December in Thornhill is the windiest month of the year. In December, the weather in Thornhill is typically freezing. Average daily high temperature in December is about zero. On average, Thornhill has 15 cloudy, 9 snowy, 5 rainy, and 2 sunny days in December. The snowfalls are usually moderate. December as well as March, April, October, and November is considered the most uncomfortable time to visit Thornhill.