Winter Tire Rubber Is Made to Stay Soft in the Cold
Rubber changes with temperature. Tire makers describe winter tires as using special rubber mixtures that are softer and more flexible, and that stay soft in freezing temperatures [1].
Why does softness matter? Soft rubber can mold itself to a cold road surface, while stiff rubber has a harder time doing that. That is a common way of picturing it. The manufacturer page we read states the result (better grip in low temperatures) but does not explain the rubber chemistry [1].
You may also see a rule of thumb that winter tires work best below about 7 °C (45 °F). We did not confirm that number in the official sources we read, so treat it as a rough idea from the tire trade, not a rule of nature.
Deep Grooves Pack Snow, and Snow Grips Snow
Look at the surface of a winter tire and you will see wide, deep grooves between blocks of rubber. In loose snow, the grooves fill up with snow. That helps because, as one manufacturer puts it, snow sticks to itself better than rubber sticks to ice [1]. The snow packed into the grooves helps the tire grab the snow on the road.
A simple way to picture it is a boot with deep lugs on the sole: the lugs fill and then dig in. The picture is only an analogy, but it fits the idea that the grooves are doing real work.
Tiny Cuts Called Sipes Give the Tread More Edges
Look closer at the rubber blocks and you will see many hair-thin slits. These are called sipes. Continental describes them as very thin slots cut into the tread block, adding traction on wet roads, snow and ice, and says winter treads use a high number of sipes together with aggressive grooves to bite into ice [1].
Each sipe adds extra edges to the block. On a slick road, edges matter more than a smooth surface.
What Winter Tires Cannot Promise on Ice
Winter tires are better on snow and ice, but how much better depends on the tire. In Canada, tires marked with a three-peak mountain and snowflake symbol have passed a snow traction test. Transport Canada says such tires must be tested on medium packed snow and score at least 110 compared with a reference tire, which it describes as 10% better [2].
Ice is a different story. Transport Canada reports that no minimum government standard for winter tire performance on ice exists in Canada, partly because it is hard to design a repeatable ice test, and partly because performance on ice can trade off against performance on snow and slush [2]. So the snowflake symbol tells you about snow, not about ice.
Details differ from country to country. Some regions require winter tires in winter and others do not, and the rules and tests mentioned here are the Canadian ones.
Ways to Explore Winter Tires Yourself
With a grown-up, ask the owner of a car parked in a safe place, such as a driveway or parking lot, if you can look at the side of a tire. Stay away from cars parked beside a road. Look for the three-peak mountain and snowflake symbol, and for letters such as M+S. Then compare the tread pattern of a winter tire and a summer tire if you can find both. Count how many thin cuts you can see in one block.
You can also try a small experiment with rubber bands: put one in the refrigerator for an hour and stretch it next to a room-temperature one. A rubber band is not tire rubber, so this only shows that rubber can feel different when it is cold. Do not touch tires on a road, and stay away from moving cars.
Another idea is to read two tire makers' websites and compare how they describe winter tires. Where do they agree, and where do their words differ?