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
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
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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