Virga

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virga

(Also called Fallstreifen, fallstreaks, precipitation trails.) Wisps or streaks of water or ice particles falling out of a cloud but evaporating 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 evaporation decreases the particle terminal fall velocity near the ends of the streaks. Under some conditions, virga are associated with dry microbursts, which are formed as a product of the evaporation.
See cloud classification.

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