Heat Index

Compare discomfort index

A measure of apparent temperature, that is, how hot the air seems to be. Everyone has experienced how a warm day with high humidity feels much hotter than another day with the same air temperature, but low humidity. Actually, in addition to temperature and humidity many other factors influence how hot we feel.

The National Weather Service determines the Heat Index using an extremely complicated model based on the work of R. G. Steadman. It basically relates air temperature to ways in which the skin, in particular the face, gains and loses energy. For example, part of the model deals with the effect humidity has on the evaportion of sweat, obviously a major way of cooling the skin.

Coming up with the heat index value in your local weather report involves much more than solving a single equation. However, curve-fitting techniques have been applied to the model's results to create an equation that uses only the air temperature and relative humidity. This equation gives a result that is within 1.3F of the real Weather Service result, and it is behind this calculator:

Heat Index (estimated, ± 1.3

Temperature 90°F

Relative Humidity 60%

Heat Index 99F

The National Weather Service gives the warnings these meanings:

Caution: Fatigue possible with prolonged exposure and/or physical activity.

Extreme Caution: Sunstroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity.

Danger: Sunstroke, heat cramps or heat exhaustion LIKELY, and heatstroke POSSIBLE with prolonged exposure and/or physical activity.

Extreme Danger: Heat/sunstroke HIGHLY LIKELY with continued exposure.

It is possible for the Heat Index to be lower than the air temperature. Experiment with the calculator and you will soon discover under what sort of conditions that occurs.

The model makes various assumptions. If they aren't true in a particular situation, a person may experience an apparent temperature even greater than the model predicts. For example, the model assumes a person standing in the shade. Standing in sunlight can make one feel 10 to 18°F hotter. The model assumes a 5-knot wind. A hot, stronger, wind will also raise the apparent temperature above the model's estimate. The model assumes a short-sleeved shirt, and so on. In other words, treat the heat index figure in a weather forecast as a helpful estimate, not as a magic shield that allows you to do more than is safe on a hot day.

In the United States, more people die each year from excessive heat than from all other weather-related causes combined. Many climatologists predict heat waves will be more common and more intense in the near future.


G. Brooke Anderson, Michelle L. Bell and Roger D. Peng.
Methods to Calculate the Heat Index as an Exposure Metric in Environmental Health Research.
Environmental Health Perspectives, v121, issue 10 (October 2013).

Discusses the variety of measures that have been called “heat index” in the literature.

Lans P. Rothfusz.
The Heat Index “Equation” (or, More Than You Ever Wanted to Know About Heat Index).
Technical Attachment 90-23 to Southern Topics (July 1990), a monthly publication of the NOAA/National Weather Service Southern Region.

A breezy but very informative description by a Weather Service professional. Available online at www.srh.noaa.gov/images/ffc/pdf/ta_htindex.PDF

R. G. Steadman.
The assessment of sultriness. Part I: A temperature-humidity index based on human physiology and clothing science.
Journal of Applied Meteorology, vol. 18, issue 7. pages 861-873. (July 1979).

The paper can be downloaded as a pdf from: www.webmail.climateknowledge.org/heat_waves/Doc1006_Steadman_Heat_Stress_Index_JApplMet_1979.pdf

R. G. Steadman.
The assessment of sultriness. Part II: Effects of wind, extra radiation and barometric pressure on apparent temperature.
Journal of Applied Meteorology, vol. 18, issue 7, pages 874–885. (July 1979).

The paper can be downloaded as a pdf from: vmtz-won-app1.cdc.gov/wonder/help/Climate/Steadman1979.PDF

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