The Kopp-Etchells Effect

Source: Webstie

http://www.michaelyon-online.com/the-kopp-etchells-effect.htm

 

When a helicopter descends into a sandy environment, the enormous downward thrust from the rotors inevitably kicks up a cloud of sand. Once in the air, the helicopter’s blades cut through this cloud, generating the halo.

 

But how?