Virga – An Atmospheric Magic Trick

Some clouds produce showers of rain. Some clouds produce none. Others can release a shower, and then, as if by magic, can make it disappear again. Welcome to the cloud feature known as virga: when moisture falls from a cloud and dissipates away again long before reaching the ground.

Streaks of virga can hang from seven of the ten main cloud types. Most often, like the virga featuring as Cloud of the Month for March, they appear beneath mid-level clouds called Altocumulus. The Altocumulus virga here were spotted over West Lakes, Australia by Darren Hallet. Why does the falling moisture in Darren’s virga just disappear like this? And why do Darren’s virga seem to bend so abruptly to the right like this?

Streaks of virga can be formed by rain, snow, or ice crystals falling from a cloud, all of which can disappear like this if they fall through air that’s dry enough or warm enough. The raindrops can evaporate (change from visible droplets to invisible water vapour), the ice can sublimate (change directly from solid ice crystals to water vapour), and the drier the air, the faster the disappearing act happens. As for the abrupt bend of the virga to one side, this happens when, beneath the cloud, the winds change distinctly with altitude, something known as wind shear.

Now you see it, now you don’t. Roll up, roll up, and be amazed by the deft sleight of hand of Altocumulus clouds with the assistance of dry, shearing winds below.

Altocumulus floccus virga spotted over West Lakes, Australia by Darren Hallet. View this in the photo gallery.