Meteorologist Kirstie Zontini looks into what the heat index is and why it is important.

Heat Index: What is it? Why does it matter?

In the winter we talk about wind chill and in the spring and summer, it is the heat index that is important, Storm Center 7 Meteorologist Kirstie Zontini said.

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Both are apparent or “feels like” temperatures. The heat index is a “feels like” temperature that looks at the actual air temperature and how much moisture is in the air, either with relative humidity or dew point. When it is hot, it can be uncomfortable. When it is hot and muggy, it is a different level for our bodies. 

The amount of moisture in the air affects how well our bodies can cool off. When you are hot, your body starts to sweat, that sweat is then evaporated from your body into the atmosphere. If the moisture content of the air is high, it is harder for that evaporation to take place, and harder then for your body to cool down. 

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Here is the actual equation for calculating heat index: 

HI = -42.379 + 2.04901523*T + 10.14333127*RH - .22475541*T*RH - .00683783*T*T - .05481717*RH*RH + .00122874*T*T*RH + .00085282*T*RH*RH - .00000199*T*T*RH*RH

As the air temperature and relative humidity increase, the heat index will as well. If it is 88 degrees with a relative humidity of 55%, the heat index would be 93 degrees. If it was 100 degrees with a relative humidity of 55%, the heat index would reach 124 degrees. This makes being outside when it is hot and humid more dangerous. 

  • 80-90 degrees: Use caution. Prolonged activity outside or exposure could be dangerous. 
  • 90-103 degrees: Use extreme caution. Lengthy exposure can cause heat cramps, heat stroke or heat exhaustion
  • 103-124 degrees: Danger! Heat cramps and heat exhaustion likely. Prolonged exposure can lead to heat stroke. 

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