Description of the Continental Polar Air Mass

According to USA Today's "The Weather Book," air masses form mainly in the polar or tropical regions, since air in the middle latitudes moves much more freely. Thus, in the United States, the weather changes as lunging air masses of the northern polar areas and the southerly tropics alternate. The continental polar air mass is aptly named, because it forms over cold land near the poles but away from oceans. It occurs in North America, Europe and Asia.

Origins

• The continental polar air mass develops over land in close proximity to or within the polar region; in the Northern Hemisphere, that is above the Arctic Circle at just over 66 degrees latitude. In North America this air mass usually originates in Alaska or northwestern Canada and then moves southward. In Europe it forms over Finland, Lapland and Russia; and in Asia, anywhere in Siberia.

Properties

• This air mass contains cool to cold air that contains little humidity and is stable. This differentiates it from polar maritime air, which is just as cold or colder but may have higher humidity, because it develops over the Arctic Ocean's open waters in the Northern Hemisphere in summer.

  Continental polar air occurs in all seasons but contains considerably colder air in winter than if it is developing in summertime. Only Arctic air masses have colder temperatures.

Modification

• As cold continental air moves south, driven by the jet stream, it modifies its temperature as it warms slowly while passing over land. In winter, the warming can be diminished if the air mass is passing over land covered in snow. It may also gain a little moisture if it passes over open bodies of water such as huge lakes or smaller seas and bays.

  In "The Weather Book," a good example of this modification is shared: A continental polar air mass of -20 to -40 degrees Fahrenheit that originates in Alaska in January may warm to 25 to 50 degrees by the time it reaches the Gulf of Mexico. Moreover, temperatures that dip to the low 20 degrees in Florida occur when snowcover blankets the American Midwest, reducing the amount of heat the air mass gains as it travels south.

Effects of Mountains

• The southerly movement of the continental polar air mass can be slowed or blocked by mountain ranges, especially those with many high montane peaks. In North America, the north-to-south line of the Rockies allows this air mass to readily plunge to the south. In Europe and Asia, both the east-to-west Alps and the Himalayas can prevent penetration of the air mass to the Mediterranean Sea or Indian subcontinent, respectively.

Aviation Conditions

• Pilots of airplanes experience different atmospheric conditions when continental polar air masses migrate southward in winter or summer. For example, if they fly over the source region of the air mass, they experience good visibility, unlimited ceilings and clear skies in winter and perhaps occasional cumulus clouds in summer.

  As the continental polar air migrates further south and rolls over warm soil or the Great Lakes, for example, lower cloud decks may form, with some flurries in winter, or just low stratus clouds in summer along the Pacific Coast in the United States.