---
title: "What Is Heat Index? Heat Stress and Hydration Glossary"
description: "The heat index measures how hot it feels when humidity is added to the actual air temperature."
url: https://jmarsolutions.multisites.ai/glossary/heat-index
publisher: JMAR Solutions
phone: +1-214-632-7780
quote: https://jmarsolutions.multisites.ai/contact
---

Measurement

# Heat index

The heat index measures how hot it feels when humidity is added to the actual air temperature. It assumes shade and light wind, so full sun makes the real risk higher.

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](https://jmarsolutions.multisites.ai/glossary/wbgt) is a better measure. Crews can pair the heat index with a [work/rest schedule](https://jmarsolutions.multisites.ai/glossary/work-rest-schedule). See the [heat index work/rest chart guide](https://jmarsolutions.multisites.ai/guides/heat-index-work-rest-chart) and our [WBGT vs heat index guide](https://jmarsolutions.multisites.ai/guides/wbgt-vs-heat-index).

[From JMAR SolutionsHydration programs and toolbox talks for crews](https://jmarsolutions.multisites.ai/hydration-programs)

## Related terms

-   [WBGT](https://jmarsolutions.multisites.ai/glossary/wbgt)
-   [HeatRisk](https://jmarsolutions.multisites.ai/glossary/heatrisk)
-   [Heat index work rest chart](https://jmarsolutions.multisites.ai/glossary/heat-index-work-rest-chart)
-   [Work/rest schedule](https://jmarsolutions.multisites.ai/glossary/work-rest-schedule)

## Sources

1.  [NWS: Heat Forecast Tools](https://www.weather.gov/safety/heat-tools)
2.  [NWS: Heat Forecast Tools](https://www.weather.gov/safety/heat-tools)

![Ironworkers silhouetted on a steel frame against a hot orange sunset](https://jmarsolutions.multisites.ai/_astro/sunset-steel.IhAMDca0_6wk53.webp)

## See it on your crew in 15 minutes

Jennifer walks your safety team through the drops, the protocol and the numbers on a short live call.

[Book a 15-minute demo](https://calendly.com/jen-jmarsolutions/15-min-meeting)