‘Heat Index’ explained

Weather Blog

This weekend, a storm system passing to our north will pull Gulf moisture into our area. This additional moisture will increase humidity across Central Texas, making the unseasonably hot temps feel even more uncomfortable… as the afternoon highs in the 90s will feel more like low 100s. But why is that?

‘Heat Index’ – what is it?

As defined by the National Weather Service, heat index is “a measure of how hot it really feels when relative humidity is factored in with the actual air temperature”. Simply stated, the more humidity = the hotter it feels.

It is calculated by a very long and complicated equation:

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 10-6T2R2
where T = ambient dry bulb temperature (°F)
R = relative humidity

To avoid any grueling algebra, the NWS created a table to help easily figure out what the heat index would be given the air temperature and relative humidity:

Heat Index Table — National Weather Service

If you know the current air temperature and dew point or humidity, you can also use the NWS ‘Heat Index’ calculator, found here: HEAT INDEX CALCULATOR

Why does it feel hotter?

On a hot day, our bodies try to regulate temperature by the process of sweating. The cooling comes as sweat evaporates off the body. When there is more moisture (more humidity) in the air, it makes it harder for that sweat to evaporate. Therefore, our bodies cannot cool efficiently making it feel hotter than what the thermometer actually reads. That’s why heat indices are also referred to as ‘Feels Like’ temps.

It’s important to keep in mind that heat index values are what it feels like in the shade on a non-windy day. If exposed to full sunshine, the heat index could climb up to 15° higher.

Heat Index explanation and equation

Why is it important?

Heat is the number one weather-related killer in the U.S. In hot and humid weather, our body’s inner temperature can increase if not careful. Dehydration will only exacerbate the effects. Excessive heat exposure can result in heat cramps, exhaustion and stroke. Ultimately, if left uncontrolled, overexposure could result in death.

For another look at how ‘Heat Index’ works, check out Meteorologist David Yeomans’ First Warning Weather University lesson.

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