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).

A beaver tail cloud with a stinger- like shape.

Electromagnetic radiation of lower frequencies and longer wavelengths than visible light (greater than 0.7 microns (? m)). Solar ultra- violet radiation is absorbed by the Earth's surface and re- emitted as infra- red radiation.

Incoming solar radiation. Solar heating; sunshine.

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

The region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms near the equator.

An increase in temperature with height. The reverse of the normal cooling with height in the atmosphere. Temperature inversions trap atmospheric- pollutants in the lower troposphere, resulting in higher concentrations of pollutants at ground levels than would usually be experienced.

Also known as the thermosphere. A layer in the atmosphere above the mesosphere extending from about 80km above the Earth's surface. It can be considered a distinct layer due to a rise in air temperature with increasing height. Atmospheric densities here are very low.

Brilliant patches of green or pink sometimes seen near the edges of high or medium- level clouds.

Lifting of air that is traveling along an upward- sloping isentropic surface. Situations involving isentropic lift often are characterized by widespread- stratiform clouds and precipitation.

A two- dimensional surface containing points of equal potential temperature.

A line of equal barometric pressure on a weather map.

A line of equal dew point temperature.

A line of equal precipitation amounts.

General term for a line of equal value of some quantity. Isobars, isotherms, etc. all are examples of isopleths.

A line of equal wind speed.

A line of equal temperature on a weather map.