Air mass that forms over extensive ocean areas of the middle to high latitudes. Around North America, these air mass system form over the Atlantic and Pacific oceans at the middle latitudes. Maritime Polar air masses are mild and humid in summer and cool and humid in winter. In the Northern Hemisphere, maritime polar air masses are normally unstable during the winter. In the summer, atmospheric stability depends on the position of the air mass relative to a continent. Around North America, Maritime Polar air masses found over the Atlantic are stable in summer, while Pacific systems tend to be unstable.