No expert at all, but it seems to be related to blade loading. The 212 didn't display the phenomenon until it lifted the load and then it did it in spades. The helo at the ski resort suddenly did it on the approach - from both the main and tail rotor. On the ground it was still burning and turning, but no cloud apparent.
I always understood that 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? So, exactly how and from what, are those rotors beating ice out of thin air?
Roger.