On a hot, summer day, it does not take long for perspiration beads to form on your forehead before dripping down your face.
Yes, sweating may seem dirty and yucky, but it’s actually a good thing!
What is happening is your body is attempting to cool itself by covering it in a layer of watery sweat. On a hot but relatively dry day, the sweat would evaporate off of your skin.
It turns out that when water changes phase from liquid to gas (which happens when it evaporates), the heat required to make the water change phase is absorbed by the sweat when it becomes gaseous water vapor — and the air near your skin cools. That air is also in contact with your body, and the cooled air also cools your skin. The result? Your body is able to protect itself from overheating — definitely a win!
But notice how I said “relatively dry day” in the explanation above. In order for the water to change phase from liquid sweat to gaseous water vapor, there needs to be “space” for the water to change phase. If the atmosphere already has a lot of water vapor present, then it is close to saturation — and not as much liquid water will be able to evaporate.
This is why on a hot, humid day, it seems like you are more sweaty: Your body is producing the same amount of sweat on a humid day compared to the dry day (even if the temperature for both days is the same); however, the water content in the sweat is able to evaporate, and cool your body on the dry day. On a humid day, the sweat can’t evaporate, and instead it accumulates into a hot, sweaty mess.
This is also why meteorologists talk about something called the heat index or issue alerts like the heat advisory.
The heat index takes into account both the actual temperature as well as the humidity. Given a particular temperature, the body is unable to cool itself the higher the humidity levels are at that temperature.
If your body is unable to stay cool, then heat illnesses will develop: muscle pains, cramps, and intense thirst are all symptoms of developing heat illnesses.
If left untreated, the symptoms could become worse and a person could succumb to heat exhaustion (heavy sweating, rapid breathing and a fast, weak pulse) or worse, heat stroke (marked by dry skin, dizziness, fatigue, and even unconsciousness as the body’s temperature rises above 104 degrees). Keep in mind that heat exhaustion and heat stroke are considered severe heat illnesses as well as medical emergencies. If someone is showing signs of heat exhaustion or heat stroke, call 911 immediately, and get the person to a cool, dry place as soon as possible.
Heat advisories are issued to let you know when the temperature as well as the humidity will reach levels high enough for a long enough period of time to produce a potentially dangerous situation, especially for those who spend a large amount of time working or living in hot and humid conditions. If that is the case, try to take frequent breaks in a cool spot (like an air conditioned room or under a shady tree) and drink plenty of water to replace fluids lost due to perspiration.
So the next time you wipe sweat from your brow, thank your body for perspiring: It’s a process that keeps you alive every summer.