Heat Index Calculator

Humid air stops sweat evaporating, so a summer afternoon feels hotter than the thermometer reads. Enter the air temperature with either the relative humidity or the dew point, and this returns the heat index in Fahrenheit and Celsius together with the National Weather Service caution category. The arithmetic is the NWS Rothfusz regression from Technical Attachment SR 90-23, including the corrections the NWS applies to very dry and very humid air. That regression is fitted from 80 F and 40% relative humidity upwards, so anything below it is flagged rather than dressed up as a heat index.