By Lee Johnson | Updated Aug 30, 2022
In scientific contexts we rely on objective measurements, but everyday weather forecasts often use the heat index to convey how hot it actually feels to the human body. This metric combines air temperature with relative humidity to reflect the combined effect of heat and moisture on the skin.
The heat index is a subjective measure of perceived temperature. While a thermometer reads a single value in degrees, the heat index tells you how hot you feel when humidity is high. Meteorologists use it to warn of heat‑related risks because a humid day can feel much hotter than the ambient temperature alone would suggest.
The National Weather Service’s heat index equation incorporates dozens of physical factors—skin radiation, clothing, sweating, and more—into a single, manageable expression. For most practical applications the following formula is used:
HI = –42.379 + (2.04901523 × T) + (10.14333127 × R) – (0.22475541 × T × R) – (0.00683783 × T²) – (0.05481717 × R²) + (0.00122874 × T² × R) + (0.00085282 × T × R²) – (0.00000199 × T² × R²)
Here, T is the air temperature in °F and R is the relative humidity expressed as a whole‑number percentage (e.g., 65 % → 65). If you have temperature in Celsius, first convert it to Fahrenheit with F = C × 9⁄5 + 32 and use the integer humidity value.
To compute the heat index manually, simply substitute your temperature and humidity into the equation above. For those who prefer automation, the formula can be entered into Excel or Google Sheets using cell references for T and R. Online heat‑index calculators are also available and provide instant results.
Keep in mind that the heat index is most meaningful when the temperature exceeds 80 °F and humidity is above 40 %. Below those thresholds, the standard temperature reading is usually sufficient.
