Evaporative cooling is one of physics's great magic tricks. Basically, when we talk about a substance's temperature, that temperature is the average temperatur

Evaporative cooling is one of physics's great magic tricks. Basically, when we talk about a substance's temperature, that temperature is the average temperature of the system. Temperature is just "how quickly are the atoms/molecules here jiggling?" When the atoms/molecules collide, some of them speed up, some slow down. That means that some atoms are "hotter" than the average temperature, and some are cooler. Liquids have a certain energy above which a molecule is ejected from the puddle. This is evaporation. Even if the average temperature (read: energy) is below that threshold energy, some molecules are moving faster, because they have collided with other molecules in just the right way. Those fast molecules get ejected. Evaporation! But now that the puddle lost the fast (read: hot, read: high energy) molecules, the average energy of the system is lower. The whole thing cools down! In dry environments, it's possible for sweat to evaporte (cool) at a rate that is faster than the rate at which your body heats up from hot colliding air. In humid environments two things happen: evaporation slows down, and you heat up faster. So there is a combination of temperature & humidity at which, if either of those parameters is exceeded, you gain heat faster than you lose it.