The heat index was developed in 1978 by George Winterling as the "humiture" and was adopted by the National Weather Service a year later. It is derived from work carried out by Robert G. Steadman. Like the wind chill index, the heat index contains assumptions about the human body mass and height, clothing, amount of physical activity, thickness of blood, sunlight and ultraviolet radiation exposure, and the wind speed. Large deviations from the typical values will result in heat-index values which do not accurately reflect the perceived temperature.