Apparent temperature, commonly known as "feels like," is a measure of how hot or cold the air actually feels to the human body, combining air temperature, humidity, and wind speed—a concept formalized by Robert G. Steadman in a 1984 paper. In hot, humid conditions above 27°C (81°F), the heat index (or humidex) shows how humidity makes the air feel much hotter by reducing sweat evaporation, while in cold conditions below 10°C (50°F), the wind chill factor accounts for how wind accelerates heat loss from the skin. A more comprehensive measure, the wet-bulb globe temperature (WBGT), also factors in solar radiation, but it is rarely used in official weather reports because it requires a specialized globe thermometer exposed to the sun. Since standard weather stations don't measure direct sunlight—which can add up to 15°F (8.3°C) to the perceived temperature—commercial services like AccuWeather's "RealFeel" and The Weather Company's "FeelsLike" use proprietary algorithms to estimate this solar effect. The Australian apparent temperature (AT), developed in the late 1970s and extended in the early 1980s, uses a mathematical model of an adult walking outdoors in the shade to incorporate humidity and wind, and its hot-weather version is what the U.S. National Weather Service calls the heat index.