The heat index is a measurement of how hot it will feel to the human body based on the ambient air temperature and relative humidity.

This heat index calculator uses three different formulas with different index coefficients. The first (heat index 1) uses the NOAA (National Weather Service) table which is the following:

T2 = pow(a, 2) H2 = pow(h, 2) C1 = [ -42.379, 2.04901523, 10.14333127, -0.22475541, -6.83783e-03, -5.481717e-02, 1.22874e-03, 8.5282e-04, -1.99e-06] heatindex1 = C1[0] + (C1[1] * a) + (C1[2] * h) + (C1[3] * a * h) + (C1[4] * T2) + (C1[5] * H2) + (C1[6] * T2 * h) + (C1[7] * a * H2) + (C1[8] * T2 * H2) result = ((heatindex1 - 32) * 5/9)

**a**= air temperature in Fahrenheit**h**= relative humidity in percent**result**= heat index in celsius

Heat index 2 is calculated with the following:

T2 = pow(a, 2) H2 = pow(h, 2) C2 = [ 0.363445176, 0.988622465, 4.777114035, -0.114037667, -0.000850208, -0.020716198, 0.000687678, 0.000274954, 0] heatindex2 = C2[0] + (C2[1] * a) + (C2[2] * h) + (C2[3] * a * h) + (C2[4] * T2) + (C2[5] * H2) + (C2[6] * T2 * h) + (C2[7] * a * H2) + (C2[8] * T2 * H2) result = ((heatindex2 - 32) * 5/9)

**a**= air temperature in Fahrenheit**h**= relative humidity in percent**result**= heat index in celsius

Heat index 3 is calculated with the following:

T2 = pow(a, 2) T3 = pow(a, 3) H2 = pow(h, 2) H3 = pow(h, 3) C3 = [ 16.923, 0.185212, 5.37941, -0.100254, 0.00941695, 0.00728898, 0.000345372, -0.000814971, 0.0000102102, -0.000038646, 0.0000291583, 0.00000142721, 0.000000197483, -0.0000000218429, 0.000000000843296, -0.0000000000481975] heatindex3 = C3[0] + (C3[1] * a) + (C3[2] * h) + (C3[3] * a * h) + (C3[4] * T2) + (C3[5] * H2) + (C3[6] * T2 * h) + (C3[7] * a * H2) + (C3[8] * T2 * H2) + (C3[9] * T3) + (C3[10] * H3) + (C3[11] * T3 * h) + (C3[12] * a * H3) + (C3[13] * T3 * H2) + (C3[14] * T2 * H3) + (C3[15] * T3 * H3) result = ((heatindex3 - 32) * 5/9)

**a**= air temperature in Fahrenheit**h**= relative humidity in percent**result**= heat index in celsius

## Why Use The Heat Index?

When the human body becomes too hot it will perspire through pores in the skin to radiate heat through evaporation. When the apparent temperature becomes too high the body can no longer effectively regulate its temperature, which can be dangerous and in extreme cases, deadly.

The heat index provides an objective method of deciding whether you might be in danger. The National Weather Service describes heat index ranges in the following way:

**Extreme Caution:**90°F - 103°F – Fatigue possible.**Danger:**103°F - 124°F – Extreme caution. Heatstroke, heat cramps, or heat exhaustion are possible.**Caution:**80°F - 90°F – Danger. Heat cramps or heat exhaustion are likely, and heat stroke is possible.**Extreme Danger:**125°F or higher – Extreme danger. Heatstroke is highly likely.