Originally Posted By: Johnny2Bad
......The reason is the winter-specific tread depth is less than the measured tread depth; the cold weather sticky portion is an outer layer.
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This is incorrect for most tires, although there are a couple of exceptions. The main reason why a snow tire loses it's effectiveness as it wears is loss of tread block flexibility and also tread depth. In the tire industry, 6/32nds is a widely accepted value for "acceptable winter performance," although I don't think it's based on any recent scientific data and not all snow tires are created equal. Also note that tread flexibility is lost with the aging of the tread compound over time and heat cycles.
