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) Ambient Dry-bulb Temperature
• (RH)  Relative Humidity.

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`

Where 

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

Weather (Meteorology) Calculators

• Heat Index: Approximates the heat index based on the temperature and relative humidity
• Canadian Humidity Index (HUMIDEX): Computes the Canadian Humidity Index (HUMIDEX) based on the dry-bulb ambient temperature and the dew-point temperature.
• Australian Apparent Temperature (AT): Computes the apparent temperature based on the dry ambient temperature, the relative humidity and the wind speed at ten meters.
• Relative Humidity: Computes the relative humidity based on the actual density of vapor and the saturated density.
• Dew Point from Relative Humidity: Computes the dew point based on the relative humidity and ambient temperature.
• Summer Simmer Index: Computes the Summer Simmer Index based on the air temperature and relative humidity.
• Wind Chill Index (North America): Computes the wind chill index based on the air temperature and the velocity of the wind.
• Wet-Bulb Globe Temperature (WBGT): Computes the wet-bulb globe temperature index based on dry-bulb (air) temperature, globe thermometer temperature and the wet-bulb temperature.
• Humature Index: Estimates the humidity index based on the temperature and dew point.
• Antoine Equation: Computes the apparent vapor pressure of pure substances based on temperature and coefficients for the substance.
• Barometric Formula (Tropospheric) calculator computes the normal barometric pressure based on the altitude (h) using the Exponential Atmosphere formula.
• Planetary Temperature: Estimates a planet's temperature based on the luminosity of its star, distance to the star, average albedo and infrared optical depth of atmosphere.
• Snow Water Equivalent (SWE): Computes the volume of liquid water contained in rain or snow pack defined by area, depth and snow type or rain.

Reference

• Heat Waves and Heat Related Illnesses 
• Wikipedia article on Heat Index: en.wikipedia.org/wiki/Heat_index