How is the heat index figured out ?

Question

In weather reports you hear the temperature is 90 degrees and the heat index is 100 .

Answer 1

The heat index attempts to combine the air temperature and relative humidity in order to come up with a measure that indicates how hot it feels.  The calculation will differ depending on the temperature scale used.

From the US NOAA site, the calculation of Heat Index (HI) for the Farenheit scale is:  

HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783x10 -3 T 2

- 5.481717x10 -2 R 2 + 1.22874x10 -3 T 2R + 8.5282x10 -4 TR 2 - 1.99x10 -6 T 2 R 2

where

T = ambient dry bulb temperature degrees Fahrenheit

R = relative humidity