REFERENCE // TOOL
Heat Index
What the heat actually does to a body once humidity is folded in.
Humidity is what makes heat dangerous: it stops sweat from evaporating. This is the NWS Rothfusz heat-index regression in °F.
Heat index (feels like)
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Risk band
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How it works
The full Rothfusz regression is used above 80°F and 40% humidity, with the NWS low-humidity and high-humidity corrections applied; below that a simpler average is used. Bands: Caution 80–90°F, Extreme Caution 90–103°F, Danger 103–125°F, Extreme Danger 125°F+. Values assume shade — full sun adds up to about 15°F.
Source: NOAA / NWS — The Heat Index Equation (verified 2026).