This article is from the Hanggliding and Paragliding FAQ, by Joao Geada with numerous contributions by others.
Hanggliders are flown in sub-zero conditions in the winter and in
the hottest deserts in the summer. Since the air temperature typically
falls by about 4 degrees (F) for every 1000 ft gain in elevation,
however, high altitude hangglider flights are frequently cold. Pilots
expecting to fly over about 12 - 14,000 ft in the summer will
generally wear warm clothing to protect against exposure.