So, what is this “heat index”?

“The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature,” the NWS explains on Weather.gov.

You know how Phoenicians regularly remind people who don’t live here that “It’s a dry heat”?

Most of us will take a dry heat over heat and humidity any day because we feel warmer since our bodies cannot cool as efficiently.

[SPECIAL SECTION: Arizona's Extreme Heat]

It’s all about evaporation. And biology.

“When the body gets too hot, it begins to perspire or sweat to cool itself off,” the NWS explains in its breakdown of the heat index. “If the perspiration is not able to evaporate, the body cannot regulate its temperature. … When the atmospheric moisture content (i.e. relative humidity) is high, the rate of perspiration from the body decreases. In other words, the human body feels warmer in humid conditions. The opposite is true when the relative humidity decreases because the rate of perspiration increases.”

As an example, the NWS says a 100-degree day with a relative humidity of 55 percent will have a heat index of 124 degrees. Drop the relative humidity to 15 percent and that 100-degree day will feel like it’s 96.

There is another factor, of course. Are you in the shade or direct sunlight?

“If you are exposed to direct sunlight, the heat index value can be increased by up to 15°F,” according to the NWS.

That’s why the agency advises people working or play outdoors during an Excessive Heat Watch or an Excessive Heat Warning to take plenty of shade breaks. It’s better to spend some time in air conditioning if you can.

The formula for determining the heat index is complicated (see below), which is why the NWS has put together a chart and created a calculator that can use either dew point or relative humidity.

Super complicated formula for those who enjoy maths

It has an error margin of plus or minus 1.3 degrees F.

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 10-6T2R2T - air temperature (F)R - relative humidity (percentage)

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