Why are Wimbledon tennis balls kept at a temperature of 68°F (20°C)?

Every year, over 54,000 Slazenger tennis balls are prepared for Wimbledon. In 2019, the official count was 54,250 balls; enough to provide 48 cans of balls for the Centre and No.1 Courts, and 24 for each outside court, daily.

Wait - prepared? Yes, because the temperature of a tennis ball affects its performance. Inside each tennis ball is a hollow core; a round, rubber-coated space that contains gas. Change the temperature of the ball and you change the pressure of the gas that’s inside it, thereby changing the ball’s physical dynamics. A hot ball behaves very differently on the court from a cool one. Warmer temperatures make the ball’s interior gas molecules expand, creating a higher bounce. Conversely, the molecules shrink at lower temps, giving less bounce; tests have shown that 68°F (20°C) delivers the optimum bounce.

Because of the effect of temperature on ball performance, all 54,000+ balls used during Wimbledon are kept at exactly 68°F. They’re also switched for a replacement set after the first seven games to make sure they aren’t worn down. During a match, balls are then switched every nine games to prevent players from having to use scuffed or damaged balls. With these strict requirements in place, the number of balls required makes a lot more sense.

Wimbledon has grass courts, which in turn create one of the lowest bounces of any surface. Balls tend to bounce long and horizontally, requiring players to stay low as they play, and be prepared for faster returns.