Rotor can be as low as 1000ft agl and can go from 2000fpm up to 2000fpm down in about 5 seconds, and thats in the UK. Imagine what it's like in the lee of real mountains. It's very sporting. Often all you see of it is a line of raggy Cu which doesn't look much at all, although I have seen a rotor cloud in which the rotation was clearly visible. I'm glad that I wasn't flying in the middle of it at the time