Three Winter / Snow Tire Myths - Ben's Blog | Tire Rack
Myth 3: Bridgestone Blizzaks only give half the amount of winter service because the bottom half of the tread is a regular all-season compound.
Like many myths, this one is a distortion of reality. The first 55% of a Blizzak tire, like the WS70 for example, uses the winter Multicell compound. There are taller wear bars that'll indicate when the end of the compound is close. Below this is not an all-season compound, but a standard winter compound. Because the Multicell compound is more flexible, using a standard winter compound for the base makes the tread more stable.
A major point that is often overlooked is that any winter / snow tire, regardless of compound, loses much of its effectiveness when the tread is too shallow. It's not recommended to go through a winter with snow tires of any brand that start the season with less than 6/32" of remaining tread depth.
Blizzak tires will provide optimum traction until they reach the point where they or any other tire should be replaced. Below that depth they're still a winter tire, but the lack of tread depth will compromise effectiveness in deeper snow.
For additional information on this line of tires, read "Bridgestone Blizzak FAQs."