Heat Index Calculator

Sweat cools by evaporating; humid air slows evaporation, so the same 92°F feels — and physiologically is — hotter at 60% humidity (feels like 105°F) than at 20%. The Rothfusz regression behind this is the NWS standard.

Above a 91°F index, the NWS advises limiting exertion; above 103°F, heat cramps and exhaustion become likely with activity. Heat is the deadliest U.S. weather hazard — hydration, shade breaks, and afternoon avoidance are the whole playbook.