Heat index
The heat index (HI) is an index that combines air temperature and relative humidity, in shaded areas, to posit a human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the shade.
For example, when the temperature is 32 °C (90 °F) with 70% relative humidity, the heat index is 41 °C (106 °F) (see table below).
The heat index is meant to describe experienced temperatures in the shade, but it does not take into account heating from direct sunlight, physical activity or cooling from wind.
High relative humidity reduces evaporation and cooling, increasing discomfort and potential heat stress.
[1][2] Like the wind chill index, the heat index contains assumptions about the human body mass and height, clothing, amount of physical activity, individual heat tolerance, 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.
[3] In Canada, the similar humidex (a Canadian innovation introduced in 1965)[4] is used in place of the heat index.
formed by the United States and Canada to resolve differences has since been disbanded.
[citation needed] The heat index of a given combination of (dry-bulb) temperature and humidity is defined as the dry-bulb temperature which would feel the same if the water vapor pressure were 1.6 kPa.
Quoting Steadman, "Thus, for instance, an apparent temperature of 24 °C (75 °F) refers to the same level of sultriness, and the same clothing requirements, as a dry-bulb temperature of 24 °C (75 °F) with a vapor pressure of 1.6 kPa.
At standard atmospheric pressure (101.325 kPa), this baseline also 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).
[1] A given value of relative humidity causes larger increases in the heat index at higher temperatures.
[6] The relative humidity threshold, below which a heat index calculation will return a number equal to or lower than the air temperature (a lower heat index is generally considered invalid), varies with temperature and is not linear.
[5] The heat index and its counterpart the humidex both take into account only two variables, shade temperature and atmospheric moisture (humidity), thus providing only a limited estimate of thermal comfort.
Additional factors such as wind, sunshine and individual clothing choices also affect perceived temperature; these factors are parameterized as constants in the heat index formula.
The other major factor is sunshine; standing in direct sunlight can add up to 15 °F (8.3 °C) to the apparent heat compared to shade.
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.
Because of this factor, it was once believed that the highest heat index reading actually attainable anywhere on Earth was 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 81 °C (178 °F).
[9] The human body requires evaporative cooling to prevent overheating.
[10] Thus a wet bulb temperature of 35 °C (95 °F) is the threshold beyond which the body is no longer able to adequately cool itself.
The columns begin at 80 °F (27 °C), but there is also a heat index effect at 79 °F (26 °C) and similar temperatures when there is high humidity.
[12] There are many formulas devised to approximate the original tables by Steadman.
Anderson et al. (2013),[13] NWS (2011), Jonson and Long (2004), and Schoen (2005) have lesser residuals in this order.
The formula below approximates the heat index in degrees Fahrenheit, to within ±1.3 °F (0.7 °C).
[1][14] This equation reproduces the above NOAA National Weather Service table (except the values at 90 °F (32 °C) & 45%/70% relative humidity vary unrounded by less than ±1, respectively).
The following coefficients can be used to determine the heat index when the temperature is given in degrees Celsius, where
An alternative set of constants for this equation that is within ±3 °F (1.7 °C) of the NWS master table for all humidities from 0 to 80% and all temperatures between 70 and 115 °F (21–46 °C) and all heat indices below 150 °F (66 °C) is:
For example, using this last formula, with temperature 90 °F (32 °C) and relative humidity (RH) of 85%, the result would be: 114.9 °F (46.1 °C).
[16] Other issues with the heat index include the unavailability of precise humidity data in many geographical regions, the assumption that the person is healthy, and the assumption that the person has easy access to water and shade.
