As the cold weather sets in, the most common thought from people is, "Are my tires okay for snow?". While that is a good question, you should also ask yourself if they are ready for the cold air. Cold temperatures affect tire traction because tires harden up in cold weather. Think, rocks for tires or sticky bubble gum. Both are extremes, but essentially, a happy place in between both is what tire manufacturers go for. A soft tire creates greater traction because it allow for greater friction between surfaces. The downside is, too soft a tire and too much friction means tires wear out fast. So, tire manufacturers have developed ways to help a tire last longer with more traction, but it is still limited.
Considering this information, when your tires are checked for sufficient tread depth, consider the age of the tire. If your truly concerned with safety on ice and snow, replace your tires if they are 5 years old or older. This will provide you with the safest combination of tread and traction.