The 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.
The 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.
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.
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