Apparent temperature, also known as "feels like", 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 who published a paper about it in 1984. It also applies, however, to indoor temperatures, especially saunas, and when houses and workplaces are not sufficiently heated or cooled.

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).

The Australian apparent temperature (AT), invented in the late 1970s, was designed to measure thermal sensation in indoor conditions. It was extended in the early 1980s to include the effect of sun and wind. The AT index used here is based on a mathematical model of an adult, walking outdoors, in the shade (Steadman 1984). The AT is defined as the temperature, at the reference humidity level, producing the same amount of discomfort as that experienced under the current ambient temperature and humidity.

The formula is:

$${\displaystyle \mathrm {AT} =T_{\mathrm {a} }+0.33e-0.7v-4.00,}$$

where:

The vapour pressure can be calculated from the temperature and relative humidity using the equation: $${\displaystyle e={\frac {\mathrm {RH} }{100}}\cdot 6.105\cdot \exp {\left({\frac {17.27\cdot T_{\mathrm {a} }}{237.7+T_{\mathrm {a} }}}\right)},}$$

where: