Temperatures this week are some of the hottest we have seen the entire year now as we get to the end of July. This is a perfect opportunity to explain what the Heat Index is and how it is all calculated, especially since you're going to hear us mention this term a lot the next couple days. You'll often hear the WDRB Weather Team mention this term during the hot summer months because it can be vital to health and any outdoor prolonged activities. 

So, what exactly is the 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 actually has to do with both the heat itself and the humidity together. The Heat Index is what the temperature feels like to the human body when relative humidity is combined with the air temperature.

The heat index is important because it has important considerations for your body's comfort. 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 first hand by rubbing some hand sanitizer on your hands, then waving your hands around, evaporating the liquid and making your hands cooler. 

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

How Is Heat Index Calculated?

Since we live in modern times now, it actually isn't too hard to determine the heat index of the air around you. Mainly because the National Weather Service has created this table to help illustrate how the Heat Index is calculated. 

Courtesy of NWS

So, to know what the heat index is, we need to know what the air temperature is, and what the relative humidity is. You won't hear us on the WDRB Weather Team talk about relative humidity percentages very often because it can become confusing. Instead, we use dewpoints, which is another measure of the moisture in the air. 

So, for example, if our air temperature is 90ºF, and the dewpoint is 65ºF, that means the relative humidity is at 44%, which makes the heat index 92ºF. This page here is an easy calculator to convert dewpoint to RH if you wanted to try it out. 

Let's try this for tomorrow. With a forecasted high of around 95ºF, and dewpoints rising to around 72ºF at some point, that puts our RH value at about 47.6%. If we look at the chart above, that means at one point tomorrow, our heat index would technically be around 106ºF. 

This graph below from the NWS can explain what temperature of heat index can do to your body, and when serious action would need to be taken.

Courtesy of NWS

For all of my math nerds 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's the equation from the NWS...

Heat Index = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783 x 10-3T2 - 5.481717 x 10-2R2 + 1.22874 x 10-3T2R + 8.5282 x 10-4TR2 - 1.99 x 10-6T2R2

T - air temperature (F)

R - relative humidity (percentage)