How does humidity impair the ability to thermoregulate?

Answer 1

Sweating is one of the main ways the body cools off. It works by taking advantage of the process of evaporation. When the skin sweats, it makes a large surface area slightly wet. Then the sweat evaporates, cooling the skin, which in turn cools the blood circulating in the blood vessels that supply the skin. This ultimately cools the body. But why does the evaporation of water on the skin cool the skin down? Excellent question. The reason it causes the skin to cool down is because the transition of water from its liquid state to its gas state (water vapor) is an "endothermic" reaction. That means that energy is sort of absorbed in that process. In this case, the "energy" is heat. So when all that water suddenly becomes vaporized, it cools down the surface of the skin. Which brings us to how humidity messes this up. The transition of water from liquid to gas is governed by a principle called the Law of Partial Pressures. This Law, simply stated, just says that if there is a lot of something in the air already, there is less room for more of it. Let me restate. On a humid day, there is a lot of water vapor dissolved in the air already. This makes it more difficult for more water vapor to get into the atmosphere. The atmosphere is already "crowded" with water vapor. On a very dry day, in contrast, there is tons of room in the dry air for water vapor to dissolve into. In the desert, where the air is very dry, you may lose a lot more water through evaporation of sweat than you would in New Orleans, where the air is thick with moisture. The partial pressure of water vapor in desert air is low, facilitating evaporation, and subsequently, cooling of the body through sweating. The partial pressure of water vapor in tropical air is high, making evaporation more difficult, and subsequently, decreasing the effectiveness of evaporation as a cooling mechanism for the body.