virga
(
Also called Fallstreifen, fallstreaks, precipitation trails.) Wisps or streaks of water or
ice particles falling out of a
cloud but vaporizing before reaching the earth's surface as
precipitation.
Virga is frequently seen trailing from
altocumulus and
altostratus clouds, but also is discernible below the bases of high-level
cumuliform clouds from which precipitation is falling into a dry
subcloud layer. It typically exhibits a hooked form in which the streaks descend nearly vertically just under the precipitation source but appear to be almost horizontal at their lower extremities. Such curvature of virga can be produced simply by effects of strong vertical
wind shear, but ordinarily it results from the fact that
droplet or
crystal vaporization decreases the
particle terminal
fall velocity near the ends of the streaks. Under some conditions, virga is associated with dry
microbursts, which are formed as a product of the
vaporization.
See cloud classification.
Term edited 14 April 2015.
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