On my way into work last week, I noticed the road crews had put salt down due to some early season ice. Here in South Bend, Indiana, we've also experienced some light flurries. However, I will not be putting on my winter tires until later in November because highs this week are expected to reach the low 60s. If I lived somewhere like Rapid City, South Dakota that has already shoveled out of one storm and is expecting another, I would be driving on snow tires.
Since tire tread depth is one of the key elements to a winter tire's performance, it is best to maximize their life by only using them when necessary. A winter tire will start to lose quite a bit of its traction once it's worn down to approximately 6/32" of remaining tread depth. Many winter / snow tires have a "snow platform" molded into their tread to show you when it's time to replace them. Also, as a tire wears, it loses its tread design features that provide its biting edges. For example, look at the Bridgestone Blizzak WS70 (photo above) at full tread depth and you will see the grooves and siping that give this tire its traction. Those will gradually disappear as the tire wears.
What makes a tire like the Blizzak so special during the winter months? It features dual tread compounds consisting of outer tread cap and underlying base compounds. When viewed through a microscope, the outer Blizzak Multicell compound resembles the appearance of Swiss cheese because it has millions of uniformly distributed microscopic pores that are constantly being exposed as the tread surface wears. In addition to providing thousands of miniature biting edges, the tire's pores help wick away the thin layer of water that often develops on top of snow packed and icy roads, allowing the biting edges to better adhere to the surface for more traction. The underlying base compound is a standard winter tread compound.
While there is no exact date for installing your winter / snow tires, factoring in the temperature and first snowfall are good indicators of when it's time. Don't own winter / snow tires? Shop by vehicle and find the set that's right for how and where you drive.