Playing conditions
Players often say the ball feels heavy on a humid day. The physics pulls both ways, and in the matches humidity on its own barely moves the numbers. Where it really bites is on the players.
Humid air is actually slightly lighter than dry air, because water vapour weighs less than the air it replaces. On its own, that would let the ball fly a touch faster, but the difference is tiny.
The felt is the other side. It soaks up moisture, and the ITF notes that high humidity can make the ball heavier. A heavier, fluffier ball comes off the racket slower. In professional tennis the balls are changed every few games, which limits how much water they can take on.
Compared with the same tournament on drier days, humid days show no steady pattern: aces move by a percent or two either way, and holding serve doesn't change. The two effects largely cancel out.
| Humidity (daily average) | Aces | Holds (pts) |
|---|---|---|
| Under 45% | +1.8% | +0.1 |
| 45–59% | +1.7% | +0.1 |
| 60–74% | -1.2% | 0.0 |
| 75% and up | +0.6% | -0.1 |
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.
Your body cools down by sweating, and sweat only cools as it evaporates. In humid air it evaporates slowly, so players overheat sooner, grips get slippery and long matches turn into tests of endurance. That's why the ATP's heat rule doesn't use temperature alone: its measure, the wet bulb globe temperature, rises with humidity.
The ATP's heat ruleEvery ball approved by the ITF is tested after a day in a lab kept at 20 °C and 60% relative humidity, so that balls can be compared fairly. Tournament conditions are rarely that tidy.