The heat index is a measure of how hot it really feels when the effects of humidity are added to high temperature. To alert the public to the dangers of exposure to extended periods of heat and the added effects of humidity a Heat Index table is used to correlate measured temperature and humidity into a apparent temperature. This website offers graduated heat maps showing this temperature.

What is the formula for calculating heat index?

The Heat Index is a calculated value based on air temperature and humidity. To calculate a specific value for a previous date, you will need to know the air temperature and humidity.
HI (Farenheit) = 42.379 + 2.04901523*T + 10.14333127*RH - 0.22475541*T*RH - 6.83783x10^-3*T^2 - 5.481717x10^-2*RH^2 + 1.22874x10^-3*T^2*RH+8.5282x10^-4*T*RH^2 - 1.99x10^-6*t^2*RH^2
Where T = air temperature in degrees Fahrenheit
RH = relative humidity

To use the heat index table below, find the temperature on the left of the chart. Read across until you reach the desired relative humidity. The number which appears at the intersection of the temperature and relative humidity is the Heat Index. Note that the Heat Index under direct sunlight will be 8 °C higher than the number shown in the chart.

What is the discomfort index?

This index evaluates the impact of heat stress on the individual taking into account the combined effect of temperature and humidity. The formula used by the SA Weather Service to calculate discomfort index is:
Discomfort Index = (2 x T) + (RH/100 x T) + 24
Where:”
T is the dry-bulb or air temperature in degrees Celsius
RH is the percentage relative humidity

This index gives the following degrees of discomfort:
90-100 - very uncomfortable
100-110 - extremely uncomfortable
110 and more - hazardous to health

Since the relative humidity of the air can be calculated from the dry-bulb and wet-bulb temperatures, the formula can also be adapted to use the wet-bulb temperature instead of the relative humidity.