Contrary to popular belief, leg hair does not grow faster in cold weather. The growth rate of human hair over the entire body is fairly consistent, and there is little evidence to show that a change in temperature affects this in any way.
Although the rate at which hair grows remains constant, there is some evidence to suggest that the rate at which hair is shed is slower in winter than in summer. A study conducted in 2009 by the University Hospital of Zürich in Switzerland found that hair loss increased during hot summer months. This may suggest that the reason leg hair appears to grow more quickly during winter is that there is less hair loss taking place than in summer.