Beater
I'm no met man but it's basically Radiation Fog. As I understand it, it is that in order for water vapour to condense into a water droplet there needs to be a condensation nucleus. This can be anything from the aircraft itself down to microscopic pollution particles. In the case of 'rotor induced fog' the air has to be very still. The air cools overnight through radiation but because there is no movement in the air there is no mixing and therefore there are no 'collisions' with condensation nuclei and fog does not form despite 100 per cent Relative Humidity. What will be experienced however, is a very heavy dew. Now along comes ****ehawk with his helicopter and not only does he stir up the air but he's also producing quite a lot of exhaust efflux (condensation nuclei) and hey presto - Radiation Fog!
JJ