Actually you can get icing right down to -40c, below that any water is likely to be frozen already and will not adhere to the aircraft. Water above zero CAN freeze if the airframe is below zero but it would be an unusual situation for this to be a problem as the airframe would normally then be warmed. Worst icing is just below zero because only a little of the water freezes on impact, the rest flows back and freezes forming a dense hard layer, when it is colder more freezes on impact which is lighter and not as hard.