Arctic Air Masses develop around the Arctic, and are characterized by being cold from surface to great heights. The boundary of this air mass is often defined by the Arctic front, a semi-permanent, semi-continuous feature. When this air mass moves from its source region, it may become more shallow in height as it spreads southward. Continental Polar air masses occur over high latitude regions. Continental polar air (cP) is formed over cold surface regions and is typically very stable with low moisture. Maritime Tropical air masses form in the tropics or subtropics over the low latitudes. Maritime tropical air is produced over oceans and is warm and humid.