The first week of the French Open has yielded as many major upsets (two) as people collapsing on the court due to sweltering conditions.

While temperatures in Europe are extremely hot for this time of year, they aren’t Sahara-in-the-middle-of-summer hot. And yet, a ball girl nearly fainted during a first round match on Monday while men’s player Jakub Mensik keeled over on Roland Garros’ red clay on Thursday after a five-set match, saying his body "just turned off.” Players have spent time between games sitting in the shade and putting bags of ice on their heads and shoulders, all in an attempt to beat the oppressive heat.

To truly understand why people on the court are struggling amid the heat, you need to look at an obscure measure known as the wet bulb globe temperature.…