Starting this time of year, our team receives one question fairly often: "When should I put my winter / snow tires on?" The basic guideline is to change your tires when the average temperature gets into the 40 degree Fahrenheit range.
Doing so will help maximize your snow tire's treadlife and ensure you're prepared for an early winter storm. A winter tire's tread depth is critical to its performance. Take a look at the pictures below, the new tire on the far left has max tread depth and you can see all the tread pattern features that make it grip the snow and ice. The middle tire is worn to about 6/32" and it is at the point where it'll start to lose its winter traction effectiveness. Finally, the tire on the far right is worn to 2/32" and will offer very poor winter performance.
If you use a summer tire, we would suggest you switch to your winter set even if winter conditions aren't forecasted because the compound on that type of tire loses much of its flexibility in the cold and could result in loss of vehicle control even if the roads are dry. To learn more about how the proper snow tire can help you this winter, read "Tread Depth - Why Too Little is Never Enough."