# Heat Index

They say that if you don’t like the weather in Nebraska, just wait five minutes. Of course they say the same thing for every place in the United States, except maybe San Diego and Miami.

According to weatherimages.org you can calculate the heat index if you know the dry air temperature and the relative humidity. All you need is this simple formula:

=ROUND(16.923+((1.85212*(10^-1))*A2)+(5.37941*B2)-((1.00254*(10^-1))*A2*B2)+
(9.41695*(10^-3)*(A2^2))+(7.28898*(10^-3)*(B2^2))+(3.45372*(10^-4)*(A2^2)*B2)-
(8.14971*(10^-4)*A2*(B2^2))+(1.02102*(10^-5)*(A2^2)*(B2^2))-(3.8646*(10^-5)*(A2^3))+
(2.91583*(10^-5)*(B2^3))+(1.42721*(10^-6)*(A2^3)*B2)+(1.97483*(10^-7)*A2*(B2^3))-
(2.18429*(10^-8)*(A2^3)*(B2^2))+(8.43296*(10^-10)*(A2^2)*(B2^3))-(4.81975*(10^-11)*(A2^3)*(B2^3)),0)

where A2 is the temperature and B2 is the relative humidity. Here’s Omaha’s calculation for yesterday and today: People have been complaining about the excessive heat for a week around here. Not me. It won’t be long until I’m shoveling my driveway, so I’m counting my blessings.

(Thanks, DA, for the link to the formula.)

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## 8 thoughts on “Heat Index”

1. Dave says:

There must be some lower limit for where this formula becomes irrelevant. For example, I opened a new workbook and pasted the formula into C2. With everything else blank, that meant Temperature=0 and Humidity=0. It gave me a heat index of 17! And if I set Humidity to 50, I got 308…

2. Rich says:

Thanks, Dick. I showed this to a co-worker:

Here response was, “Heat Index? Am I sweating????”

LOL

3. Scott says:

Thanks for the formula, now I can actually give people an indication of how hot it really can get in South Dakota! I’m sick of people saying it’s all “dry heat!”

4. m.evans says:

Does anyone know FOR SURE if this formula doesn’t work over a certain value? I have been trying various combinations and am getting the feeling it doesn’t work unless temperature is over 80 deg. F…

5. m.evans says:

*under a certain value* oops

6. Tushar Mehta says:

@m.evans: Searched the ‘net? I looked up ‘heat index calculator’ (w/o the quotes). One of the links was to a National Weather Service page at http://www.hpc.ncep.noaa.gov/html/heatindex.shtml

Search the page source and you will see the JavaScript code to calculate the heat index for any temperature and relative humidity.

I don’t know how valid those results are because a temp of 120F and relative humidity of 90% returns a heat index of 336F!

And, of course, there’s Wikipedia: http://en.wikipedia.org/wiki/Heat_index

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