What is the heat index?

The heat index is the apparent temperature — how hot it feels to a human body — when air temperature and relative humidity are combined. High humidity reduces the body's ability to cool itself through perspiration, making the air feel hotter than a thermometer reads.

What formula does NOAA use?

NOAA uses the Rothfusz regression equation, a multi-term polynomial that takes Fahrenheit temperature (T) and relative humidity (RH) as inputs. For T < 80°F or mild conditions, the simpler Steadman formula is used. Adjustments are applied for very low or very high humidity extremes.

Why is the heat index only valid above 80°F / 27°C?

The Rothfusz formula is calibrated for shade conditions with light wind above 80°F. Below that threshold the humidity effect is minor and a simpler linear estimate suffices. NOAA does not publish heat index values below 80°F.

What are the danger levels?

NOAA classifies heat index as: Caution (80–90°F), Extreme Caution (91–103°F), Danger (103–124°F), and Extreme Danger (125°F+). Fatigue and heat cramps are possible in the Caution zone; heat stroke becomes likely above 103°F.

Does wind affect the heat index?

No. The heat index formula assumes light-wind, shaded conditions. Direct sunlight can add 10–15°F to the heat index; strong wind lowers it (that effect is captured separately by wind chill at cold temperatures).

Heat Index (NOAA)

Name: Heat Index (NOAA)
Availability: InStock
Author: Nham Vu

Enter air temperature and relative humidity to get the NOAA heat index (feels-like temperature) with a danger-level classification.

Inputs

Air Temperature (°F)

Relative Humidity (%)

Result

Enter temperature and humidity, then click Calculate.

NOAA Heat Index Danger Levels

Level	Heat Index (°F)	Possible Heat Disorders
Caution	80–90°F (27–32°C)	Fatigue possible with prolonged exposure and activity.
Extreme Caution	91–103°F (33–39°C)	Heat cramps and heat exhaustion possible.
Danger	103–124°F (39–51°C)	Heat cramps and exhaustion likely; heat stroke possible.
Extreme Danger	125°F+ (52°C+)	Heat stroke highly likely with continued exposure.

Summary

Enter air temperature and relative humidity to get the NOAA heat index (feels-like temperature) with a danger-level classification.

How it works

Enter the air temperature in °F or °C.
Enter the relative humidity as a percentage.
The tool applies the NOAA Rothfusz regression formula to compute the heat index.
For low-humidity or mild conditions, the simpler Steadman formula is used instead.
The result is classified into Caution, Extreme Caution, Danger, or Extreme Danger.

Use cases

Assess outdoor safety during hot, humid summer days.
Plan athletic events or workouts in high-temperature conditions.
Determine whether to issue heat advisories for a location.
Understand why 90°F with 90% humidity feels far hotter than 90°F with 20% humidity.
Cross-check official weather-service heat index values.
Educational demonstrations of the Rothfusz regression formula.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu