Why are tennis balls changed so frequently?
Modern tennis balls consist of an inner rubber core, filled with pressurised air, and a outer cover, made of wool or nylon, which is bound to the core by means of a heated press. According to the International Tennis Federation (ITF), when a tennis ball is dropped, vertically, onto a smooth, granite block from a height of 100”, or 8’4”, it should rebound to a height between 53” and 58”.
However, after a period of serious play – typically about three hours – the cover of a tennis ball may become fluffed up, so that the ball does not fly through the air as fast as a new one, and the ball may lose internal pressure, so that it does not bounce as high. Obviously, both these factors can adversely affect the control and accuracy of a tennis player, so tennis balls are routinely changed after every seven and nine games, alternately, throughout the course of a tennis match. The first ball change takes place after just seven games to allow for the warm-up. High humidity increases the moisture content of a tennis ball, and hence its mass, and high temperature can cause the ball to bounce higher, so ‘new’ balls are stored in a courtside refrigerator, maintained at a constant 68°F, or 20°C, to keep them in optimal condition.