The hot tennis balls should bounce the greatest, followed by the room temperature balls, then the refrigerated balls, and the frozen tennis balls should bounce the least. The increased velocity of the air inside the ball due to its temperature affects its rebound quality. A hot ball will return faster because the air inside it is moving more quickly than that in a cooler ball. Here are four things that affect the bounce of a tennis ball: temperature, humidity, air pressure, and wind speed. If you change one thing about a ball but not the others, you'll get a different result. For example, if you freeze a ball, it will be harder and feel like a solid object rather than a rubber sphere when you hit it. But if you test it out on the court under normal playing conditions, it will still bounce quite a bit lower than a normally-aired ball. There are two ways to increase the bounce of a ball: ice it or vacuum it. Iceing reduces friction between the strings and the ball, which means less energy is required to strike it and more power can be applied when hitting it hard. This is why ice hockey players can hit the puck so hard; they don't need to worry about their hands getting hurt from all the collisions.