Heat index is a calculated temperature that uses temperature and relative humidity to determine how hot the air actually “feels.” It is calculated using a regression equation developed by the U.S. National Weather Service that blends temperature and humidity into a single perceived value.
In practical terms, the heat index helps you decide when outdoor conditions may become dangerous. A heat index chart groups ranges of the heat index into caution categories, showing when fatigue, heat cramps, heat exhaustion, or heat stroke become more likely. You can also use a heat index calculator to get a precise value for the heat index for your location by entering temperature and relative humidity. Weather apps often present it alongside the forecast high.
Key points to remember about heat index:
- It increases as humidity rises because sweat evaporates less efficiently.
- It assumes shade and light wind; direct sunlight can add up to several degrees to the “feels like” temperature beyond the basic heat index.
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It is most applicable when temperatures are above roughly 80°F (27°C) and relative humidity is above 40 percent. Below those levels, the effect is smaller.
How is the "feels like" temperature calculated beyond temperature and humidity?
The “feels like” temperature is a broader comfort metric than the heat index. It integrates multiple factors to estimate the perceived temperature on human skin. In hot conditions, it typically relies on the heat index framework (temperature plus relative humidity). However, the “feels like” number can also incorporate solar radiation (sun exposure), wind speed, clothing assumptions, and sometimes surface reflectivity. For example, direct sunshine can raise the “feels like” reading well above the shade-based heat index, while a breeze can lower it by enhancing sweat evaporation. Some advanced “feels like” models weigh these variables dynamically, so two locations with the same temperature and humidity can have different “feels like” values depending on wind and sun.
Differences between heat index, feels like temperature, and wind chill
- Heat index: A hot-weather metric that combines air temperature and relative humidity to show how hot it feels in the shade with light wind. In short, heat index is the shaded, low-wind perceived temperature for warm to hot conditions.
- Feels like temperature: A general-purpose perceived temperature used by many weather services. In heat, it often mirrors the heat index but may adjust for sun and wind. In cold, it typically mirrors wind chill. It is designed as a single value that reflects overall thermal comfort or stress.
- Wind chill: A cold-weather metric that combines air temperature and wind speed to show how cold it feels. Higher wind speeds increase heat loss from exposed skin, making it feel colder than the thermometer reading, even though humidity plays a smaller role in cold conditions.