Heat Index Formula

Definition: The head index is a thermodynamic value that relates air temperature and humidity. This measurement is made in places that simulate the feeling or perception that human body can feel. Thus, it is also called the "felt air temperature" because it is not necessarily the weather or environment temperature, it is the sensation that the body can feel.

General Formula: There are some formulas to calculate the heat index, but the most used is:

Where HI = heat index (in degrees Fahrenheit), T is ambient temperature in Fahrenheit and R is the relative humidity. Meanwhile, c₁-c₉ are constants with the value of:

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.

Use: It is used to approximate the perception of the human body of the humidity and heat in order to create healthy or public plans that can preserve the human and animal safety. For example, if the temperature is 30 °C but the humidity is 80%, the heat index will be around 40 °C and especially recommendation should be made to people in order to keep them isolated or hydrated at this temperature.

Example: Calculate the HI for a temperature of 85 °C and 86%.

First, convert 85 °C is 185 °F and substituent each term is the equation:

We have a HI of 1274 °F.

Considerations: The heat index is calculated in shade areas, thus it should be considered that for an area in which the temperature is 27 °C and the humidity 50%, the heat index is around 28°C but in shade. The areas in this environment that are exposed to sunlight could have a higher or lower heat index. Also, people doing different activities can perceive different heat and not necessarily feel the heat index calculated for that day or place.