Temperature can affect the bounce of a squash ball because it can change the pressure inside the ball and the resilience or flexibility of the rubber, according to the Department of Physics at the University of Illinois.
Air pressure inside a squash ball increases with higher temperature as the air expands within a confined space. As the ball bounces, the pressurised air is compressed, but springs back readily with little loss of energy, making the ball bounce higher in warmer temperatures.
Resilience of Rubber
Rubber stiffens when it cools. When it warms up, it becomes more elastic and resilient, meaning that it springs back more readily on impact. The greater resilience of the rubber also contributes to a higher bounce in warm temperatures.
A squash ball is made of rubber that is not particularly resilient at room temperature, so squash balls need warming before play. Squash balls can be made faster or bouncier by increasing their content of elastic polymers, according to The Squash Player.