Feels Like
103°F
Celsius
39°C
Risk
Extreme Caution
Heat Index (Feels Like)
103°F
39°C
Added Heat
+13°F
Risk Level
Extreme Caution
Extreme Caution
Heat cramps and exhaustion possible. Limit outdoor exertion.
Inputs
Result
Rothfusz regression gives 103°F. Risk level: Extreme Caution. Heat cramps and exhaustion possible.
Inputs
Result
Even moderate humidity at 100°F pushes heat index into Danger zone. Heat stroke likely with exertion.
The NWS uses the Rothfusz regression equation, a polynomial formula combining air temperature and relative humidity. At 90°F and 65% humidity, the heat index is about 103°F—a 13-degree increase from humidity alone.
| Air Temp | 40% RH | 60% RH | 80% RH |
|---|---|---|---|
| 85°F | 83°F | 90°F | 97°F |
| 90°F | 88°F | 100°F | 113°F |
| 95°F | 96°F | 110°F | 133°F |
The NWS defines four danger zones: Caution (80–90°F), Extreme Caution (91–103°F), Danger (104–125°F), and Extreme Danger (126°F+). Heat stroke becomes likely above 104°F heat index.
Your body cools itself by evaporating sweat. High humidity slows evaporation, so your body cannot cool efficiently. At 100% humidity, sweat does not evaporate at all, making even moderate temperatures dangerous.
Heat index is calculated for shady conditions with light wind. In direct sun, the effective heat index can be 15°F higher. Always add 10–15°F when working or exercising in full sun.
Weather and outdoor calculators
Last Updated: Mar 20, 2026
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