Why Do You Sweat?

Evaporative Cooling, Summer Heat and the Heat Index

Heat Index Chart
Courtesy of NOAA

But it's a dry heat!

Most have heard this statement about summer heat at some point. But what does it mean? The Heat Index is another name for apparent temperature. By definition, the Heat Index is the relationship between temperature and humidity which can produce detrimental effects on the body. When high temperatures are combined with high humidity, look out! It feels VERY warm!

Why do you sweat?
Most people know sweating is a process your body uses to cool down.

You body is always trying to maintain an even body temperature. Sweating reduces body heat through a process known as evaporative cooling. Just like getting out of a pool in the summer time, a small wind will be enough movement across your wet skin to create cooling.

Try This Simple Experiment

  1. Lick the back of your hand.
  2. Blow gently across your hand. You should already feel a cooling sensation.
  3. Now, blot your hand dry and use the opposite hand to feel the actual temperature of your skin. It will actually be cooler to the touch!

During the summer, humidity in certain areas of the world is very high. Some people even refer to the weather as 'muggy' weather. High relative humidity means the air is holding a lot of water. But there is a limit to the amount of water air can hold. Think of it this way...If you have a glass of water and a pitcher, no matter how much water is in the pitcher, you simply cannot make a glass "hold" more water.

Just to be fair, the idea of air "holding" water can be seen as a common misconception unless you look at the full story on how water vapor and air interact. There is a wonderful explanation of the common misconception with relative humidity from Georgia State University.
Relative Humidity is a "Glass Half Full".
Because the air can only "hold" so much water (which increases with rising temperatures), we report relative humidity in a percentage value.
A glass half full of water is compared to a relative humidity of 50%. That same glass filled within an inch of the top rim may have a relative humidity of 90%. Learn to calculate Heat Index in this simple activity.

Going back to the idea of evaporative cooling, if there is no where for water to evaporate to, then it stays on your skin surface. In other words, when the relative humidity is very high, there is only a little room in that glass for more water.

If the Heat Index is High in Your Area...
When you sweat, the only way you cool down is through evaporation of water from your skin. But if the air is holding too much water already, the sweat stays on your skin and you get little to no relief from the heat.

A high Heat Index value shows a small chance of evaporative cooling from the skin. You even feel like it is hotter outside because you can't rid your skin of the excess water. In many areas of the world, that sticky, humid feeling is nothing more than...

Your Body Says: Wow, my sweating mechanism is not cooling my body very well because the high temperatures and high relative humidity combine to create less than ideal conditions for the evaporative cooling effects of water from surfaces.
You and I say: Wow, it is hot and sticky today. I better get in the shade!
Either way you look at it, the Heat Index is designed to keep you safe in the summertime. Keep on alert for all signs of summer heat illnesses and know the danger zones!