It’s Not the Heat…It’s the Humidity!

Excessive Heat by Midweek in Central Indiana and much of the country. This heat index forecast for Wednesday courtesy of NOAA/HPC.

There’s no better time to explain that often spoken phrase than now.  With a week’s worth of “excessive heat” in the forecast, I thought you might like to know what goes into the heat index and how a “dry heat” as opposed to a “moist/humid heat” can make all the difference in the world as to how you feel when you step outside!

First – I have have to acknowledge the Heat Index Forecast from the Hydrometeorological Prediction Center (say that 5 times fast!)  Central Indiana is poised for midweek heat indices of 105-110!  It will feel even hotter to our west, though, where portions of the Plains and Upper Midwest will feel like 110-115!

The heat index is derived from a long and drawn-out math equation that combines temperature plus humidity to yield “apparent” temperature, or how your body feels as a result of the heat and humidity.  If you’re bored and would like to compute it for yourself…here you go!

Formula for finding heat index


HI = Heat Index
T = Temperature (° F)
RH = Relative Humidity (%)

What?  Not interested in punching all those numbers into your calculator?  OK – here’s the “easy” way to compute the heat index courtesy of the National Weather Service at this link:

In a nutshell, the higher the temperature and the higher the Relative Humidity or Dew Point (both measure how moist the air is), the higher the heat index.

For example: Let’s say it’s 90 degrees with a dew point of 65.  The heat index = 94.  Look at the difference as dew points get higher:

Temp: 90 + Dew Point: 70 = Heat Index 97
Temp: 90 + Dew Point: 75 = Heat Index 101
Temp: 90 + Dew Point: 80 = Heat Index 107

Anytime the Heat Index exceeds 105, that’s considered the danger zone…and that’s what prompts an excessive heat watch or warning.

I’ve been watching all my friends in Minnesota (that’s where I used to live and work before moving to WTHR) suffer through 80-degree dew points today.  And that air is heading here to Indiana in the next couple days.  So ready or not, prepare to sweat.  And if you need a refresher on hot weather safety, check out this link:

Stay cool and stay safe!
-Meteorologist Chikage Windler Follow Chikage on TwitterBecome a fan on Facebook