|Virga - Fall Streaks - Imaged near Manchester, England by Pete Burgess. ©Pete Burgess, shown with permission.|
|Fall streaks or virga are cloud precipitation that does not reach the ground.
The streaks can be raindrops, snowflakes or even halo forming ice crystals.
When the air beneath the cloud is warmer or otherwise less humid the falling drops evaporate.
The fall streak’s characteristic hooked shape arises because drops fall progressively slower as they shrink. For small drops in laminar flow the downward velocity varies as the square of their diameter - Thus the downward velocity reduces rapidly as the drops get smaller.
While this is happening the wind carries them sideways. When the drops have almost completely evaporated they are hardly falling at all and the streak’s tail is almost horizontal.
Wind speed changes with height (shear) can also contribute to the streak’s shape.