What the Heat Index Means

This heat index calculator applies the National Weather Service Rothfusz regression for temperatures of 80 °F and above, with the standard corrections for humidity below 13% or above 85%. Below 80 °F it uses the simplified apparent-temperature formula. The result is the temperature the air feels like once humidity is considered.

Why Humidity Changes the Feeling

Sweating cools the body by evaporation. Humid air is already saturated with water vapor, so sweat evaporates slowly and the cooling effect fades. That is why a 95 °F day at 55% humidity feels like 110 °F.

Staying Safe

Above 90 °F, heat cramps and exhaustion become possible; above 103 °F, heat stroke risk rises sharply. Hydrate, take shade breaks, and reschedule strenuous outdoor work when the index climbs into the Danger zone.