PART 2: WEATHER CONDITIONS
Now that we have established the physical premises, let’s get to the court. Let’s address the meteorology of tennis, starting from the three main features of the air: temperature, humidity, and pressure, which contribute to varying ρ (air density).
Humidity, Temperature and Pressure
It’s a pretty widespread notion that humid air is heavier to breathe, and that it makes it harder to hit a hard forehand as well. This assumption is actually wrong: humid air, which has more water vapour molecules (H20, molecular weight 18), is lighter than dry air, which is made up almost entirely of nitrogen (N2, pm 28) and oxygen (02, pm 32) molecules. Therefore, it is easier for the ball to plough through humid air.
The effect of humidity on ball speed is not only the opposite of what everyone thinks, but it’s negligible as well. Considering the weather conditions under which tennis is usually played, an increase in humidity from 0 to 100% on a day when it’s 21°C would produce a decrease in air density of only 1% and increase the speed of a forehand by less than half a km/h. The effect would become a little more appreciable at higher temperatures, but these are conditions in which tennis isn’t usually played, as matches played in such climates are usually stopped due to the heat rule.
Furthermore, humidity also has a very slight effect on the diameter of the ball (it reduces it, but in a barely measurable way, and it’s therefore irrelevant to our evaluation) and on its weight, which increases, since moist balls absorb some water molecules. It cannot, however, substantially modify the equations we have explained above. On the other hand, temperature affects atmospheric density much more. If it goes from 10 to 38°C, the density of the air decreases by about 10%, and hits can gain up to 3-5 km/h in speed. Even atmospheric pressure – as it decreases – can lower the density by a lot, but on our planet the pressure is almost unvaried, so tennis does not reap the benefits of this principle (perhaps when tennis will move to Mars we will have this discussion again).
CONCLUSIONS (AND ‘HEAVY’ CONDITIONS)
To sum it up: when it’s hot, the ball travels faster and the difference in play is noticeable, when it’s humid the ball becomes (a little) faster but the difference is almost imperceptible. On the other hand, in cold conditions players experience a certain difficulty in generating speed with the ball that is perfectly in line with what has been illustrated, a difficulty that is further exacerbated it drizzles a little, as it happened at the 2020 French Open, which was played in very harsh autumnal conditions. If a few drops come down from the sky and the air is humid, the ball can actually get heavier. However, if we apply the mass increase subjected to a light drizzle to the equations above, we don’t get an effect that is important enough to explain the ‘natural selection’ that a day of tennis played in heavy conditions can exert, a day in which only the fittest can find any solace in the consistency of their shot-making.
The hypothesis of Ed Salmon is that ‘heavy’ balls complicate players’ plans more in terms of fatigue and strain. If the percentage of extra force that needs to be exerted on the ball to generate the same speed is minimal on a single shot, it is no longer so over a set or an entire match. For this reason, players who are less athletically prepared fall prey to surmenage sooner than those who can hit for hours on end, even when fuzzy balls are as hard as stones and as big as watermelons.
ALTITUDE
Before calling it a day, let’s talk a little about the variable that above all seems to affect playing conditions, i.e. altitude. From the almost 600 meters above sea level in Madrid to the over 2500 meters in Bogotá, there some series of tournaments that are played in conditions where the air is a bit thinner and therefore less dense. As already explained, the expectation is that the ball will travel faster, and it does indeed happen – percentage-wise, the increase in speed is greater than the one attributable to temperature.
This is a computer simulation comparing the speed increase of a flat shot and a topspin shot (4000 rpm).
Let’s start with a flat forehand shot at about 130 km/h above sea level in Madrid and Bogotá respectively. The computer tells us that in Colombia the groundstroke is 6.5 km/h faster when it bounces (with respect to the same shot hit at sea level) and more than 11 km/h faster when it reaches the hapless opponent. The bouncing difference is minimal, while the air rarefaction also affects the length of the shot, which is more difficult to keep in the court (because the air exerts less friction).
Before analysing what happens to a topspin forehand played at 130 km/h, it must be underlined that much more power is required to generate the same speed and at the same time a lot of topspin, which is why only Nadal and a few other mesomorphic specimens can do it. Anyway, in this case the Colombian forehand that arrives at the opponent is almost 13 km/h faster, a very significant difference. The other side of the coin, though, is that that forehand, unhinged in the rarefied South American air, would end up three meters long. So, it’s easier to hit fast backhands in Bogota, but it is more difficult to keep them in play; so, it is necessary to calibrate shots a little more. One unquestionable fact remains: if you want to beat that friend of yours who drives you to exasperation by junkballing every single stroke back to you – causing endless amounts of unforced errors – you should think about taking him… to Bogotá, rather than waiting for the summer sun. Maybe you should go there a month earlier, though: first of all, to acclimatise – breathing at that altitude is harder – and then to tame your forehand!
Don’t miss the third and last instalment: as promised, we’ll talk (almost) only about clay.
Article by Alessandro Stella; translated by Alice Nagni; edited by Tommaso Villa
