Virga is a meteorological phenomenon where precipitation falls from a cloud but evaporates or sublimates before it can reach the ground. The word virga originates from the Latin term for “streak” or “branch,” describing its visual appearance. This atmospheric event often leads to a situation where weather radar shows rain, but the ground remains dry. Virga is frequently observed in arid climates and high-altitude regions where dry air layers are common below cloud bases.
Defining the Appearance of Virga
Virga presents as distinct, wispy streaks trailing downward from the base of a cloud, resembling a ghostly tail or a falling curtain. These translucent shafts of moisture are the visible precipitation that fade or dissipate into nothingness well above the surface, instead of connecting to the ground like normal rainfall.
The appearance can vary depending on the cloud type and altitude. Virga is commonly seen trailing from mid-level clouds like altocumulus or altostratus, but can also descend from higher cirrocumulus or lower cumulus clouds. When the sun angles through the evaporating streaks, the virga can become dramatically illuminated. The streaks may appear straight and vertical, but strong winds can sometimes cause them to curve into hook-like shapes.
The Meteorological Process of Virga Formation
The formation of virga is governed by the presence of a deep layer of warm and dry air situated between the cloud base and the ground. Precipitation, which starts as either liquid water droplets or ice crystals within the cloud, falls into this air mass with low relative humidity. The air’s capacity to hold water vapor is not yet saturated, which creates the conditions for the falling moisture to change phase.
As the liquid droplets fall through the dry air, they undergo evaporation, turning directly into water vapor. If the precipitation begins as ice crystals, it may transition directly from solid to gas through the process of sublimation. Both processes require energy, which is absorbed from the surrounding air, facilitating the rapid disappearance of the precipitation.
Atmospheric Effects Caused by Virga
The physical process of evaporation and sublimation has significant consequences for the air beneath the cloud. The conversion of water from liquid or solid to a gaseous state requires a substantial amount of latent heat, which is drawn from the surrounding environment. This absorption of heat causes a localized and sometimes significant cooling of the air mass directly below the virga.
The cooled air becomes denser than the surrounding air, leading to the creation of a strong downward current known as a downdraft. In cases of intense virga, this rapidly sinking, cold air can become concentrated, resulting in a dangerous phenomenon called a dry microburst. When a dry microburst hits the ground, it spreads out violently in all directions, capable of producing sudden, high-speed surface winds that can pose hazards, particularly to aviation. The outflow of a microburst can also create a gust front, a boundary of cooler, gusty air that influences local wind patterns.