Heat index

The heat index tells you how hot it feels when humidity joins the air temperature. The National Weather Service gives an example: at 96°F and 65% humidity, the heat index is 121°F.

The index assumes shade and light wind. NWS says full sunshine can raise heat index values by up to 15°F, so a jobsite in the sun is hotter than the number. For work in direct sun or near heat sources, WBGT is a better measure. Crews can pair the heat index with a work/rest schedule. See the heat index work/rest chart guide and our WBGT vs heat index guide.