exactly
In warm conditions, the cloud disappears because the humidity is quickly absorbed by the surrounding air.
In case of an ice cloud (deposition), the ice crystals will apparently hang around longer and fall to the ground
No expert at all, but it seems to be related to blade loading.
More blade loading -> greater pressure changes -> more vapor. Same with the clouds on the fighter jets, they only appear during high-G manoeuvres.
in the very, very cold conditions as in the 212 clip, the humidity is, by definition, very, very low - otherwise it would snow, wouldn't it?
The absolute amount of water might be low, but relative humidity can still be very high.