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 often due to compressional heating, because the air pressure increases closer to the ground.
Image credit: ©Daniel Pardini – The setting sun lights up virga over the northern suburbs of Perth, Dec 2015.
Virga can cause varying weather effects, because as rain is changed from liquid to vapour, it removes heat from the air. In some instances, these pockets of colder air can descend rapidly, creating a dry microburst which can be extremely hazardous to aviation.
Virga also has a role in seeding storm cells whereby small particles from one cloud are blown into neighbouring supersaturated air and act as nucleation particles for the next thunderhead cloud to begin forming. When virga is occurring, you will often see precipitation on the rain radar, but the ground will be dry.
Virga from a decaying thunderstorm in the wheatbelt region of Western Australia.
This article is reproduced with permission and adapted under the Creative Commons Attribution-ShareAlike Licence. http://creativecommons.org/licenses/by-sa/3.0/
Article source: https://en.wikipedia.org/wiki/Virga