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The Heat Index Explained

by Steve Templeton

KMOV.com

Posted on June 22, 2009 at 10:06 PM

Updated Wednesday, Nov 4 at 3:29 PM


Heat Index = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR -
6.83783x10-3T2- 5.481717x10-2R2 + 1.22874x10-3T2R + 8.5282x10-
4TR2 - 1.99x10-6T2R2

where T = ambient dry bulb temperature (°F)
R = relative humidity (integer percentage).

Well that explains it!
Uhh....not really.

Actually, that is a version of the equation behind our heat index calculations. While that looks complex, we only have to input our humidity and temperature forecast and a computer does the rest.

So, the heat index is a math equation?

Sort of, yeah. The goal is to quantify what the average human body feels like in excessive heat and humidity. You see, humans cool down by sweating and then the sweat evaporates into the air. This evaporation is a cooling process, and if enough beads of sweat are constantly evaporating off your skin, it helps cool you down.

However, when it's excessively humid the sweat doesn't evaporate as efficiently and thus you don't cool as efficiently. This is why we overheat and why we can be drenched in sweat while doing anything physical outside on these hot and humid days.

So, the heat index is more of a man made number taking into account heat and humidity. It's a great indicator of dangerous levels that mean we can overheat quickly. Most people will want to take caution when the heat index is over 100 degrees and dangerous levels for the average person are from 105 degrees and higher.

Keep in mind the heat index is calculated based on shady conditions. It can be 10 to 15 degrees hotter if you're in the full sunshine!

I hope this helps explain the term we hear so much during our St. Louis Summer and especially all of this week and weekend.

Steve Templeton


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