Mate, the windfarms aren't much good, they turn at an RPM decided by computers although obviously the wind speed will affect the max achievable RPM. Some will even be stopped!
As far as smoke goes, I usually estimate horizontal from the chimney as 25-30 knots and work progressively less until it goes straight up at less than 5 kts. I don't really fly in surface winds above 30kts so can't tell you about signs when it's greater than that!
HTH.
Blunty