Normal tires that people use on the roads have grooves in them, known as the tread. These grooves allow the tire to channel water away from the tire as they ride down the road. As your tire gets older, the rubber compound wears down and the treads become lower. In wet conditions, your ability to stop decreases the lower your tread becomes. If the tread is too low, your car can hydroplane on the wet road. This happens when the tire isn't making enough contact with the road and is instead skimming across the water on the road.
NASCAR tires have a completely smooth tread. These tires are designed to have as much contact on the track as possible. During a normal race, the cars are driving at speeds around 200 mph, and handling the forces of cornering and the angle of the track. At these speeds and conditions, NASCAR drivers need the greatest amount of surface area on the tire in order for the car to stay on the track and handle properly.
NASCAR tires endure something called the slip angle during a race. When a NASCAR race car comes around the corner of a track, the tires will point in one direction, while the wheel goes in a slightly different direction [source: Leslie-Pelecky]. This causes the tires on the car to skid along the track slightly, allowing the driver to maximize his or her turning force. If a driver goes above the normal slip angle, about two to four degrees, he or she risks allowing the tires to skid too much on the track, which can lead to serious loss of control. Wet track conditions increase this risk.
Having said all that, NASCAR does have rain tires, but they're rarely used. Most track and weather conditions aren't conducive to using the tires and maintaining a safe and competitive race simultaneously. Let's go on to the next page and take a look at how rain directly affects track conditions.