Skip Navigation Links 
NOAA logo - Click to go to the NOAA homepage National Weather Service   NWS logo - Click to go to the NWS homepage
The Weather Prediction Center



Follow the Weather Prediction Center on Facebook Follow the Weather Prediction Center on Twitter
NCEP Quarterly Newsletter
WPC Home
Analyses and Forecasts
   National High & Low
   WPC Discussions
   Surface Analysis
   Days ½-2½ CONUS
   Days 3-7 CONUS
   Days 4-8 Alaska
   Flood Outlook
   Winter Weather
   Storm Summaries
   Heat Index
   Tropical Products
   Daily Weather Map
   GIS Products
Current Watches/

Satellite and Radar Imagery
  GOES-East Satellite
  GOES-West Satellite
  National Radar
Product Archive
WPC Verification
   Medium Range
   Model Diagnostics
   Event Reviews
   Winter Weather
International Desks
Development and Training
WPC Overview
   About the WPC
   WPC History
   Other Sites
Meteorological Calculators
Contact Us
   About Our Site is the U.S. Government's official web portal to all federal, state, and local government web resources and services.

The Heat Index Equation
The computation of the heat index is a refinement of a result obtained by multiple regression analysis carried out by Lans P. Rothfusz and described in a 1990 National Weather Service (NWS) Technical Attachment (SR 90-23).  The regression equation of Rothfusz is
HI = -42.379 + 2.04901523*T + 10.14333127*RH - .22475541*T*RH - .00683783*T*T - .05481717*RH*RH + .00122874*T*T*RH + .00085282*T*RH*RH - .00000199*T*T*RH*RH
where T is temperature in degrees F and RH is relative humidity in percent.  HI is the heat index expressed as an apparent temperature in degrees F.  If the RH is less than 13% and the temperature is between 80 and 112 degrees F, then the following adjustment is subtracted from HI:
ADJUSTMENT = [(13-RH)/4]*SQRT{[17-ABS(T-95.)]/17}
where ABS and SQRT are the absolute value and square root functions, respectively.  On the other hand, if the RH is greater than 85% and the temperature is between 80 and 87 degrees F, then the following adjustment is added to HI:
ADJUSTMENT = [(RH-85)/10] * [(87-T)/5]
The Rothfusz regression is not appropriate when conditions of temperature and humidity warrant a heat index value below about 80 degrees F. In those cases, a simpler formula is applied to calculate values consistent with Steadman's results:
HI = 0.5 * {T + 61.0 + [(T-68.0)*1.2] + (RH*0.094)}
In practice, the simple formula is computed first and the result averaged with the temperature. If this heat index value is 80 degrees F or higher, the full regression equation along with any adjustment as described above is applied.

The Rothfusz regression is not valid for extreme temperature and relative humidity conditions beyond the range of data considered by Steadman.

NOAA/ National Weather Service
National Centers for Environmental Prediction
Weather Prediction Center
5830 University Research Court
College Park, Maryland 20740
Weather Prediction Center Web Team
Privacy Policy
About Us
Career Opportunities
Page last modified: Thursday, 12-May-2022 19:37:55 UTC