Weather conditions you encounter when driving are a key factor in determining if you need new tires. Simply checking the tread and thinking it looks safe or is not down to the treadwear indicator is not enough to ensure safe driving. The reality is that weather and road conditions are critical factors because of the differing amounts of tread volume or "void space" required for adequate traction.Dry conditions: In warm or moderately dry conditions, your tires need very little void space to provide strong grip. Think of a racing slick and how they grip well without tread. However, an exception to this is how temperatures affect different tread compounds. Summer tires get too hard when temperatures drop below 40 degrees Fahrenheit. This means that in the winter, even when there is no snow or ice on the roads, summer tires do not provide strong enough traction regardless of tread depth.
Wet conditions: Wet road conditions are arguably the most dangerous because many drivers don't adjust their speed appropriately as they do in snow and on ice. Wet traction and hydroplaning resistance is proportionate to void space - the deeper the water, the more void space is needed to evacuate water. Watch "What Honest Abe Doesn't Tell You About Minimum Tread Depths" to see how tread depth affects your vehicle's stopping time.
Snow and ice: Winter weather conditions are the most demanding and require the deepest tread for optimal performance. Tires must combine three fundamental features to deliver good wintertime performance, including an appropriate tread design, pliable tread compound and sufficient tread depth. If any one of these fundamental characteristics is absent, the other two, regardless of their ability can't deliver the intended results.
The bottom line is that if you are evaluating your tires and measuring tread depth, it is important to consider weather conditions. And when it comes time to replace your tires, read "When Should I Replace My Tires?" to ensure your tires always have the appropriate amount of tread depth.