Heat index

The heat index (HI) or humiture is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature — how hot it feels. The result is also known as the "felt air temperature" or "apparent temperature". For example, when the temperature is 90 °F (32 °C) with very high humidity, the heat index can be about 105 °F (41 °C).

The human body normally cools itself by perspiration, or sweating, which evaporates and carries heat away from the body. However, when the relative humidity is high, the evaporation rate is reduced, so heat is removed from the body at a lower rate, causing it to retain more heat than it would in dry air. Measurements have been taken based on subjective descriptions of how hot subjects feel for a given temperature and humidity, allowing an index to be made which relates one temperature and humidity combination to another at a higher temperature in drier air.

History

The heat index was developed in 1978 by George Winterling as the "humiture" and was adopted by the USA's National Weather Service a year later. It is derived from work carried out by Robert G. Steadman. Like the wind chill index, the heat index contains assumptions about the human body mass and height, clothing, amount of physical activity, thickness of blood, sunlight and ultraviolet radiation exposure, and the wind speed. Significant deviations from these will result in heat index values which do not accurately reflect the perceived temperature.

In Canada, the similar humidex is used in place of the heat index. The humidex differs from the heat index in using the relative humidity rather than the dew point.

The heat index is defined so as to equal the actual air temperature when the partial pressure of water vapor is equal to a baseline value of 1.6 kPa. At standard atmospheric pressure (101.325 kPa), this baseline corresponds to a dew point of 14 °C (57 °F) and a mixing ratio of 0.01 (10 g of water vapor per kilogram of dry air). This corresponds to an air temperature of 25 °C (77 °F) and relative humidity of 50% in the sea-level psychrometric chart.

At high temperatures, the level of relative humidity needed to make the heat index higher, than the actual temperature, is lower than at cooler temperatures. For example, at approximately 27 °C (80 °F), the heat index will agree with the actual temperature if the relative humidity is 45%, but at about 43 °C (110 °F), any relative-humidity reading above 17% will make the heat index higher than 43 °C.

The formula described is considered valid only if the actual temperature is above 27 °C (80 °F), dew point temperatures greater than 12 °C (54 °F), and relative humidities higher than 40%. The heat index and humidex figures are based on temperature measurements taken in the shade and not the sun, so extra care must be taken while in the sun. The heat index also does not factor in the effects of wind, which lowers the apparent temperature.

Sometimes the heat index and the wind chill are denoted collectively by the single term "apparent temperature" or "relative outdoor temperature".

Meteorological considerations

Outdoors in open conditions, as the relative humidity increases, first haze and ultimately a thicker cloud cover develops, reducing the amount of direct sunlight reaching the surface. Thus, there is an inverse relationship between maximum potential temperature and maximum potential relative humidity. Because of this factor, it was once believed that the highest heat index reading actually attainable anywhere on Earth is approximately 71 °C (160 °F). However, in Dhahran, Saudi Arabia on July 8, 2003, the dewpoint was 35 °C (95 °F) while the temperature was 42 °C (108 °F), resulting in a heat index of 78 °C (172 °F).

Table of Fahrenheit heat index values

This table is from the U.S. National Weather Service. Percent figures are relative humidity.

°F	90%	80%	70%	60%	50%	40%	°C
80	85	84	82	81	80	79	27
85	101	96	92	90	86	84	30
90	121	113	105	99	94	90	32
95		133	122	113	105	98	35
100			142	129	118	109	38
105				148	133	121	41
110						135	43

Effects of the heat index (shade values)

Celsius	Fahrenheit	Notes
27–32 °C	80–90 °F	Caution — fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps
32–41 °C	90–105 °F	Extreme caution — heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke
41–54 °C	105–130 °F	Danger — heat cramps and heat exhaustion are likely; heat stroke is probable with continued activity
over 54 °C	over 130 °F	Extreme danger — heat stroke is imminent

Note that exposure to full sunshine can increase heat index values by up to 8 °C (14 °F).

Formula

Here is a formula for approximating the heat index in degrees Fahrenheit, to within ±1.3 °F. It is the result of a multivariate fit (over temperatures at least 80°F and relative humidity at least 40%) to a model of the human body.

$\mathrm{HI} = c_1 + c_2 T + c_3 R + c_4 T R + c_5 T^2 + c_6 R^2 + c_7 T^2R + c_8 T R^2 + c_9 T^2 R^2\ \,$

where

$\mathrm{HI}\,\!$ = heat index (in degrees Fahrenheit)

$T\,\!$ = ambient dry-bulb temperature (in degrees Fahrenheit)

$R\,\!$ = relative humidity (in percent)

$c_1 = -42.38, \,\!$ $c_2 = 2.049, \,\!$ $c_3 = 10.14,\,\!$ $c_4 = -0.2248, \,\!$ $c_5 = -6.838 \times 10^{-3},\,\!$ $c_6 = -5.482 \times 10^{-2},\,\!$ $c_7 = 1.228\times 10^{-3}, \,\!$ $c_8 = 8.528 \times 10^{-4}, \,\!$ $c_9 = -1.99 \times 10^{-6}.\,\!$

An alternative set of constants for this equation which is within 3 degrees of the NWS master table for all humidities from 0 to 80% and all temperatures between 70 and 115 °F and all heat indexes < 150 °F is C1 = 0.363445176, C2= 0.988622465, C3=4.777114035, C4=-0.114037667, C5=-0.000850208, C6=-0.020716198, C7=0.000687678, C8=0.000274954, C9=0 (unused).

See also

• Humidex
• Wind chill

External links

• Description of wind chill & apparent temperature Formulae in metric units
• Heat Index Calculator Calculates both °F and °C