From the blog "Walsherific Blogging!"

First, let us bow our heads and give brief thanks: "We thank thee, creator of roads and corners. Bringer of snow and handbrakes. You alone make driving fun." And now, lets talk about driving in the snow! Effectively, snow lowers the coefficient of static friction between the tires and the road surface. This means that for the same weight of car and contact patch with the road, The road can provide less "push" against the tires, so they're more likely to spin instead of roll. This