What happens when rain doesn’t reach the ground?

You look up at a cloud above and notice wispy streaks descending below the cloud base, but no rain ever hits your head! What you are witnessing is what meteorologists call "virga". Virga is a name given to precipitating rain or ice that evaporate during the downward journey becoming water vapor before reaching the ground. Other unique names for the phenomenon are “Fallstreifen”, “fallstreaks”, “precipitation trails”, and even “jelly fish clouds”!

Besides being a popular photogenic cloud, especially during times of sunset or sunrise, there are a few forecasting considerations to note when seeing virga. Fundamentally, weather prediction relies on accounting for fluxes of energy transfer throughout the atmosphere. Water molecules forced to cycle through different phase changes upon mixing with a spectrum of environmental moisture content and temperatures is a key player in that regard!

During times of active virga, as liquid droplets (and ice crystals if present) are transformed back to gaseous water vapor state the evaporative cooling effect occurring through the layer of falling virga will alter the vertical temperature profile below the cloud.

At the times the cooling effect can reach the ground! Similar to the mechanisms allowing dry thunderstorm microburst winds, excessive and widespread virga showers have the potential to generate gustier winds at the surface when chilled air below the cloud loses buoyancy and is forced to drop altitude. Visually, the wispy streaks may be confined to a part of the sky, but weather modification beyond the immediate cloud is possible with cool sinking air! Even if gustier winds don’t reach the ground the localized fluctuations in vertical air mass buoyancy become important for aviation turbulence awareness.

Additionally, over a period time, a persistent episode of virga would gradually extend the depth of precipitation surviving below the cloud. Essentially, the sacrifice of initial precipitation to evaporation increases humidity (water vapor content), which makes it easier for ongoing precipitation to penetrate lower towards to the surface. Under this scenario, an eventual transition to rain/snow at the surface during a long-duration virga event is possible favoring mountainous terrain first, while being delayed for the surrounding lower valleys.

Furthermore, increasing coverage of virga, especially from cumulus clouds, may be a sign that a larger scale weather pattern shift is underway with a trend for deeper moisture and greater instability. For instance, should clear skies one day quickly turn virga filled the next, a regional uptick in shower and thunderstorm activity may be imminent!

Photo Credit: Jonny William Malloy