The physics of how sweat cools us down is pretty interesting. I can’t remember the specific physics formula that calculates the cooling effect of evaporation, but as water molecules leave the skin, they take some heat energy with them, leaving the body slightly cooler. But how well this system works is also dependent on the weather and air conditions that the sweat would potentially evaporate away into.
In Gulp, Mary Roach describes some of the important factors for this process. “When the air around you is saturated with moisture, your sweat – most of it, anyway – has no where to evaporate to. It beads on your skin and beads down your face and back. More to the point, it doesn’t cool you.” The water in your sweat has to evaporate away for cooling to take place. When the air is too humid for the water to evaporate away into (an over simplification of the physics I’m sure) then you can’t take advantage of the cooling potential of sweat.
Roach continues, “It’s the humidity, but it’s also the heat. When the air is cooler than 92 degrees Fahrenheit, the body can cool itself by radiating heat into the cooler air. Over 92 – no go.” Hot air rises through convection, allowing cooler air to replace the hotter air, cooling you off as slightly cooler air replaces the hotter air around you. At a certain point however, there is no cooler air moving in to replace the hot air coming from your body. Roach also writes, “a breeze cools you by blowing away the penumbra of swampy air created by your body. If the air that moves in to take its place is cooler and drier, so, then, are you.”
Sweating is an incredible ability that helps keep us cool, but its efficiency is dependent on the weather outside our body. We can sweat all we want, but if the air that is around us isn’t cooler and drier than we are, we won’t enjoy the benefits of sweating. We won’t dry off in the air, we won’t cool down, and we will be gross and swampy.