You're a smart, savvy motorist who made the decision years ago to run dedicated winter / snow tires for your vehicle. You have been enjoying the superior performance in snow and on ice, while other cars go sliding through intersections and get swallowed by the looming Ditch Monster.
What you may not be aware of however, is the amount of tread depth needed to provide adequate snow and ice traction. Even the best performing winter / snow tire needs proper tread depth to provide good traction in winter driving conditions. It's important to inspect your winter tires each fall to ensure they are ready to handle the months of upcoming cold, snow and ice.
In order to provide snow traction, a tire needs more tread depth than it would for rainy or dry conditions. If the tire doesn't have enough volume of space to retain snow, the tire slips and slides across the snowy surface. When your tires have snow retained into the tread, snow-to-snow contact creates traction. See in the picture below how even the half-worn tire in the middle doesn't have the volume needed for good snow traction.
Sheer ice and hard-packed snow traction require two properties to maintain adequate traction, soft tread compound and tread sipes. These sipes open up to provide hundreds of biting surfaces that grip into the ice and packed snow, while the soft compound allows these sipes to open in extreme cold temperatures. This is why an all-season tire with sipes will not perform as well in freezing weather, the tread hardens and the sipes are less effective. See the photo of the the last tire on the right (below) to see what the sipes look like when they're almost worn down completely.
To gain a better understanding of the importance of your tire's tread depth this winter, read "Tread Depth - Why Too Little is Never Enough."