How to Calculate Heat Index
What is Heat Index?
The heat index (apparent temperature or "feels like") combines air temperature and relative humidity to indicate how hot it actually feels to the human body. High humidity prevents sweat from evaporating, reducing the body's cooling ability.
Formula
Heat index ≈ −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T×RH + ... (complex formula)
- HI
- Heat Index (°F)
- T
- Air Temperature (°F)
- RH
- Relative Humidity (%)
Step-by-Step Guide
- 1Uses the Rothfusz regression equation (NWS standard)
- 2Valid for temperatures ≥ 80°F (27°C) and humidity ≥ 40%
- 3Calculated from 9-term polynomial regression
- 4Danger levels: Caution >90°F, Extreme >103°F, Danger >125°F
Worked Examples
Input
95°F, 60% humidity
Result
Heat index ≈ 108°F — Danger zone
Input
95°F, 20% humidity
Result
Heat index ≈ 87°F — Caution zone (desert-like)
Frequently Asked Questions
Is heat index the same as "feels like" temperature?
Heat index and "feels like" are similar concepts, but heat index specifically accounts for humidity's effect on the body's cooling through sweating.
At what heat index is it dangerous?
Heat index above 103°F creates risk for heat exhaustion. Above 125°F, heat stroke risk increases dramatically even for healthy people.
Why does humidity matter in hot weather?
Humidity prevents sweat from evaporating, which is your body's primary cooling mechanism. High humidity traps body heat and raises core temperature faster.
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