Heat Index Formula
Calculate the apparent temperature from air temperature and humidity.
Understand how hot it really feels outside.
The Formula
- 0.00683783T² - 0.05481717R² + 0.00122874T²R
+ 0.00085282TR² - 0.00000199T²R²
The heat index combines air temperature and relative humidity to show how hot it actually feels. High humidity prevents sweat from evaporating, making the body feel much hotter.
Variables
| Symbol | Meaning |
|---|---|
| HI | Heat index — apparent temperature (°F) |
| T | Air temperature (°F) |
| R | Relative humidity (%) |
Note: This formula is valid when T ≥ 80°F (27°C) and R ≥ 40%. For Celsius, convert first: °F = °C × 9/5 + 32.
Example 1
Temperature is 95°F (35°C) with 50% humidity
Plugging T = 95, R = 50 into the formula:
HI ≈ -42.4 + 194.7 + 507.2 - 1066.0 - 61.6 - 137.0 + 55.5 + 38.5 - 0.9
HI ≈ 107°F (42°C) — Danger zone: heat exhaustion likely
Example 2
Temperature is 85°F (29°C) with 80% humidity
Plugging T = 85, R = 80 into the formula:
HI ≈ 97°F (36°C) — Extreme Caution zone
When to Use It
Use the heat index formula when:
- Assessing outdoor safety for workers or athletes
- Planning outdoor events in hot, humid weather
- Issuing heat warnings and advisories
- Understanding the health risk of combined heat and humidity