Quick answer
The Heat Index is an apparent temperature that combines air temperature and relative humidity to describe how hot conditions feel, under shaded, light-wind conditions.
It is not the actual air temperature and does not fully account for direct sunshine or strenuous activity.
Why humidity changes how hot it feels
Sweat helps cool you when it evaporates. In humid air, evaporation is less effective, so the body has a harder time losing heat. A humid afternoon can therefore feel more oppressive than a drier afternoon with the same thermometer reading.
Relative humidity describes how close air is to saturation at its current temperature. Dew point offers another way to understand moisture. Read it alongside temperature: a humidity percentage alone does not tell you how hot conditions will feel.
Common misunderstanding
- “The Heat Index is the temperature in the sun.” It is based on shaded conditions; direct sun adds another load.
- “Dry heat is always safe.” Very high air temperatures can be dangerous even with low humidity. The index also does not measure UV exposure, so a lower heat value does not remove the need for sun protection.
Simple example
For an illustrative afternoon with an air temperature of 90°F (32°C) and relative humidity of 70%, the NWS heat-index calculation gives roughly 106°F (41°C). That is in the danger range. A thermometer in properly measured shade would still read 90°F.
90°F + 40% RH
Heat Index: about 91°F.
Read humidity alongside temperature.
90°F + 55% RH
Heat Index: about 97°F.
Sweat evaporates less effectively.
90°F + 70% RH
Heat Index: about 106°F.
The thermometer still reads 90°F.
RH = relative humidity. These examples assume shade and light wind.
You might change a long outdoor chore into shorter sessions with cooling breaks, or choose another time. The index helps explain why “only 90” can be a misleading description of the burden on the body.
Heat Index categories and their meaning
| Heat Index | Category | General meaning |
|---|---|---|
| 80–90°F | Caution | Fatigue is possible with prolonged activity. |
| 90–103°F | Extreme caution | Heat illness becomes a greater concern. |
| 103–124°F | Danger | Heat illness is likely with prolonged exposure or activity. |
| 125°F or higher | Extreme danger | Heat stroke is highly likely. |
These chart bands describe general risk with prolonged exposure and/or physical activity; they are not personal safety thresholds. Published chart boundaries may be rounded. Health, exertion, acclimatization, and time outside affect risk. Heat can be hazardous below a warning threshold too.
Why the Heat Index can change
Heat Index changes with temperature and humidity. Sunshine and activity add concerns beyond the number itself.
Changing temperature
Temperature and humidity work together. The day’s highest values may occur at different times.
Moisture in the air
Higher humidity makes sweat evaporation less effective, adding to the heat burden.
Shade versus sun
Standard values assume shade and light wind. Direct sun can add substantial heat load.
Different forecast values
A current reading, daily maximum, and provider’s “feels like” value may describe different things.
Sunshine, WBGT, and forecast definitions
Heat Index values assume shade and light wind. The NWS notes that full sunshine can increase apparent heat by up to 15°F (about 8°C). That is a possible additional effect, not a fixed correction to add to every forecast.
For outdoor work and sports, wet-bulb globe temperature (WBGT) is another heat-stress measure that considers more environmental factors, including sunlight and wind. Follow activity-specific guidance and local alerts rather than treating Heat Index as a complete exposure assessment.
A forecast can contain several different numbers: the air temperature, a calculated apparent temperature, and a daily maximum. Match the time and definition before comparing them. Daily high temperature and the day’s highest humidity do not necessarily occur together.
SkyAndTime’s Weather page uses the provider’s apparent-temperature value for “feels like.” This may use a different method from the NWS Heat Index. The Wind Chill guide explains a related cold-weather measure; the UV Index measures radiation instead of heat.
How to use a heat forecast
Use the latest forecast and local heat alerts to plan an outdoor activity.
Choose cooler periods
Check the hourly forecast and move strenuous activities when practical.
Plan cooling breaks
Arrange access to a cool indoor space, drink water regularly, and take breaks.
Consider the whole day
Hot nights can limit cooling. Check on people who may need help.
Take heat seriously
Never leave children or pets in parked vehicles. Seek urgent help for signs of serious heat illness.
A forecast is a planning aid, not a guarantee that any duration of exposure is safe.
Frequently asked questions
Is Heat Index the same as temperature?
No. Temperature measures the air; Heat Index combines temperature and relative humidity to estimate apparent heat under specified conditions.
Does the Heat Index include direct sunlight?
Standard Heat Index values assume shade and light wind. Direct sunlight can increase the heat burden, so use extra care outdoors.
Is Heat Index the same as the UV Index?
No. Heat Index describes apparent heat. UV Index describes sunburn-producing ultraviolet radiation. Check both for outdoor plans.
Sources & further reading
Official educational references checked September 27, 2026. Examples are illustrative, not live forecasts.
