virga n : light wispy precipitation that evaporates before it reaches the ground (especially when the lower air is low in humidity)

English

Noun

1. In the context of "weather": Rain or snow that is dissipated in falling and does not reach the ground, commonly appearing in trails descending from a cloud layer.

For the genus of grass skipper butterflies, see Virga (butterfly). See Virga (planetary geology) for virgae, a type of surface feature found on Titan''.

In meteorology, virga is an observable streak or shaft of 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 and in temperate climates. It is also common in the Southern United States during summer.

Virgae can cause very interesting weather effects, because as rain is changed from liquid to vapor form, it removes heat from the air due to the high heat of vaporization of water. In some instances, these pockets of colder air can descend rapidly, creating a dry microburst which can be extremely hazardous to aviation.

Virgae also have a role in seeding storm cells whereby small particles from one cloud are blown into neighboring 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, and winds may push the bottom ends of the virga so it falls at an angle, making the clouds appear to have commas attached.

The word virga is derived from Latin meaning twig or branch and a popular backronym in meteorology is "Variable Intensity Rain Gradient Aloft."

Extraterrestrial versions

Sulfuric acid rain in the atmosphere of Venus evaporates before reaching the ground due to the immense heat near the surface. Similarly, virga happens on gas giant planets such as Jupiter.

See also

• Fall streaks
• Aviation safety

External links

• National Science Digital Library - Virga
• Picture: Virga near Denver, Colorado.
• "Viewing the Vagaries and Verities of Virga" Alistair B. Fraser and Craig F. Bohren, Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania, 2 November 1992 and 25 January 1993.