Heat index

Common card · No. 316595

The heat index (HI) is a measure of temperature as perceived by humans, combining actual air temperature and relative humidity, in shaded areas; it takes into account that elevated humidity makes hot ambient temperature feel higher than the same temperature with lower humidity, as evaporative cooling of sweat is less effective. For example, when the temperature is 32 °C (90 °F) but relative humidity is high at 70% it feels as hot as 41 °C (106 °F) (the heat index) at low humidity (see § Table of values). The heat index relates to sensed temperatures in the shade without taking into account heating by direct sunlight and physical activity, or cooling by wind. The human body normally cools itself largely by evaporation of sweat. High relative humidity reduces evaporation and cooling, increasing discomfort and potential heat stress. Different individuals perceive heat differently on the basis of body shape, metabolism, level of hydration, pregnancy, other physical conditions, acclimatisation...

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