You probably hear it mentioned all the time in the summer. The heat index is a measurement used to tell you what it feels like outside when you take the air temperature and factor in the humidity. The higher the heat index value, the hotter it feels. It’s similar to the wind chill temperature in the winter.

Why does it feel hotter? Naturally, our bodies will cool down by sweating. Sweating cools us down by taking the heat from our body and using it to evaporate the sweat. By transferring your body heat to the sweat droplets, you feel cooler.

When there is a lot of humidity in the air, the evaporation process of sweat takes more time to complete. This keeps the sweat droplets on your body for a longer period of time and makes it feel hotter.

Calculating the heat index

Heat index chart from the National Weather Service (NWS)

You can calculate the heat index in 3 easy steps by using the chart above.

  1. On the top row, find the outside air temperature.
  2. Using the left most column, find the relative humidity.
  3. Move down from the air temperature and across from the relative humidty. The number you stop at is the current heat index temperature.

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