Heat Wave Risk Calculator
The heat index tells you how hot it actually feels when you factor in humidity. High humidity impairs the body's ability to cool itself by sweating, making the effective temperature far higher than the thermometer reading. The NOAA National Weather Service uses the Rothfusz polynomial regression equation to calculate the heat index, which is the standard used for all NWS heat advisories and warnings across the United States. Enter air temperature in degrees Fahrenheit and relative humidity percentage to calculate the heat index and associated NWS health risk category.
NOAA Rothfusz heat index equation
HI = -42.379 + 2.04901523×T + 10.14333127×RH
- 0.22475541×T×RH - 0.00683783×T2
- 0.05391553×RH2 + 0.00122874×T2×RH
+ 0.00085282×T×RH2 - 0.00000199×T2×RH2
Where T is temperature in deg F and RH is relative humidity in percent. Valid when heat index exceeds 80 deg F. Source: NOAA NWS Heat Index Technical Note (Rothfusz, 1990, NWS Southern Region Technical Attachment, SR/SSD 90-23).
NWS heat index risk categories
- Below 80 deg F (27 deg C): No heat risk. Comfortable conditions for most people.
- 80 to 90 deg F (27 to 32 deg C): Caution. Fatigue possible with prolonged exposure and activity.
- 90 to 103 deg F (32 to 39 deg C): Extreme Caution. Heat cramps and heat exhaustion possible.
- 103 to 124 deg F (39 to 51 deg C): Danger. Heat cramps and heat exhaustion likely; heat stroke possible.
- Above 124 deg F (51 deg C): Extreme Danger. Heat stroke highly likely with continued exposure.
Heat wave risk calculator: frequently asked questions
What is the heat index and how is it calculated?
The heat index (HI) is an apparent temperature that combines air temperature and relative humidity to quantify how hot it feels to the human body. The NOAA NWS uses the Rothfusz regression equation: HI = -42.379 + 2.04901523 x T + 10.14333127 x RH - 0.22475541 x T x RH - 0.00683783 x T^2 - 0.05391553 x RH^2 + 0.00122874 x T^2 x RH + 0.00085282 x T x RH^2 - 0.00000199 x T^2 x RH^2, where T is in deg F and RH is the relative humidity percent.
When should the simple vs the Rothfusz heat index equation be used?
NOAA uses the Rothfusz regression equation when the heat index is above 80 deg F and humidity is above 40%. For temperatures between 80 and 112 deg F, two adjustment equations are applied on top of the Rothfusz result: one for very low humidity (below 13% RH when T is between 80 and 112 deg F) and one for very high humidity (above 85% RH when T is between 80 and 87 deg F). This calculator applies the Rothfusz equation, which is valid for the main range of heat advisory conditions.
What are the NWS heat illness risk categories?
NWS heat index risk categories: Below 80 deg F (27 deg C): No heat-related risk for most people. 80 to 90 deg F (27 to 32 deg C): Caution; fatigue possible with prolonged exposure and activity. 90 to 103 deg F (32 to 39 deg C): Extreme Caution; heat cramps and heat exhaustion possible. 103 to 124 deg F (39 to 51 deg C): Danger; heat cramps and heat exhaustion likely; heat stroke possible. Above 124 deg F (51 deg C): Extreme Danger; heat stroke highly likely with prolonged exposure.
What constitutes an official NWS heat advisory vs. heat warning?
NWS issues Heat Advisories when the heat index is expected to reach 100 to 104 deg F for at least 2 hours. An Excessive Heat Warning is issued when the heat index is expected to be 105 deg F or higher for at least 2 hours. An Excessive Heat Watch is issued 24 to 72 hours in advance of watch criteria. These thresholds may vary by region depending on local climate normals (a heat index of 95 deg F in the Pacific Northwest may trigger a warning that would not be issued in Phoenix).
What is the difference between heat index and wet bulb temperature?
The heat index estimates how hot it feels in the shade with low wind. Wet bulb temperature (WBT) is the lowest temperature achievable by evaporative cooling of a wetted thermometer, and is a direct measure of the human body's ability to cool itself by sweating. A wet bulb temperature above 35 deg C (95 deg F) is considered lethal for any human activity lasting more than a few hours, even at rest. WBT incorporates both temperature and humidity and is more directly related to physiological heat stress limits than the heat index.
Official sources
- NOAA NWS Heat Index: weather.gov/ama/heatindex.
- NWS Excessive Heat Watch/Warning/Advisory Criteria: weather.gov/safety/heat.
- Rothfusz, R.L. (1990). NWS Technical Attachment SR/SSD 90-23: The Heat Index (or, More Than You Ever Wanted to Know About Heat Index). NOAA.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.