Heat Index Explained — What ‘Feels Like’ Really Means
What the heat index is, why humid days feel hotter, and how to read the Weather Service chart — in the National Weather Service’s own words.
What the heat index is
The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature.
There is a direct relationship between the air temperature and relative humidity and the heat index: as the air temperature and relative humidity increase, the heat index increases. As they decrease, the heat index decreases.
Why humid days feel hotter
When the body gets too hot, it perspires — it sweats — to cool itself off. Evaporation is a cooling process: when perspiration evaporates off the body, it reduces the body’s temperature.
When relative humidity is high, the rate of evaporation from the body decreases. The body feels warmer, because it cannot regulate its temperature as well.
When relative humidity is low, perspiration evaporates faster. The body actually feels cooler in dry, arid conditions.
How to read the chart
To find the heat index, you need two numbers: the air temperature and the relative humidity.
Find where the temperature and the humidity cross on the chart. That crossing point is the heat index.
Here are the Weather Service’s own examples:
Air temperature 100°F + humidity 55% → heat index 124°F
An air temperature of 100°F with relative humidity of 55% gives a heat index of 124°F.
Air temperature 100°F + humidity 15% → heat index 96°F
An air temperature of 100°F with relative humidity of only 15% gives a heat index of 96°F. The apparent temperature can actually be lower than the air temperature when humidity is low.
Air temperature 96°F + humidity 65% → heat index 121°F
An air temperature of 96°F with relative humidity of 65% gives a heat index — how hot it feels — of 121°F.
Shade warning: add up to 15°F in direct sun
The heat index values in the chart are for shady locations.
If you are exposed to direct sunlight, the heat index value can be increased by up to 15°F.
What the numbers mean: the four bands
The Weather Service sorts heat index values into four bands. These are the Weather Service’s own words for what each band can do:
| Band | Heat index | Possible effects |
|---|---|---|
| Caution | 80°F–90°F | fatigue possible with prolonged exposure and/or physical activity |
| Extreme Caution | 90°F–103°F | heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity |
| Danger | 103°F–124°F | heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity |
| Extreme Danger | 125°F or higher | heat stroke highly likely |
Heat indices meeting or exceeding 103°F can lead to dangerous heat disorders with prolonged exposure and/or physical activity in the heat.
The full chart and calculator
For the full heat index chart and calculator, visit the National Weather Service at weather.gov.
Sources
National Weather Service (weather.gov) — What is the heat index?
Checked November 2026