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It’s Hot, But the Heat Index Means It Feels Even Hotter

Are you just dying for this heat wave to end? Do you find yourself reluctant to go outside because the temperature is over 95 degrees Fahrenheit? You might think that’s hot—but it will feel even hotter. Let me be clear: There are two very different definitions of “hot.” You could use “hot” to refer to the air temperature or you could use “hot” to talk about how you feel. And that is the problem: We’re humans, not blocks of aluminum. Our well being and our comfort level depend on more than just the temperature of the surrounding air.

It’s More Than Just Temperature

It happens to me quite often. I’ll be outside at some soccer match and then I’ll make a comment like this:

“Sure, it’s pretty hot—but it’s not too bad here in the shade, especially with that breeze blowing.”

The human body has two things that make it different than a brick. First, humans have stuff going on in the inside of their bodies that can increase their temperature (you know, like exercise). Second, humans have a method for decreasing their temperature—such as with sweat (or perspiration if you prefer).

But let’s assume that you’re a stationary human, not exercising. Your body temperature is probably around 98°F (37°C) and this is likely higher than the air temperature. In this case, there are several things that can change the way you feel.

  • Sunlight vs. shade. If you’re in direct sunlight, this will increase your temperature—it doesn’t even matter if you’re a human or a block of aluminum. There is energy in sunlight—I guess that’s obvious.
  • Relative humidity. Humans sweat. Some humans sweat more than others. Either way, the sweat is liquid water and it takes energy for this liquid to make a phase transition to a gas. This energy comes from the human body, which helps to decrease its temperature. However, if the relative humidity is high then the sweat-water doesn’t evaporate you just end up with wet clothes.
  • Wind. A slight breeze won’t change the temperature of inanimate objects, but it will decrease the temperature of wet objects. With a little bit of wind you can get a higher rate of sweat evaporation for a human—this is how a fan works.

Put these three factors together and you could measure the “feel” temperature. Even if you could do that, this feel-temp would just be an estimate. Every human interprets sensations differently. What might feel hot to you might feel cool to me.

The Heat Index

When you check the weather in the morning, many sites will report both the temperature and the heat index. The heat index is a way to get a measure of the “feel temperature.” It attempts to compensate for the effects of humidity in the air by calculating an equivalent temperature. The heat index takes the current air temperature and the relative humidity and returns the temperature it would feel ike if you were at a relative humidity of 20 percent.

OK, how about a quick example. Suppose it is 94°F with a relative humidity of 75 percent. This is approximately what it would feel like with a temperature of 124°F and a relative humidity of 20 percent. Yes, at that high of a temperature, it wouldn’t feel quite so bad at such a low humidity since your sweat would actually do its job instead of just getting you wet.

But what is the relative humidity? It is a measure of the amount of water vapor (water in gas form) in the air. The actual value is the ratio of the current partial pressure of water vapor to the maximum partial pressure of water vapor in the air. What happens at the maximum partial pressure of water vapor? At this point, there is enough water vapor in the air that the molecules can interact with each other and condense into liquid water. You’ve probably seen this before: It’s what we call fog, and it would probably be 100 percent relative humidity (although fog can also occur at lower humidity).

Just for fun, here is a device you can use to measure relative humidity.

It’s basically a box with two thermometers. One of the thermometers has a cloth “sock” on it that you can wet. Then there is a fan that blows air over the two thermometers. Of course the fan will have no effect on the dry thermometer, but the wet one will decrease in temperature. The lower the humidity, the greater the decrease in temperature of the wet bulb. Once you have these “wet” and “dry” temperatures, you can estimate the relative humidity.

OK, back to the heat index. There is actually a formula that takes both the air temperature and the relative humidity and returns the heat index. You can find that equation on this Wikipedia page—but it’s not really fun to look at. Instead, let’s make a plot.

There are two input variables, so the best thing would be to make a 3D surface plot. However, I’m going to plot the heat index vs. relative humidity for different air temperatures since that’s easier to understand. Here is what I have.

Really, you could imagine all sorts of ways to calculate the relative humidity with a program. For instance, you could make super simple calculator like this:

That’s fun, right?

OK, that’s it for the heat index. Remember that this only takes into account the “feel” temperature based on the relative humidity and air temperature. It assumes that you are in the shade with little or no wind. If you have to go outside, try to stay in the shade. At least in the shade it will feel like the heat index. In the sun, it will feel super crazy hot.

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