The continental polar air masses of North America originate over north-central Canada and are characterized by low temperature and low moisture content. These air masses form tongues of cold air, which periodically extend south and east from the source region to produce anticyclones accompanied, in winter, by low temperatures and clear skies. Over the Arctic Ocean and its bordering lands of the arctic zone there develops the arctic air mass, which is extremely cold and stable. When this air mass invades the United States, it produces a severe cold wave.
Over the North Pacific and Bering Straits originate the maritime polar air masses. With ample opportunity to absorb moisture both over the source region and throughout their travel southeastward to the west coast of North America, these air masses are characteristically cool and moist, with a tendency in winter to become unstable, giving heavy precipitation over coastal ranges.
Another maritime polar air mass of the North American region originates over the North Atlantic Ocean. It, too, is cool and moist.
Note that the polar air masses originate in the sub arctic latitude zone not in the polar latitude zone. The meteorological definition of the word 'polar' for air masses has long been in use and has international acceptance, hence cannot be changed to confirm with the latitude zones.
Of the tropical air masses the commonest visitor to the central and eastern states is the maritime tropical air mass from the Gulf of Mexico. It moves northward, bringing warm, moist, unstable air over the eastern part of the country. In the summer, particularly, this air mass brings hot, sultry weather to the central and eastern United States. This air mass gives frequent thunderstorms. Closely related is a maritime tropical air mass from the Atlantic Ocean east of Florida, over the Bahamas.
During the summer there originates over northern Mexico, western Texas, New Mexico, and Arizona a tropical continental air mass which is hot and dry. This air mass does not travel widely, but governs weather conditions over the source region.
Over the Pacific Ocean in the cell of high pressure located to the southwest of Lower California is a source region of another maritime tropical air mass. It visits the United States only in winter and affects only the California coast.
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