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Physics of SweatingWhy Evaporating Sweat Cools our Bodies in Hot Weather
According to the principles of thermal physics evaporation is a cooling process. Hence sweating is an important mechanism for keeping us cool in hot weather.
Sweating is uncomfortable, makes us sticky, and makes us smell funky, but it cools the body in hot weather. Let's look at the physics of why we sweat. Evaporation as a Cooling ProcessWhen we boil water to turn it into steam, we must add heat to bring the water to the boiling point. We must then add more heat to evaporate the water. Even though the temperature does not go up, heat energy is required to turn liquid water into water vapor. In general when water evaporates it requires heat energy. The amount of heat energy required is called the latent heat of vaporization. If the water is not sitting on a stove that supplies the energy, the energy must come from someplace else. When water that is not boiling on a stove evaporates, it gets the needed heat energy from the nearby environment. Hence evaporation is a cooling process that cools the nearby environment. Sweating and EvaporationWhen we sweat, our skin and clothing become covered with water. If the atmospheric humidity is low, this water evaporates easily. The heat energy needed to evaporate the water comes from our bodies. So this evaporation cools our bodies, which have too much heat. For the same reason splashing water on ourselves when it is hot feels good. Being wet during cold weather, however can excessively chill us because of this same evaporation effect. Sweating is therefore the human body's primary cooling mechanism. Because this mechanism uses water, we need to replace lost fluids by drinking more fluids in hot weather. This is especially true after exercising or working in hot weather. Humidity Breezes and Perceived HeatWhen it is very humid, our sweat does not evaporate as easily. With the body's primary cooling process not working efficiently, we feel hotter. That is why a hot humid day is more uncomfortable than a hot dry day. As the air near our bodies absorbs evaporating sweat, the humidity very close to our skin increases, so our sweat does not evaporate as easily. If there is a breeze or wind, the air near our bodies is replaced by dryer air that can continue to absorb evaporating sweat. Hence a breeze on a hot day has a pleasant cooling effect. On a cold day it produces a wind chill effect making it seem much colder than it actually is. Despite the fact that sweating can make us feel unpleasantly sticky, the principles of thermal physics make sweating a very important mechanism for cooling the body in hot weather. Further ReadingBloomfield, Louis A., How Things Work, 3rd ed., Wiley, 2006.
The copyright of the article Physics of Sweating in Physics is owned by Paul A. Heckert. Permission to republish Physics of Sweating in print or online must be granted by the author in writing.
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