Wind chill is the apparent temperature felt on exposed skin due to the combination of air temperature and wind speed. Except at higher temperatures, where wind chill is considered less important, the wind chill temperature (often incorrectly called the "wind chill factor") is always lower than the air temperature, because any wind increases the rate at which moisture evaporates from the skin and carries heat away from the body. The phase change of water (in sweat) from liquid to vapor requires that the molecules reach a higher energy state. That energy is acquired by absorbing heat from surrounding tissue by conduction (see heat transfer).
Air movement increases the rate at which the temperature of an object reaches the temperature of the ambient air. Humans feel this increased rate of heat transfer as wind chill.