virga

Virga is a meteorological phenomenon in which precipitation falls from a cloud but evaporates or sublimates before reaching the ground. It appears as wispy streaks or shafts extending downward from the cloud, giving the impression of rain or snow that never makes it to the surface. Virga occurs when falling droplets or ice crystals pass through a layer of air that is unsaturated or drier than the cloud, causing rapid evaporation or sublimation.

Virga can involve liquid water droplets (rain virga) or ice crystals (ice virga). In some cases, snow or graupel can sublimate as they descend, producing snow virga. The effect is most common beneath high-based clouds such as cirrus, cirrostratus, and altostratus, and is also observed beneath thunderstorm anvils or other cloud systems where a dry layer lies below the base.

Meteorologists monitor virga because it signals a lack of moisture reaching the ground and can indicate atmospheric stability in the lower layers. In addition, the evaporation of precipitation can cool the surrounding air, potentially generating gusts or microbursts at the surface where drier air intercepts the descending air mass. Virga is often mistaken for rain by observers on the ground, since the visible streaks resemble precipitation trajectories.

The term virga comes from Latin virga, meaning a rod or wand, reflecting the shaft-like appearance of the falling precipitation.

See also: precipitation, thunderstorm dynamics, microbursts, atmospheric moisture.