Weatherwatch: Virga cloud’s wispy curtain is an illusion of rain

One of the joys of walking in the countryside is seeing a big cloud in the middle distance with what appears to be burst of rain falling on someone else’s head while you remain in the sunshine or at least dry. These wispy curtains, that look like waterfalls, are known as virga clouds and are particularly noticeable and attractive if caught by the evening sun.

But the impression that the rain from them is reaching the ground is an illusion. The rain, or often ice, that you are seeing is falling but as it hits warmer air below it evaporates (or sublimes). Sublime is the word chemists use to describe the reaction when a solid substance turns to vapour as it is heated.

Cloud watchers often refer to virga as jellyfish clouds because they have a similar appearance to a jellyfish floating in the sea, with its translucent tentacles hanging from its more substantial body.

So while no one on the ground gets wet as a result of this phenomenon, there is a danger to aircraft passengers from within these clouds. If an aircraft passes through at the point when the rain turns to vapour, the plane can unexpectedly hit a column of air accelerating downwards as it cools, causing severe air turbulence.