I've driven hundreds of miles with the top down, heater cranked, windows up and a knit hat on when it is in the high 30's and low 40's. No problem, very comfortable.
I remember coming back from a Concours at the Joliet Autobahn a couple years ago with the top down and it was in the mid 90's. That was smoking hot!
So my top down range is 37-94!!!