The heat index is calculated via looking up the "effective temperature" in a table of air temperature and humidity values, and then adding a bunch more degrees because it feels WAY hotter than that.

"Heat Index", by Randall Munroe, licensed under Creative Commons Attribution-NonCommercial 2.5 License

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The heat index is calculated via looking up the "effective temperature" in a table of air temperature and humidity values, and then adding a bunch more degrees because it feels WAY hotter than that.