When people in the sport talk about “heavy” playing conditions, humidity is the atmospheric variable they mention most often. It comes up during rainy weather like at this year’s Roland Garros, in the sauna-like conditions that often prevail on the US east coast during the hardcourt season, and has even been blamed for slowing down play when the roof closes at Wimbledon. Conversely, one popular justification for the widespread belief that balls tend to fly further than normal at Indian Wells is the dry desert air.
There are two problems with such theories. Most importantly, they’re backwards. Contrary to common intuition, humid air is less dense than dry air, and therefore creates less drag on a ball. This is a consequence of the ideal gas law, which implies that at a given pressure and temperature, a given volume of any gas contains the same number of molecules. Dry air consists almost entirely of nitrogen molecules (N2, molecular weight 28) and oxygen molecules (O2, molecular weight 32). Diluting it with molecules of water vapor (H2O, molecular weight 18) makes it lighter.
The other problem with blaming humidity for slowing down or speeding up play is that, in the conditions under which most tennis matches are played, the effect of humidity on air density is very small. At 70°F (21°C), a change in the humidity from 0% to 100% decreases the air density by a mere 1%, which increases the speed at which a fast groundstroke arrives at the opposite baseline by less than 0.3 mph (0.5 km/h).
At very high temperatures, the effect is larger because hot air at 100% humidity holds more water than cooler air at 100% humidity. But heat combined with high humidity is so hard on the human body that WTA rules, at least, require suspension of play under such conditions (see below).
The reason humid air feels heavy to our senses isn’t because of its density or viscosity—it’s because the more water is already in the air, the less sweat evaporates from our skin. That forces our bodies to sweat harder and do more work to keep cool, leaving less energy available for athletic performance. Taken to the extreme, it causes dehydration and excess body temperature, which can cause the body’s mechanisms to malfunction altogether.
Humidity does have some effect on the surface characteristics and mass of the ball, as well as the air density, but these effects turn out to be very small and largely cancel each other out. More on this later.
Cool temperatures and showers during the first week of Roland Garros this year have had a mass of pundits and players talking about the “heavy” playing conditions. Some said those condi…
www.google.com
