Heat index adjusts that temperature based upon relative humidity to a number that relates to "how hot it feels." As the human body cools through perspiration, high humidity prevents evaporation and results a higher heat index.

Also note that being in direct intense sunlight may increase the heat index by 10-15 degrees Fahrenheit.

Formula:

• T = Temperature in °F
• R = Relative Humidity in %
• Heat Index = -42.379 + (2.04901523 × T) + (10.14333127 × R) - (0.22475541 × T × R) - (6.83783×10 ^-3 × T ² ) - (5.481717×10 ^-2 × R ² ) + (1.22874×10 ^-3 × T ² × R) + (8.5282×10 ^-4 × T × R ² ) - (1.99×10 ^-6 × T ² × R ² )

Reference:

• National Weather Service - Weather Prediction Center. Heat Index Calculator. http://www.hpc.ncep.noaa.gov/html/heatindex.shtml. Updated April 19, 2013. Last accessed May 09, 2013.