Heat Index

The heat index--a reflection of what the weather feels like--is a temperature that is a mixture of actual temperature, mixed with what the humidity is to arrive at a more accurate number. The formula is as follows:
Here is a formula for approximating the heat index in degrees Fahrenheit, to within ±1.3 °F. It is useful only when the temperature is at least 80 °F and the relative humidity is at least 40%.
HI = c_1 + c_2 T + c_3 R + c_4 T R + c_5 T^2 + c_6 R^2 + c_7 T^2R + c_8 T R^2 + c_9 T^2 R^2
where
HI = heat index (in degrees Fahrenheit)
T = ambient dry-bulb temperature (in degrees Fahrenheit)
R = relative humidity (in percent)
c_1 = -42.379
c_2 = 2.04901523
c_3 = 10.14333127
c_4 = -0.22475541
c_5 = -6.83783 x 10^{-3}
c_6 = -5.481717 x 10^{-2}
c_7 = 1.22874 x 10^{-3}
c_8 = 8.5282 x 10^{-4}
c_9 = -1.99 x 10^{-6}
You can see why the table fell in popular use...
After one day where the outside temperature was 105 and the humidity was 50%, I began to really get what this heat index is all about. It was impossible to walk outside in any sort of sunlight by mid-morning without sweating profusely, even for a few minutes.
I did not remember the heat index as a factor in my childhood--which is a true memory, because it was described in 1978 (which falls clearly outside of what might be realistically classified as 'childhood' for me)--but I also grew up in Pasadena, where it might reach 105 on occasion but it was a very dry heat, feeling pretty much like 105 when it occurred. Now that I have logged 20+ years in the Midwest, I have an entirely different view of what is tolerable, temperature-wise, and the recent weather has been well outside what I find acceptable.