These air masses move around the America they can begin to acquire additional attributes.In winter an arctic air mass(cold and dry) can move over the ocean, picking up some warmth and moisture from the warmer ocean and becoming a maritime polar air mass. If that same polar air mass moves south from Canada into the southern U.S. it will pick up some of the warmth of the ground, but due to lack of moisture it remains very dry. This is called a continental polar air mass.