Heat index

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, and even psychological factors. Measurement of perceived temperature has been based on reports of how hot subjects feel under controlled conditions of temperature and humidity. Besides the heat index, other measures of apparent temperature include the Canadian humidex, the wet-bulb globe temperature, the "relative outdoor temperature", and the proprietary "RealFeel". A term sometimes translated as "heat index" but not accounting for humidity is the national thermal indicator—indicateur thermique national (ITN)−of France, an average of temperature readings at 30 mainland weather stations used to track temperature trends and define heatwaves.