Source / pinned edition
The Heat Index Equation
Source of package us.nws.heat_index · NWS Weather Prediction Center, Heat Index Equation
- work
- urn:us:nws:heat-index#NWS_HEAT_INDEX
- number
- NWS Weather Prediction Center, Heat Index Equation
- kind
- urn:us:nws:heat-index#guidance
Page last modified 12 May 2022
- edition
- urn:us:nws:heat-index#NWS_HEAT_INDEX_2022
- language
- en
- officiality
- unofficial
- materialisation
- PINNED_UNOFFICIAL_COPY
- life cycle
- adopted: 2022-05-12; in_force: [object Object]
Publications
| URI | Type | documentHash | Retrieved |
|---|---|---|---|
| https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml | text/plain; charset=utf-8 | sha256:71eaa2e170304f1b8b0716e0b147ab7fe80bbd21308e9a2a4c36874c05141e5b | 2026-09-17T00:00:00Z |
Fragments — 1
equation · section
en · unofficial · sha256:050a98f19a5c71d5b38d73815214fa5a4f5e5aa40993925eb6093336fe21682d
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.The hash beside the text is the sha256 of its exact bytes. It is the check that the text shown matches the one pinned in the package: a divergence fails the oracle’s static analysis.