A squash ball will bounce higher as it heats up. In relation to the rising temperature of the ball itself, the air inside the ball essentially becomes increasingly pressurized. This results in the ball becoming more resilient, effectively firmer, which causes it to have more bounce when it impacts the racket, wall or floor.
As a squash ball is deformed upon impact with a surface, energy is expended. Initially, much of this energy is lost as dissipated heat. As the ball is repeatedly hit by a racket and bounced against the walls and floor of the squash court, the air inside the ball becomes warmer, the rubber of the ball itself becomes firmer and more heat is stored rather than lost. Squash Player explains that eventually this state of affairs will reach equilibrium, usually at around 45 degrees Celsius, and the heat stored by the ball will equal that which is lost during each impact. The time it takes to reach this equilibrium depends on the original resilience of the ball (squash balls are manufactured with varying levels of basic firmness to complement the experience level of the player), the temperature of the court and the ability of the players to keep the ball in play.