Instrument face indicating the value of the heat index at the intersection of its two hands (indicating temperature and relative humidity), with a chart showing windchill according to the temperature (indicated) and wind speed (acquired by some other method)
Apparent temperature, also known as "feels like",[1][2] is the temperature equivalent perceived by humans, caused by the combined effects of air temperature, relative humidity and wind speed. The measure is most commonly applied to the perceived outdoor temperature. Apparent temperature was invented by Robert G. Steadman[3] who published a paper about it in 1984.[4] It also applies, however, to indoor temperatures, especially saunas, and when houses and workplaces are not sufficiently heated or cooled.
The heat index and humidex measure the effect of humidity on the perception of temperatures above +27 °C (81 °F). In humid conditions, the air feels much hotter, because less perspiration evaporates from the skin.
The wind chill factor measures the effect of wind speed on cooling of the human body below 10 °C (50 °F). As airflow increases over the skin, more heat will be removed. Standard models and conditions are used.
The wet-bulb globe temperature (WBGT) combines the effects of radiation (typically sunlight), humidity, temperature and wind speed on the perception of temperature. It is not often used, since its measurement requires the use of a globe thermometer exposed to the sun, which is not included in standard meteorological equipment used in official weather conditions reporting (nor are, in most cases, any other explicit means of measuring solar radiation; temperature measurement takes place entirely in a shade box to avoid direct solar effects). It also does not have an explicit relationship with the perceived temperature a person feels; when used for practical purposes, the WBGT is linked to a category system to estimate the threat of heat-related illness.[5]
Since there is no direct measurement of solar radiation in U.S. observation systems, and solar radiation can add up to 15 °F (8.3 °C) to the apparent temperature, commercial weather companies have attempted to develop their own proprietary apparent temperature systems, including The Weather Company's "FeelsLike" and AccuWeather's "RealFeel". These systems, while their exact mechanisms are trade secrets, are believed to estimate the effect of solar radiation based on the available meteorological data that is reported (such as UV index and cloud cover).
A generalized view of the heat index showing how the perception of heat by the human body increases with temperature but more rapidly at higher humidity levels.
