Tire Tread-Depth is the thickness of the rubber remaining on your tire tread. Tire tread is measured in 32nds of an inch and most tires start out with 10/32nds or 5/16th of an inch of rubber to grip the road.
As you drive, the rubber wears down and becomes less effective at controlling your vehicle. Once your tire tread is below 1/8th of an inch of rubber, most industry experts suggest preparing to replace.
If you are down to 1/16th of an inch, your tires are considered worn out, and most states require immediate replacement. Driving in inclement weather like this could put you in a bad situation without much warning.