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Weather IQ: Heat Index Explained

In the southeast, it is humid during the summertime, which means most days have a heat index. Chris Mulcahy simplifies what is really a complex formula.

CHARLOTTE, N.C. — Heat and humidity are two words that are mentioned in every Charlotte forecast during the summer, but these two things also control the heat index, or how hot we feel. 

Our bodies start to feel hotter once it begins feeling sticky outside (which is when the dew point hits 60 degrees). 

The Heat Index

The Heat index derives from this simple formula:

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 

Or it can be more simply calculated by using charts based on temperature and humidity.

Credit: NWS

You can also calculate the heat index by using the dew point.

🌩️ If you like weather, watch Brad Panovich and the WCNC Charlotte Weather Team on their YouTube channel, Weather IQ. 🎥

Credit: NWS


Example: If it is 100° and the dew point is a sticky 60, it will feel like 101°. But a tropical dewpoint of 75 degrees will make it feel like 114°.

Here is a fact most people don’t know. Heat index charts are based on shady locations.  

If you are exposed to direct sunlight, the heat index value can be increased by up to 10 to 15° Fahrenheit.  

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Why We Feel Hotter?

As heat and humidity increase, so does the heat index. Or what our bodies think the temperature is. 

When your body gets too hot, you will sweat. That sweat will evaporate which is a cooling process.  

But warmer air holds more moisture which only makes it feel hotter. The high moisture content in the air slows our skin's evaporation. This causes our internal temperature to rise.  

This can lead to different levels of heat disorders due to prolonged exposure, including heat cramps, heat exhaustion and heat stroke. 

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So when is it considered dangerously hot? 

The simple answer is once heat indices reach 103-105°. Here are three ranges to know. 

  • When heat indices are 90 to 103° it is encouraged to use extreme caution. 
  • A heat index from 103-124 degrees is the danger level. Heat cramps or exhaustion are likely and in the worst-case scenario, heat stroke is possible. 
  • Anything 125+ is extremely dangerous and heat stroke is highly likely.  

Contact Chris Mulcahy at cmulcahy@wcnc.com and follow him on FacebookTwitterInstagram, and TikTok.

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