Virga is precipitation that falls from a cloud and evaporates or sublimates before it reaches the ground. It appears as wispy streaks or curtains hanging below the cloud base, often curved and trailing off into clear air.
How it forms
For virga to appear, the air below the cloud has to be dry enough that falling drops or ice crystals lose mass to the environment faster than they can reach the surface. As drops shrink, they slow and drift with the wind, which gives virga its trailing, hooked, or comma-shaped appearance.
The same evaporation that shrinks the drops also cools the surrounding air. That parcel becomes denser than the air around it and begins to sink. In a well-mixed dry layer with a lot of falling precipitation, the descent can accelerate into a downdraft with substantial momentum by the time it reaches the surface.
How forecasters use it
Virga is a quick visual read on the sub-cloud environment. Streaks curving off into nothing beneath high-based cumulus or altocumulus indicate a deep dry layer under the cloud. That configuration, especially over the High Plains and the interior West, is a classic setup for dry microburst wind events.
In severe-weather forecasting, virga under high-based storms is a cue to watch for sudden gust events rather than sustained rainfall. On satellite and radar, hints of virga show up as reflectivity aloft that fades before reaching the lowest tilts.
Important limits
Not all virga produces damaging winds. Weak precipitation into a modestly dry sub-cloud layer can evaporate quietly with no operationally significant downdraft. It takes both meaningful precipitation loading and a deep enough dry layer to generate a strong dry microburst.
Virga can also be misread as ordinary light rain that will reach the surface, especially near sunset when perspective flattens the scene. Watching whether the streaks reach the ground over several minutes is the reliable test.