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Calculate feels-like temperature in heat with NWS danger levels and safety tips.
The heat index is the apparent temperature that combines air temperature and relative humidity to indicate how hot it truly feels to the human body, calculated using the Rothfusz regression equation.
| Heat Index | Level | Risk |
|---|---|---|
| Below 80°F / 27°C | None | No significant risk |
| 80–90°F / 27–32°C | Caution | Fatigue with prolonged exposure |
| 91–103°F / 33–39°C | Extreme Caution | Heat cramps, exhaustion possible |
| 104–125°F / 40–51°C | Danger | Heat cramps/exhaustion likely, stroke possible |
| Above 125°F / 52°C | Extreme Danger | Heat stroke highly likely |
Formula
HI = c₁ + c₂T + c₃R + c₄TR + c₅T² + c₆R² + c₇T²R + c₈TR² + c₉T²R²HI = Heat index in °F
T = Air temperature in °F
R = Relative humidity (%)
c₁–c₉ = Rothfusz regression coefficients (NWS)
Worked Example
Heat index at 90°F and 65% humidity
Did you know? Heat is the #1 weather-related killer in the United States, causing more deaths annually than hurricanes, tornadoes, and floods combined (source: NOAA).
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