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 x T) + (10.14333127 x R) - (0.22475541 x T x R) - (6.83783x10 ^-3 x T ² ) - (5.481717x10 ^-2 x R ² ) + (1.22874x10 ^-3 x T ² x R) + (8.5282x10 ^-4 x T x R ² ) - (1.99x10 ^-6 x T ² x R ² )

Reference:

http://www.crh.noaa.gov/jkl/-n=heat_index_calculator