Virga Rain Over Evaporation: Why Precipitation Disappears Mid-Air

Virga rain over evaporation occurs when precipitation falls from a cloud but evaporates or sublimes before reaching the ground. This happens when falling hydrometeors move through a dry layer of air below the cloud base, where low relative humidity allows the moisture to disappear before it reaches the surface.

Key takeaways

• Virga is a visible shaft of rain or ice falling from precipitating clouds that vanishes before reaching the ground.

• Dry air below the cloud base, low relative humidity, and adiabatic heating all help evaporating rain streaks disappear mid-air.

• Virga streaks may be made of liquid droplets or frozen hydrometeors, so both evaporation and sublimation can play a part.

• Virga can be linked to downdrafts and microbursts, which are major aviation hazards.

• Cloud types that can show virga include altocumulus and cirrocumulus, but they are not called jellyfish clouds; jellyfish clouds is a misnomer, as the trailing look is characteristic of virga in altocumulus floccus and cirrocumulus, but no cloud is officially named 'jellyfish clouds' due to this feature.

Virga rain over evaporation explained

Meteorologists use the term virga for precipitation that falls from a cloud but does not reach the ground. According to Wikipedia, this observable shaft of precipitation is essentially a "dry storm." The streaks can look like a shaft of rain hanging below the cloud base, even though the air underneath is too dry for the drops or ice crystals to survive the full descent.

The main driver is atmospheric humidity. When relative humidity is low below the cloud, the falling moisture evaporates into the surrounding air. If the precipitation is frozen, sublimation can also occur. In both cases, the cloud may look active while the surface stays dry.

Virga during a sunset over Saratov in south-west Russia. By Виктор Алексеев - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=112499661

Virga is often easiest to spot under precipitating clouds with a clearly defined dry slot beneath them. The streaks may fade before touching the ground, leaving a thin grey curtain that hangs under the cloud base. That is why people often ask why rain can be seen overhead but not felt at the surface.

What happens inside the dry layer?

As hydrometeors fall into drier air, evaporation cools the air around them. That cooling can affect the local air parcel, but the surrounding air still has to supply the energy needed for the phase change. If enough moisture is removed, the visible streaks thin out and vanish before reaching the ground.

What is the difference between virga and sublimation?

Virga is the visible phenomenon. Sublimation is one of the processes that can create it. Evaporation affects liquid drops, while sublimation turns ice directly into vapour. Both can produce evaporating rain streaks beneath a cloud.

How virga rain over evaporation appears on BOM radar

A diagram demonstrating how dry air causes virga on the radar by evaporating precipitation beneath the radar beam scan.

The Bureau of Meteorology uses radar and other observing tools to track precipitation. Radar can detect hydrometeors aloft even when the rain never reaches the ground. That means an area may show precipitation echoes while the surface remains dry because the falling moisture evaporates before it arrives.

This is why virga can be confusing for anyone reading a radar image without checking the full weather situation. Radar shows what is happening in the cloud and below it, but it cannot directly show every change in the air column. A dry layer, low atmospheric humidity, and a cloud base well above the ground can all favour virga.

What does virga look like on radar?

On radar, virga may look similar to ordinary rain echoes because the beam still detects precipitation aloft. The difference is at the surface. If the air below the cloud is dry enough, gauges and observers on the ground may record little or no rainfall.

Cloud types that can produce virga

Virga can appear under a range of types of clouds, especially higher-based clouds where precipitation has more time to fall through dry air. Altocumulus and cirrocumulus can both show virga streaks. Some clouds with long hanging streaks are sometimes called jellyfish clouds because the falling trails resemble tentacles.

These clouds are often easiest to notice when the air beneath them is dry enough for the streaks to vanish part way down. The result is a soft curtain of rain that never reaches the ground.

Why does virga look like wisps?

Virga looks wispy because the falling moisture gets smaller as it descends. The streak narrows as drops evaporate or ice crystals sublimate in drier air, so the lower part of the trail fades first.

Why virga matters for aviation

Red-tinted virga streaks hanging from altocumulus clouds during an outback sunset.

Virga is important for aviation because the same dry-air conditions that cause it can also support strong downdrafts and microbursts. According to the Bureau of Meteorology, microbursts are the most violent form of wind shear produced by thunderstorms and can be less than 4 km across. That makes them a serious hazard during take-off and landing.

A microburst can create a sudden change in wind speed and direction near the ground. BoM notes that wind shear can involve horizontal changes of as much as 180 degrees at ground level following a gust front, and wind gusts may exceed 50 knots during wind shear events associated with gust fronts. Pilots treat this as an aviation hazard because it can alter lift and aircraft performance very quickly.

Virga does not always mean a microburst will form, but it can be a warning sign that the air below the cloud is dry and unstable. That is why forecasters and pilots pay close attention to virga streaks beneath thunderstorms and other precipitating clouds.

Virga, downdrafts and dry thunderstorms

When precipitation evaporates in a dry layer, the cooling can help create downdrafts. If those downdrafts strengthen near the surface, they may produce gusty outflow and, in some storms, microbursts. In dry thunderstorm setups, lightning can reach the ground while very little rain does, which raises the risk of bushfire ignition.