Weather Blog: Understanding the heat index and its effect on a hot summer afternoon

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We talk a lot about heat index values, but other than saying “you take the temperatures and the dewpoints and combine them” it’s not often explained why on a hot and humid summer day it feels 5-7 degrees warmer than what the thermometer says.

Well sit down and get your notebooks out! It’s time for a little science lesson!

To actually calculate the heat index you need a very long complicated equation called the Rothfusz Regression… but the National Weather Service has simplified it into this chart-

Courtesy: NWS

The heat index measures how the air really feels when you take the dewpoint (or relative humidity) into account. The reason why 90° with a dewpoint of 50° feels a bit more manageable than 90° with a dewpoint of 70° is because the bodies main cooling mechanism is sweat. When we have low dewpoints that sweat evaporates off of your skin quickly allowing you to cool off, but when the air is already saturated with moisture (and dewpoints are high) the evaporation process takes a lot longer and you’re body temperature rises because the sweat and heat isn’t being wicked away.

For today’s forecast, and really through the rest of the week dewpoints remain high in the upper 60’s to low 70’s, and with temperatures in the 90’s each afternoon heat index values easily soar into the mid to upper 90’s even nearing 100°!

Staying hydrated and finding a spot to cool down will be SO important this week, especially if you’re spending a lot of time outdoors! Here are a few other safety tips to beat the heat!

Stay cool!

-Skytracker Meteorologist Haley Bouley

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