What is Heat Index?

Heat index is often referred to as humiture, and is similar to wind chill in its attempt to measure the perceived rather than the actual temperature. For example, an air temperature of 83°F with a relative humidity of 70% would result in an estimated 88°F perceived temperature. This difference in perceived and actual temperature is the result of a mixture of air temperature, relative humidity, and wind speed.
The perception of heat is subjective, and can be affected by various factors such as menopause, pregnancy, and the effects of drugs or withdrawal, as well as differences in hydration, body shape, and metabolism. A higher relative humidity affects normal body cooling by reducing the rate of evaporation of sweat. The human body cools itself through perspiration, where heat is removed from the body as a result of the evaporation of sweat. The lower rate of evaporation subsequently lowers the rate at which the body cools, increasing the perception of heat. This perception of heat is what the heat index seeks to measure, and while it can technically be used indoors, it is most often used in reference to the outside temperature.

How to Calculate Heat Index?

Like the Wind Chill Temperature Index, the heat index used by the National Weather Service (NWS) in the United States is based on many assumptions such as body mass, height, clothing, individual physical activity, blood thickness, and wind speed. As such, depending on how significantly these assumptions vary from the reality of an individual, heat index estimates may not accurately reflect the perceived temperature. The equation used by the NWS to estimate heat index was developed by George Winterling in 1978, and is meant to be valid for temperatures of 80°F or higher, and relative humidity of 40% or more.

Potential effects of heat index

As described above, the heat index is the temperature equivalent perceived by humans as a result of air temperature, relative humidity, and wind speed. This temperature can have potentially severe medical effects. Under high air temperature and humidity (high heat index) conditions, perspiration is hindered due to reduced evaporation as a result of high humidity. Sweat is the human body's physiological response to high temperatures, and is an attempt to lower body temperature through the evaporation of sweat. When this is hindered, overheating and dehydration can occur, with varying severity.

The Heat Index Equation

The computation of the heat index is a refinement of a result obtained by multiple regression analysis carried out by Lans P. Rothfusz and described in a 1990 National Weather Service (NWS) Technical Attachment (SR 90-23). The regression equation of Rothfusz is

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

where T is temperature in degrees F and RH is relative humidity in percent. HI is the heat index expressed as an apparent temperature in degrees F. If the RH is less than 13% and the temperature is between 80 and 112 degrees F, then the following adjustment is subtracted from HI:

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

where ABS and SQRT are the absolute value and square root functions, respectively. On the other hand, if the RH is greater than 85% and the temperature is between 80 and 87 degrees F, then the following adjustment is added to HI:

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

The Rothfusz regression is not appropriate when conditions of temperature and humidity warrant a heat index value below about 80 degrees F. In those cases, a simpler formula is applied to calculate values consistent with Steadman's results:

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

In practice, the simple formula is computed first and the result averaged with the temperature. If this heat index value is 80 degrees F or higher, the full regression equation along with any adjustment as described above is applied.

The Rothfusz regression is not valid for extreme temperature and relative humidity conditions beyond the range of data considered by Steadman.