Heat Index

Not Reviewed
Equation / Last modified by KurtHeckman on 2019/07/19 12:53
`HI = `
rklarsen.Heat Index

The Heat Index calculator approximates the heat index based on  the temperature and relative humidity.

INSTRUCTIONS: Choose units and enter the following:

  • (Tf)  This is the ambient dry-bulb temperature
  • (RH)  Relative humidity.  This is a percentage with values between 0 and 100.

Heat Index (HI): The calculator returns the heat index in degree Fahrenheit (°F).  However, this can be automatically converted to compatible units via the pull-down menu.

The Math / Science

The Heat Index formula is the result of a multivariate fit (temperature equal to or greater than 80 °F and relative humidity equal to or greater than 40%) to a model of the human body. This equation reproduces comparable results to the widely published NOAA National Weather Service table for Heat Index.
The result is a Heat Index that reflects the effect of relative humidity and temperature on the human body.  In the shade, the following values of heat index are known to have the following effects on the human body:

  • 80-91 F : Caution: fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps.
  • 90-105 F: Extreme caution: heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke.
  • 105 - 130 F: Danger: heat cramps and heat exhaustion are likely; heat stroke is probable with continued activity.
  • Over 130 F: Extreme danger: heat stroke is imminent.

The formula for Heat Index is:

    `HI= -42.379 + 2.04901523*Tf + 10.14333127*RH - 0.22475541* Tf * RH -6.83783*10^(-3)*Tf^2 -5.481717*10^(-2)*RH^2 + 1.22874*10^(-3)*Tf^2*RH + 8.5282*10^(-4)*Tf*RH^2-1.99*10^(-6) * Tf^2 * RH^2 `


  • HI = Heat Index
  • Tf = Temperature
  • RH = Relative Humidity

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