Utility functions for calculating Heat Index (HI).

Calculate heat index (HI) from air temperature and relative humidity.

Heat index is derived from original work carried out by Robert G. Steadman [1], which defined heat index through large tables of empirical data. The formula here approximates the heat index to within +/- 0.7C and is the result of a multivariate fit [2]. Heat index was adopted by the US’s National Weather Service (NWS) in 1979.

Note

[1] Steadman, R. G. (July 1979). “The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science”. Journal of Applied Meteorology. 18 (7): 861–873.

[2] Lans P. Rothfusz. “The Heat Index ‘Equation’ (or, More Than You Ever Wanted to Know About Heat Index)”, Scientific Services Division (NWS Southern Region Headquarters), 1 July 1990. https://www.weather.gov/media/ffc/ta_htindx.PDF

Parameters
• ta – Air temperature [C]

• rh – Relative humidity [%]

Returns

hi – Heat index [C]

Get the category of warning associated with a given heat index (HI).

Categories are used by the US National Weather Service (NWS) and National Oceanic and Atmospheric Administration (NOAA) to issue the following warnings:

• 0 = No Warning. Satisfactory temperature. Can continue with activity.

• 1 = Caution: Fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps.

• 2 = Extreme caution: Heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke.

• 3 = Danger: Heat cramps and heat exhaustion are likely. Heat stroke is probable with continued activity.

• 4 = Extreme danger: Heat stroke is imminent.

Parameters

hi – Heat index [C]

Returns

category – An integer indicating the level of warning associated with the heat index. Values are one of the following:

• 0 = No Warning

• 1 = Caution

• 2 = Extreme Caution

• 3 = Danger

• 4 = Extreme Danger