Fog that forms over water is commonly referred to as sea fog or lake fog. It forms when warm, moist air flows over relatively colder waters. Sea or lake fog can occur over the Atlantic and Pacific Oceans, the Gulf of Mexico, the Great Lakes and other bodies of water. Fog is common along the U.S. Pacific coastline year round because the water is typically much colder than the nearby land. Sea fog is a type of advection fog, and therefore can move into land areas and result in hazards to motorists. Sometimes radiation fog that forms over land can move over bays, harbors, inlets, the intra-coastal and nearby ocean waters. While this is not pure sea fog, it can also be a concern for mariners. The National Weather Service issues Dense Fog Advisories when fog over water reduces visibility to 1 mile or less.