My experience is 20-80F air temp, but that's not necessarily relevant... 40F, windy and raining feels WAY colder (and is much more dangerous hypothermia-wise) than 20F on a calm frosty morning.
I've woken up in the tent at 13.2k ft on the Whitney summit ridge at 20F with a decent layer of ice on the inside, but not been cold. I've camped in Crabtree meadows in a big storm at 40F and been shivering until I dried out.
So really, what matters is the weather forecast... see links on the left!
