To measure and calculate the heat index, you need to have the values for two variables:

the air temperaturethe amount of humidity in the air (this is measured by a percent value)For example, let's say the air temperature on a given day is 90 degrees Fahrenheit with an air moisture percentage of 75%. This results in a heat index of 109 °F.

### Regression Equation of Rothfusz

To calculate the heat index value, you have to plug these variables into an equation known as the regression equation of Rothfusz:

##### HI = -42.379 + 2.04901523 · T + 10.14333127 · RH - .22475541 · T · RH - .00683783 · T · T - .05481717 · RH · RH + .00122874 · T · T · RH + .00085282 · T · RH · RH - .00000199 · T · T · RH · RH

It's a bit much, we know. In this equation:

- HI stands for the heat index
- the Ts represent the temperature
- and the RHs represent the percentage of moisture in the air.

### Adjustments To The Equation

However, this equation alone is not enough. If the temperature and humidity fall within certain ranges, you have to use additional equations to find additional values called "adjustments."

#### Humidity Below 13%

If the RH of a given day is less than 13% and the T is between 80 and 112 degrees Fahrenheit, you have to subtract this adjustment from the HI you calculated using the Rothfusz equation:

##### ADJUSTMENT = [(13 - RH) / 4] · SQRT{[17 - ABS(T - 95.)] / 17}

In this equation, SQRT represents a square root function and ABS represents the absolute value.

#### Humidity Above 85%

For different circumstances, there is yet another adjustment value you have to calculate.

If the RH of a given day is greater than 85% and the temperature is between 80 and 87 degrees Fahrenheit, this adjustment is added to your initial heat index calculation:

##### ADJUSTMENT = [(RH - 85) / 10] · [(87 - T) / 5]

#### Heat Index Below 80 Degrees

Lastly, if you used the Rothfusz equation and calculated a heat index below 80 degrees, you either did it wrong or the RH and T were **too low** for that equation to be useful.

In the latter instances, you should use a much simpler equation:

##### HI = 0.5 · {T + 61.0 + [(T - 68.0) · 1.2] + (RH · 0.094)}