Question 1

What is the heat index?

Accepted Answer

The heat index is the temperature the body feels when relative humidity is combined with the air temperature. When humidity is high, sweat evaporates more slowly, so the body cannot cool itself and the air feels hotter than the thermometer reads.

Question 2

What formula does this calculator use?

Accepted Answer

This tool uses the NWS Rothfusz regression: HI = -42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² − 0.00000199·T²·RH², where T is in °F and RH is relative humidity in percent.

Question 3

When is heat index dangerous?

Accepted Answer

A heat index of 90°F (32°C) starts to cause fatigue; 103°F (39°C) brings possible sunstroke; 125°F (52°C) or higher is highly dangerous with heatstroke likely. The calculator shows the NWS risk category for your inputs.

Question 4

Below what temperature is heat index not defined?

Accepted Answer

The Rothfusz heat index is only meaningful for air temperatures at or above 80°F (26.7°C) and relative humidity of 40% or higher. Below those thresholds, the calculator reports the actual temperature because the body cools itself efficiently.

NWS heat index risk categories