Playing conditions
Heat changes the ball and the air it flies through. On hot days serves come through quicker and there are more aces. Here is why, and how much it shows up in the matches.
Most balls on tour are pressurised. As the ball warms up, the gas inside pushes harder and the rubber gets springier, so it comes off the racket and the court faster and bounces higher. The ITF, which approves every tournament ball, notes that high temperatures make the ball bounce higher.
Warm air is also thinner than cool air, so the ball loses less speed on its way over the net, the same effect that makes the ball fly at altitude.
Against air at 20 °C, the temperature at which the ITF tests balls. A 35 °C day thins the air almost as much as Madrid's altitude does (about 5%).
On hotter days, servers hit more aces than at the same tournament on cooler days, and hold serve a little more often. Cool days go the other way.
| Hottest of the day | Aces | Holds (pts) |
|---|---|---|
| Under 20 °C | -4.2% | -0.4 |
| 20–24 °C | -1.6% | -0.1 |
| 25–29 °C | +1.2% | +0.2 |
| 30 °C and up | +4.5% | +0.4 |
Each match is compared with the same tournament on its other days, so the court, the balls and most of the field stay the same. Outdoor ATP matches with serve statistics since 2019, venues below 550 m. Holds: the change in service games held, in percentage points.
From 2026 the ATP measures heat stress with the wet bulb globe temperature (WBGT), which combines temperature, humidity, sun and wind. When it reaches 30.1 °C during the first two sets of a best-of-three match, either player can ask for a 10-minute cooling break after the second set. Above 32.2 °C, play is suspended. The WTA has had a heat rule for decades.
Why humidity makes heat harder to play inDeepCourt's court speed reading measures how fast each tournament actually played, conditions included, so a hot week shows up in it.
How court speed is worked out