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Heat Index Formula

The Heat Index is a measure of heat which feels when relative humidity is factored in with the air temperature.

The heat index formula is based upon the temperature and relative humidity to indicate the air temperature perceived by the body. For instance, if the temperature is 32°C and 70% relative humidity, the heat index is 41°C. This heat index has an implied humidity of 20%. It’s the value of the relative humidity for which the heat index formula shows 41°C. A heat index temperature of 32°C holds implied relative humidity of 38%.

The heat index formula is expressed as,

The heat index formula is expressed as,
HI= c1+c2T+c3R+c4TR+c5T2+c6R2+c7T2R+c8TR2+c9T2R2

In this formula,
HI = heat index in degrees Fahrenheit,
R = Relative humidity,
T = Temperature in ∘F,
c1 = -42.379,
c2= -2.04901523,
c3 = -10.14333127,
c4 = -0.22475541,
c5 = -6.83783 x 10−3,
c6 = -5.481717 x 10−2,
c7 = -1.22874 x 10−3,
c8 = 8.5282 x 10−4,
c9 = -1.99 x 10−6
It is found that whenever HI values exceed above 40∘C during different temperature and humidity ranges, there is a threat of heatstroke.

Example 1
Determine the heat index if the temperature is 197∘F and the relative humidity is 80%.
Solution:
Given:
T = 197∘F,
R = 80%

Substituting the values in the given formula,
HI= c1+c2T+c3R+c4TR+c5T2+c6R2+c7T2R+c8TR2+c9T2R2

Heat Index = 1410.25 oF

Therefore, the heat index is 1410.25 Fahrenheit.

Example 2
Determine the heat index for the relative humidity of 86% and the temperature of 85oC.
Solution:
Given:
T = 85∘C
= 185∘F,
R = 86%

Substituting the values in the given formula,
HI= c1+c2T+c3R+c4TR+c5T2+c6R2+c7T2R+c8TR2+c9T2R2

Heat Index = 1273.47 oF