What is the heat index?

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Jean Rowe asked me, "What is with this 'feels-like temp?" When I was a kid I don't ever remember the term heat index. When the thermometer read 107 it was 107."
In the 1970s, predominately in the southeastern United States, doctors noticed there were a lot more people getting heat cramps, heat exhaustion and heat stroke compared to the rest of the country. Meteorologist George Winterling knew that humidity and heat are much more dangerous than just heat. In 1978, he came up with a formula showing the human-perceived equivalent temperature, or what it "feels like." The National Weather Service started using the heat index formula one year later. Here is what it looks like:
Heat Index = -42.379 + (2.04901523 x T) + (10.14333127 x R) – (0.22475541 x T x R) – (6.83783×10-3 x T2) – (5.481717×10-2 x R2) + (1.22874×10-3 x T2 x R) + (8.5282×10-4 x T x R2) – (1.99×10-6 x T2 x R2)
T = Temperature in °F
R = Relative Humidity in %

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