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How the heat index works

It has been hot here in Middle Tennessee, especially over the last week.

Since July 6, 10 out of the last 11 days have hit at least 90 degrees and the humidity doesn’t help either.

Middle Tennessee has consistently seen dewpoints in the low 70s, which is very muggy, tropical like air. Combine the two factors and it can feel much hotter than what the actual air temperature is.

It will be another hot week, in fact, the hottest of 2017 so far. Temperatures will range from the low 90s to mid 90s in a lot of locations.

Higher humidity levels will also be building back in as the week progresses and we are expecting Heat Index values as high as 105 degrees. So how is the Heat Index calculated and what does it mean for you?

The Heat Index is a calculation between actual air temperatures and relative humidity values. Relative humidity is a mathematical equation between the dewpoint and the air temperature, meaning that the closer the dewpoints are to the air temperature the higher the relative humidity.

The tricky thing about relative humidity is as the temperature changes so does the relative humidity, even if the dewpoint is the same. For example, if you have a dewpoint of 70 degrees and an air temperature of 75 degrees, the relative humidity would be about 85 percent. If you have the same dewpoint of 70 degrees but increase the temperature to 90 degrees, the relative humidity would only be 52 percent. Both of those days would feel very humid, which is why the relative humidity can be deceiving.

The Heat Index is another way of saying what the temperature outside actually feels like. The higher the relative humidity and the higher the air temperature, the higher the Heat Index values will be. For example, a relative humidity of 50 percent at 90 degrees will make it feel like 95 degrees. But a relative humidity of 50 percent at 96 degrees will make it feel like 108 degrees.

Below is a chart that calculated the Heat Index based on Relative Humidity and Temperature.

Image: National Weather Service


Normally the National Weather Service will issue alerts when Heat Index values exceed 105 to 110 degrees for at least two days. Advisories are not expected but extra caution may be needed.

For Middle Tennessee this week, the Heat Index will start in the low 90s and slowly build into the low 100s into Wednesday and Thursday. Heat Index values will most likely say in the low 100s right into the weekend.

With values this high, extra caution should be taken. Limit the amount of strenuous activity you do outside and stay hydrated. Check on the young and elderly, especially anyone who does not have air conditioning. If you work outside, take multiple breaks and also stay hydrated. Make sure to never leave your kids or pets in the car for any period of time.

Courtesy of WKRN’s News 2

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