Because higher temperatures result in higher pressure, a tennis ball bounces higher when it is hot. When the gas molecules expand in the tennis ball, it causes more energy and a higher bounce.
When the temperatures decreases, it has the opposite effect on the tennis ball's bounce. The gas molecules contract and move around much more slowly. In the winter season, tennis players often string their rackets 2 pounds lower to compensate for the change in pressure in the tennis ball. A fully inflated tennis ball often appears deflated or softer if there is a sudden change in temperature outside.