Heat Index Calculator
This calculator calculates the heat index or apparent temperature.
The heat index is a measure of the perceived temperature experienced by the human body. It takes into account both the air temperature and the relative humidity, providing a more accurate representation of the conditions experienced by an individual. In high humidity conditions, for example, the air temperature may feel much hotter than it would in drier conditions. While Humindex is used in Canada, the Heat Index is used in US.
The calculator takes as input the air temperature and relative humidity, and calculates the heat index. This information can be used to make informed decisions related to outdoor activities, such as scheduling work or leisure activities, and to monitor potential heatrelated health risks, especially during extreme heat events.
There are two versions: for Fahrenheit and for Celsius degrees. You can also find the Heat Index formulas below the calculators.
Heat Index formulas
The Heat Index (or apparent temperature) is based on R.G. Steadman's work published in 1979 under the title "The Assessment of Sultriness, Parts 1 and 2.", so it is the result of extensive biometeorological studies. Originally, results were in tables.
To arrive at an equation that uses more conventional independent variables, a multiple regression analysis was performed on the data from Steadman's tables. It was done by Lans P. Rothfusz and described in his work "The Heat Index "Equation" (or, More Than You Ever Wanted to Know About Heat Index) in 1990. You can find it here, for example.
So, the sequence of calculations is:

Heat Index is calculated using this formula:
,
where T  air temperature in Fahrengeit and RH  relative humidity in percents. 
If RH is less than 13% and air temperature is between 80 and 112F, the following adjustment is subtracted from HI:
,
where ABS  absolute value. 
If RH is more than 85% and air temperature is between 80 and 87F, the following adjustment is added to HI:
 If resulting HI is less than 80F, it is neglected and new Heat Index is calculated using siimpler formula, which approximates results from R. G. Steadman
Usually, HI is calculated using simpler formula (4), then averaged with air temperature. If the result is more than 80F, full formulas should be used.
However, these formulas are not usable for temperature and humidity outside of the range used by Steadman. For temperature, it ranges from 20 to 50 degrees Celsius. As for humidity, after the 30C graph is not linear, it is better to look it up for yourself, in Steadman, R.G., 1979: The assessment of sultriness. Part I, for example, here.
Source: National Weather Service
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