Virga

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This article is about the meteorological term. See Virgae for a type of surface feature found on Titan.

Nimbostratus virga

In meteorology, virga is precipitation that falls from a cloud but evaporates before reaching the ground. At high altitudes the precipitation falls mainly as ice crystals before melting and finally evaporating; this is usually due to compressional heating because the air pressure increases closer to the ground. It is very common in the desert.

Virga can cause very interesting weather effects, because as rain is changed from liquid to vapour form, it removes much heat from the air due to the high heat of vaporization of water. These small pockets of extremely cold air then descend rapidly, creating a microburst which can be extremely hazardous to aviation.

Virga also has a role in seeding storm cells, where light particles from one cloud are blown into neighbouring supersaturated air and act as nucleation particles for the next thunderhead cloud to begin forming.

Virga can produce dramatic and beautiful scenes, especially during a red sunset. The red light can be caught by the streamers of falling precipitation, while aloft winds push the bottom ends of the virga so it falls at an angle, making the clouds appear to have commas attached.

Virga is a Latin word for a branch or twig, and hence for objects made from it, as a broom, a staff, or a rod (hence the English word virge).

It also is an acronym for "Variable Intensity Rain Gradient Aloft", meaning the rain gradient varies in intensity dependent upon altitude. As the precipitation evaporates as it falls, its intensity lessens, hence; virga.