Advection fog forms as warmer, moist air moves over a cold ground. The air is cooled to saturation by the cold from the ground below cooling the air above. Unlike radiation fog, advection fog may form under cloudy skies and with moderate to strong winds. Initial stability is relatively unimportant since low level cooling makes the air stable near the ground, allowing the fog to form. Once formed, it may move across the landscape, pushed by low level winds. Advection fog can last for several days and is most common in the U.S. on the West Coast.