What is a heat index? How is it calculated? | Weather Blog

We are now starting to see this transition from cooler weather to warmer and more humid conditions as we head into mid-May. Therefore, it won’t be long before we start talking about heat index or, as some of you may know, “like” temperature.

You’ll often hear the WDRB weather team mention the Heat Index during the hot summer months, as it can be vital to health and any extended outdoor activity.

So what exactly is heat index?

Most of you have probably said “it’s not the heat, it’s the humidity” at least once in your life when you step outside and immediately feel like you’re breathing with gills. This statement is partially valid, but really concerns both the heat itself and the humidity. heat index is what the temperature feels like to the human body when relative humidity is combined with air temperature.

The heat index is important because it has important considerations for the comfort of your body. Obviously, when the body gets too hot, it sweats to cool us down. Your sweat evaporates and cools you down because evaporation is technically a cooling process. You can see this firsthand by rubbing some hand sanitizer on your hands, then waving your hands around, evaporating the liquid, and cooling your hands.

So what does the heat index have to do with perspiration? Well, when the moisture content of the atmosphere (relative humidity) is high, it slows down the rate of evaporation to help cool our bodies.

How is the heat index calculated?

Since we live in modern times, it’s actually not too difficult to determine the heat index of the air around you. Mainly because the National Weather Service created this chart to help illustrate how the Heat Index is calculated.

Courtesy of NWS

So, to know what is the heat index, we need to know what is the air temperature and what is the relative humidity. You won’t hear us on the WDRB weather team talking about relative humidity percentages very often, because it can get confusing. Instead, we use dew points, which are another measure of air humidity.

So, for example, if our air temperature is 90ºF, and the dew point is 65ºF, this means the relative humidity is at 44%, which makes the heat index 92°F. This page here is a simple calculator to convert dew point to RH if you wanted to try it.

Let’s try this for today. With an expected high of around 88°F and dew points reaching around 67°F at some point, this puts our relative humidity value at around 50%. If we look at the graph above, that means that at some point today, our heat index would technically be around 91°F.

This chart below from the NWS can explain what heat index temperature can do to your body and when serious action should be taken.

heat index effects nws.PNG

Courtesy of NWS

For all my math geeks out there, there is technically an equation that can give you a pretty close approximation to the heat index, but technically has an error of +/- 1.3ºF.

Here is the NWS equation…

Heat Index = -42.379 + 2.04901523T + 10.14333127R – 0.22475541TR – 6.83783 x 10-3J2 – 5.481717 x 10-2R2 +1.22874×10-3J2R+8.5282×10-4TR2 – 1.99×10-6J2R2

T – air temperature (F)

R – relative humidity (percentage)

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