Cryosphere glossary
atmospheric phenomenon
as commonly used in weather observing practice, an observable occurrence of particular physical significance within the atmosphere; from the viewpoint of weather observations, the atmospheric phenomena include all hydrometeors (precipitation types and fogs), blowing snow, thunderstorms, tornadoes, waterspouts, and others.
atmospheric pressure
pressure (force per unit area) exerted by the atmosphere on any surface by virtue of its weight; it is equivalent to the weight of a vertical column of air extending above a surface of unit area to the outer limit of the atmosphere.
atmospheric radiation
longwave (infrared) radiation emitted by or being propagated through the atmosphere.
aurora
luminous phenomena, in the form of arcs, bands, draperies, or curtains in the high atmosphere over high latitudes; auroras are related to magnetic storms and the influx of charged particles from the sun, the phenomena are called aurora borealis in the northern hemisphere and aurora australis in the southern hemisphere.
avalanche
mass of snow which becomes detached and slides down a slope, often acquiring great bulk by fresh addition as it descends.
Image
An avalanche in motion.
Richard Armstrong, National Snow and Ice Data Center
average value
arithmetic mean (m) of a number (n) of values (x1, x2, ... xn), defined by the equation: m = sxi/n; annual average value is calculated from 12 monthly means; daily average (or mean) value is calculated from 24 hourly readings of a meteorological element, or often from the average of the daily maximum and minimum values (for example, of temperature); monthly average is usually calculated as the average of the daily average values.
Azores high
the semipermanent subtropical high over the North Atlantic Ocean, especially when it is located over the eastern part of the ocean; the same high over the western part of the Atlantic is called the Bermuda high; on mean charts of sea level pressure, this high is one of the primary centers of action in northern latitudes.