Air density decreases with temperature. For a given windspeed you need smaller sails in winter. From 35 to 5°C it goes from 1,14 to 1.27 so you get an equivalent "push" with a 7.7 sail (roughly).
Air density also drops with altitude. I've seen people windsurfing at lake Mont Cenis in the Alps (~2500m) with very large sails though it was quite windy. They told me that otherwise they woul not plane at all.