Heat Index
The heat index is how hot it feels when relative humidity is combined with the air temperature. Humid air slows the evaporation of sweat, so the body cools less efficiently and feels hotter than the thermometer reads. At 100°F with 55% relative humidity, the heat index is 124°F.
Why It Matters
The heat index is the number the National Weather Service uses to describe heat stress on the body. The same 100°F day feels like 96°F at 15% relative humidity and 124°F at 55%.
Heat illness can occur at lower values too, depending on sun, exertion, exposure time, and individual health.
National Weather Service Heat Index Classifications
| Classification | Heat index | Effect on the body |
|---|---|---|
| Caution | 80°F to 90°F | Fatigue possible with prolonged exposure or physical activity |
| Extreme Caution | 90°F to 103°F | Heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure or physical activity |
| Danger | 103°F to 124°F | Heat cramps or heat exhaustion likely, and heat stroke possible, with prolonged exposure or physical activity |
| Extreme Danger | 125°F or higher | Heat stroke highly likely |
Heat index values are for shady locations. Direct sunshine can raise the heat index by up to 15°F.
Frequently Asked Questions
What heat index is dangerous?
The National Weather Service classifies a heat index of 103°F to 124°F as Danger, when heat cramps or heat exhaustion are likely and heat stroke is possible with prolonged exposure or activity. At 125°F or higher, Extreme Danger, heat stroke is highly likely.
Is the heat index measured in the sun or the shade?
The shade. Heat index values assume a shady location, and direct sunshine can raise the value by up to 15°F.
What is the difference between heat index and wet-bulb globe temperature?
The heat index combines air temperature and humidity for a shady spot. Wet-bulb globe temperature (WBGT) is a broader measure of heat stress that also accounts for wind speed, sun angle, and cloud cover.
Related Terms
Dew Point
The dew point is the temperature to which air must be cooled to become saturated, so that water vapor begins to condense into dew or fog. The higher the dew point, the more moisture is in the air. In summer, a dew point of 55°F or lower feels dry and comfortable, and 65°F or higher feels muggy.
Relative Humidity
The ratio of the current absolute humidity to the maximum possible humidity at the current temperature, expressed as a percentage. 100% relative humidity means the air is fully saturated. Relative humidity changes with temperature even if moisture content stays the same.
Humidity
The amount of water vapor present in the air. Absolute humidity measures the actual water content, while relative humidity expresses how close the air is to saturation at its current temperature, shown as a percentage.
Wet-Bulb Temperature
Wet-bulb temperature is the lowest temperature air can reach by evaporating water into it. It is always at or below the air temperature, and the drier the air, the larger the gap between the two. Forecasters use it to judge heat stress and how much falling rain or snow can cool the air.
Wind Chill
Wind chill is how cold the air feels on exposed skin once wind is taken into account. Wind strips heat from the body faster than still air does, so the wind chill temperature is lower than the air temperature. At 0°F with a 15 mph wind, the wind chill is -19°F, and exposed skin can freeze in 30 minutes.
UV Index
The UV index is a forecast of the intensity of skin-damaging ultraviolet radiation expected to reach the ground at solar noon, when the sun is highest. It is 0 at night and rises to 11 or higher, and the higher the number, the faster unprotected skin and eyes are harmed. Sun protection is recommended at 3 and above.
Heat Index: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
In-depth guide
The Heat Index Explained: How Hot It Really Feels and How to Stay Safe
Learn what the heat index is, how temperature and humidity combine into a 'feels like' temperature, how to read the NWS heat index chart, and how to stay safe in extreme heat.
Weather Education
Watch vs Warning vs Advisory vs Outlook: NWS Weather Alerts Explained
Learn the difference between a weather watch, warning, advisory, and outlook, what a Special Weather Statement means, and when to prepare versus take immediate action.
Weather Safety
Wildfire Smoke and AQI: What the Numbers Actually Mean for Your Lungs
Learn what the Air Quality Index really measures, why PM2.5 from wildfire smoke is so harmful, why the AQI can lag the sky outside, and what to do at each level to protect yourself.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Tool
Heat Index Calculator
A free tool for working this out with your own numbers.
Tool
Wet Bulb Calculator
Check heat stress conditions for sports, outdoor work, and summer safety.
Guide
'Feels Like' Temperature Explained
What apparent temperature really means and how wind chill and heat index are calculated.
Use Case
Weather App for Runners
Time runs around heat, humidity, and air quality with hour-by-hour conditions.
Sources
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Red Flag Warning
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Kp Index
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In the WeatherAI App
WeatherAI shows a feels-like temperature with current conditions, and Standard and Pro subscribers can set a custom notification rule on feels-like temperature, checked at a time of day they choose.
Availability, coverage, update frequency, and alert delivery vary by data source, region, subscription plan, device permissions, and service conditions. WeatherAI supplements official sources and is not an emergency notification service. For life-safety decisions, rely on your national weather service, local emergency management, and official warning channels.