What is the difference between the heat index and the temperature?

The air temperature is what a thermometer reads in the shade; the heat index adds the effect of humidity to estimate how hot it feels to a human body. The NWS heat index is only defined at or above 80°F and 40% relative humidity, and it assumes shade — direct sun can add up to about 15°F.

At what heat index is it dangerous?

NWS categories: 80–90°F is "caution," 90–103°F is "extreme caution" (heat cramps or exhaustion possible), 103–124°F is "danger" (heat exhaustion likely, heat stroke possible), and 125°F or above is "extreme danger" (heat stroke highly likely). Heat is the deadliest weather hazard in an average U.S. year.

How is the heat index calculated?

It is computed from air temperature and relative humidity (which depends on the dew point) using an NWS regression equation. The official value assumes shade and a light wind; full direct sunlight can raise the real feels-like value by up to roughly 15°F.

What Is the Heat Index? — The "Feels Like" Temperature

What Is the Heat Index? — The "Feels Like" TemperatureThe heat index ("feels like" temperature) combines air temperature and humidity to show how hot it feels to the body. The NWS caution/danger categories, the full-sun caveat, and how it differs from temperature.The heat index — often shown as the "feels like" temperature — is how hot it actually feels to the human body when relative humidity is combined with the air temperature. The body cools itself by sweating, and sweat cooling depends on evaporation. When the air is already humid, sweat evaporates slowly, so you feel hotter than the thermometer reads. A 96°F afternoon at a 75°F dew point can feel like 110°F or more.What goes into itThe heat index is computed from air temperature and relative humidity (which is driven by the dew point). The NWS heat index is only defined for temperatures at or above 80°F and relative humidity at or above 40%. The official value also assumes shade and a light wind — in full direct sunlight it can run up to about 15°F higher than the forecast value.NWS heat index categories

80–90°F: Caution — fatigue possible with prolonged exposure

90–103°F: Extreme caution — heat cramps and heat exhaustion possible

103–124°F: Danger — heat exhaustion likely, heat stroke possible

125°F and above: Extreme danger — heat stroke highly likely

Heat is the deadliest weather hazard in an average U.S. year. The heat index is what drives Heat Advisories and Excessive Heat Warnings, cooling-center activation, and outdoor-event decisions. The heat index rates a single hour; for a multi-day, health-based view that blends temperature with local climatology, see NWS HeatRisk.

Frequently asked questions

What is the heat index?: The heat index, or "feels like" temperature, is how hot it actually feels when relative humidity is combined with the air temperature. Because the body cools by evaporating sweat, and humid air slows evaporation, high humidity makes a given temperature feel hotter — a 96°F day at a 75°F dew point can feel like 110°F or more.
What is the difference between the heat index and the temperature?: The air temperature is what a thermometer reads in the shade; the heat index adds the effect of humidity to estimate how hot it feels to a human body. The NWS heat index is only defined at or above 80°F and 40% relative humidity, and it assumes shade — direct sun can add up to about 15°F.
At what heat index is it dangerous?: NWS categories: 80–90°F is "caution," 90–103°F is "extreme caution" (heat cramps or exhaustion possible), 103–124°F is "danger" (heat exhaustion likely, heat stroke possible), and 125°F or above is "extreme danger" (heat stroke highly likely). Heat is the deadliest weather hazard in an average U.S. year.
How is the heat index calculated?: It is computed from air temperature and relative humidity (which depends on the dew point) using an NWS regression equation. The official value assumes shade and a light wind; full direct sunlight can raise the real feels-like value by up to roughly 15°F.

Part of the BloomWX learn library — beginner-friendly explainers covering every surface of the BloomWX weather dashboard. Open BloomWX to see live data for any U.S. county.