# Heat index

The heat index (HI) or humiture or humidex (not to be confused with the Canadian humidex) 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. Heat is removed from the body by evaporation of that sweat. However, relative humidity reduces the evaporation rate because the higher vapor content of the surrounding air does not allow the maximum amount of evaporation from the body to occur. This results in a lower rate of heat removal from the body, hence the sensation of being overheated. This effect is subjective; its measurement has been based on subjective descriptions of how hot subjects feel for a given temperature and humidity. This results in a heat index that relates one combination of temperature and humidity to another one at higher temperature and lower humidity.

## 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.[1] It is derived from work carried out by Robert G. Steadman.[2][3] 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.[4]

In Canada, the similar humidex is used in place of the heat index. While both the humidex and the heat index are calculated using dew point, the humidex uses a dew point of 45 °F (7 °C) as a base, whereas the heat index uses a dew point base of 57 °F (14 °C). Further, the heat index uses heat balance equations which account for many variables other than vapor pressure, which is used exclusively in the humidex calculation. A joint committee formed by the United States and Canada to resolve differences has since been disbanded.

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).[2] 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%.[5] 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", "relative outdoor temperature", or "feels like".

## 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 dew point was 35 °C (95 °F) while the temperature was 42 °C (108 °F), resulting in a heat index of 78 °C (172 °F). This is comparable to the temperatures that are recommended to kill bacteria in many meat products, and it is common in a sauna. High heat-index values also indicate that intense thunderstorms are approaching, depending on the intensity of the cold front, causing more violent storms.[6]

## Table of Heat Index values

This table is from the U.S. National Oceanic and Atmospheric Administration.

80 82 84 86 88 90 92 94 96 98 100 102 104 106 108 110 temperature (°F) Relative Humidity (%) 80 81 83 85 88 91 94 97 101 105 109 114 119 124 130 136 80 82 84 87 89 93 96 100 104 109 114 119 124 130 137 81 83 85 88 91 95 99 103 108 113 118 124 131 137 81 84 86 89 93 97 101 106 112 117 124 130 137 82 84 88 91 95 100 105 110 116 123 129 137 82 85 89 93 98 103 108 114 121 128 136 83 86 90 95 100 105 112 119 126 134 84 88 92 97 103 109 116 124 132 84 89 94 100 106 113 121 129 85 90 96 102 110 117 126 135 86 91 98 105 113 122 131 86 93 100 108 117 127 87 95 103 112 121 132
Caution
Extreme Caution
Danger
Extreme Danger

To find the Heat Index temperature, look at the Heat Index chart above. For example, if the air temperature is 96°F and the relative humidity is 65%, the heat index—how hot it feels—is 121°F.

This table is an approximation of the Heat Index, using the formula and first set of constants below, converted to Celsius.

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 temperature (°C) Relative Humidity (%) 27 28 29 30 31 32 34 35 37 39 41 43 46 48 51 54 57 27 28 29 30 32 33 35 37 39 41 43 46 49 51 54 57 27 28 30 31 33 34 36 38 41 43 46 49 52 55 58 28 29 30 32 34 36 38 40 43 46 48 52 55 59 28 29 31 33 35 37 40 42 45 48 51 55 59 28 30 32 34 36 39 41 44 48 51 55 59 29 31 33 35 38 40 43 47 50 54 58 29 31 34 36 39 42 46 49 53 58 30 32 35 38 41 44 48 52 57 30 33 36 39 43 47 51 55 31 34 37 41 45 49 54 31 35 38 42 47 51 57 32 36 40 44 49 54
Caution
Extreme Caution
Danger
Extreme Danger

## Effects of the heat index (shade values)

CelsiusFahrenheitNotes
27–32 °C80–90 °FCaution: fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps.
32–41 °C90–105 °FExtreme caution: heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke.
41–54 °C105–130 °FDanger: heat cramps and heat exhaustion are likely; heat stroke is probable with continued activity.
over 54 °Cover 130 °FExtreme danger: heat stroke is imminent.

Exposure to full sunshine can increase heat index values by up to 8 °C (14 °F).[7]

## Formula

The formula below approximates the heat index in degrees Fahrenheit, to within ±1.3 °F. It is the result of a multivariate fit (temperature equal to or greater than 80°F and relative humidity equal to or greater than 40%) to a model of the human body.[8][9] This equation reproduces the above NOAA National Weather Service table (except the values at 90°F & 45%/70% relative humidity vary unrounded by less than -1/+1, respectively).

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

where

$\mathrm{HI}\,\!$ = heat index (in degrees Fahrenheit)
$T\,\!$ = ambient dry-bulb temperature (in degrees Fahrenheit)
$R\,\!$ = relative humidity (percentage value between 0 and 100)
$c_1 = -42.379, \,\!$ $c_2 = 2.04901523, \,\!$ $c_3 = 10.14333127,\,\!$ $c_4 = -0.22475541, \,\!$ $c_5 = -6.83783 \times 10^{-3},\,\!$ $c_6 = -5.481717 \times 10^{-2},\,\!$ $c_7 = 1.22874 \times 10^{-3}, \,\!$ $c_8 = 8.5282 \times 10^{-4}, \,\!$ $c_9 = -1.99 \times 10^{-6}.\,\!$

An alternative set of constants for this equation that 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: $c_1 = 0.363445176, \,\!$ $c_2 = 0.988622465, \,\!$ $c_3 = 4.777114035, \,\!$ $c_4 = -0.114037667, \,\!$ $c_5 = -0.000850208, \,\!$ $c_6 = -0.020716198, \,\!$ $c_7 = 0.000687678, \,\!$ $c_8 = 0.000274954, \,\!$ $c_9 = 0 \,\!$ $(c_9 \,\!$ $unused).$

A further alternate is this:[10]

$\mathrm{HI} = c_1 + c_2 T + c_3 R + c_4 T R + c_5 T^2 + c_6 R^2 + c_7 T^2 R + c_8 T R^2 + c_9 T^2 R^2 + c_{10} T^3 + c_{11} R^3 + c_{12} T^3 R + c_{13} T R^3 + c_{14} T^3 R^2 + c_{15} T^2 R^3 + c_{16} T^3 R^3\ \,$

where

$c_1 = 16.923, \,\!$ $c_2 = 0.185212, \,\!$ $c_3 = 5.37941,\,\!$ $c_4 = -0.100254, \,\!$ $c_5 = 9.41695 \times 10^{-3},\,\!$ $c_6 = 7.28898 \times 10^{-3},\,\!$ $c_7 = 3.45372\times 10^{-4}, \,\!$ $c_8 = -8.14971 \times 10^{-4}, \,\!$ $c_9 = 1.02102 \times 10^{-5},\,\!$ $c_{10} = -3.8646 \times 10^{-5},\,\!$ $c_{11} = 2.91583 \times 10^{-5},\,\!$ $c_{12} = 1.42721 \times 10^{-6},\,\!$ $c_{13} = 1.97483 \times 10^{-7},\,\!$ $c_{14} = -2.18429 \times 10^{-8},\,\!$ $c_{15} = 8.43296 \times 10^{-10},\,\!$ $c_{16} = -4.81975 \times 10^{-11}.\,\!$

For example, using this last formula, with temperature 90 °F (32 °C) and relative humidity (RH) of 85%, the result would be: Heat index for 90 °F, RH 85% = 114.9.