Snow is kind of like dew. Don't ice crystals need a "nucleus" on which to form, usually a dust particle or something? In that sense it is a bit like frost forming on a surface - just a very, very small surface. And Faldage is right about the temperature being different "up there" in the cloud. That's probably the temperature that counts for the initial formation of snow. I've certainly experienced snow when the ground temperature is above freezing. Obviously where the snow originated it was below freezing!