I may as well mention another variable: radiant temperature.

Imagine you are attending a spring wave camp in some Nordic country. It =
is a beautiful, cold, clear day without a trace of cloud. The sun gives =
little warmth as its rays are so slanted. In these conditions, even if =
outside air temperature is, let's say, +2=BAC, you are already in risk, =
because the sky vault has a radiant temperature some ten degrees lower =
than the local air temperature, and will be chilling all exposed =
surfaces. This is the same mechanism that causes frost on clear nights. =
Manufacturers have to take into account such worst case scenarios for =
certification purposes.

In a midsummer Arizona day things will be quite different. The blazing =
sun, re-irradiation from cumulus clouds and even from the warm ground, =
will ensure that the mean radiant temperature to which the glider is =
subjected is higher than the air temperature at altitude. Therefore, a =
(few) degrees below zero will still be safe.