Because as the water molecules get heated by the sun they begin to move faster and faster until they evaporate from the liquid and become water vapour. On a cold day this process is slower because the molecules usually move slower and find it trickier to evaporate from the liquid.
2 years ago
Last edited at 2:51PM on 3/28/2012
Because evaporation of water requires heat, so hot air (which has higher amount of heat) will evaporate water faster than the cold one. And the opposite for the condensation process, when hot steam meets a colder air it condensate faster than meeting the normal air.