![]() ![]() The other major factor is sunshine standing in direct sunlight can add up to 15 ☏ (8.3 ☌) to the apparent heat compared to shade. Wind passing over wet or sweaty skin causes evaporation and a wind chill effect that the heat index does not measure. Wind, for example, is assumed to be 5 knots (9.3 km/h). 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 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. The threshold is commonly set at an arbitrary 40%. 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. It has been suggested that the equation described is valid only if the temperature is 27 ☌ (81 ☏) or more. For example, at approximately 27 ☌ (81 ☏), the heat index will agree with the actual temperature if the relative humidity is 45%, but at 43 ☌ (109 ☏), any relative-humidity reading above 18% will make the heat index higher than 43 ☌. Ī given value of relative humidity causes larger increases in the heat index at higher temperatures. At standard atmospheric pressure (101.325 kPa), this baseline also corresponds to a dew point of 14 ☌ (57 ☏) and a mixing ratio of 0.01 (10 g of water vapor per kilogram of dry air). This vapor pressure corresponds for example to an air temperature of 29 ☌ (84 ☏) and relative humidity of 40% in the sea-level psychrometric chart, and in Steadman's table at 40% RH the apparent temperature is equal to the true temperature between 26–31 ☌ (79–88 ☏). Quoting Steadman, "Thus, for instance, an apparent temperature of 24 ☌ (75 ☏) refers to the same level of sultriness, and the same clothing requirements, as a dry-bulb temperature of 24 ☌ (75 ☏) with a vapor pressure of 1.6 kPa." 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. A joint committee formed by the United States and Canada to resolve differences has since been disbanded. 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. While both the humidex and the heat index are calculated using dew point, the humidex uses a dew point of 7 ☌ (45 ☏) as a base, whereas the heat index uses a dew point base of 14 ☌ (57 ☏). In Canada, the similar humidex (a Canadian innovation introduced in 1965) is used in place of the heat index. Significant deviations from these will result in heat index values which do not accurately reflect the perceived temperature. ![]() ![]() 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. The heat index was developed in 1979 by Robert G. Besides the heat index, other measures of apparent temperature include the Canadian humidex, the wet-bulb globe temperature, "relative outdoor temperature", and the proprietary " RealFeel". Measurement of perceived temperature has been based on reports of how hot subjects feel under controlled conditions of temperature and humidity. Different individuals perceive heat differently due to body shape, metabolism, level of hydration, pregnancy, or other physical conditions. High relative humidity reduces evaporation and cooling, increasing discomfort and potential heat stress. ![]() The human body normally cools itself by evaporation of sweat. The heat index is meant to describe experienced temperatures in the shade, but it does not take into account heating from direct sunlight or physical activity, or cooling from wind. For example, when the temperature is 32 ☌ (90 ☏) with 70% relative humidity, the heat index is 41 ☌ (106 ☏) (see table below). 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. ![]()
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