An air mass is a vast body of air with consistent temperature and humidity characteristics throughout its horizontal expanse, adapting its properties from the surface over which it forms. A continental polar (cP) air mass is cold and dry, originating over large land areas at high latitudes.
Key Ingredients for Formation
The formation of a continental polar air mass requires specific atmospheric and geographical conditions. A large, flat landmass is necessary, allowing the air to settle and acquire uniform properties. High latitudes provide the cold temperatures needed for these air masses to develop.
During winter, long periods of darkness in these high-latitude regions contribute to significant radiative cooling of the surface. This process cools the overlying air by conduction. Stable, high-pressure systems are also crucial, promoting calm conditions and minimizing vertical air movement, allowing the air to remain in contact with the cold surface long enough to become uniformly chilled and dry. This extended contact is essential for the air mass to fully develop its cold, dry nature.
Primary Formation Regions
Continental polar air masses predominantly form in two major global regions: northern Canada and Siberia. These areas serve as ideal source regions due to their necessary geographical features and climatic conditions. Both locations are vast, relatively flat landmasses at high latitudes, experiencing prolonged darkness and extreme cold during winter.
In northern Canada, particularly over the interiors of Canada and Alaska, cP air masses frequently originate. The intense winter cold and stable atmospheric conditions allow the air to become very cold, dry, and stable. Similarly, northeastern Siberia is a significant source region, often associated with the Siberian anticyclone, a semipermanent high-pressure system formed by intense surface cooling. This produces some of the coldest and driest air masses in the Northern Hemisphere.
Influence on Weather Patterns
Once formed, continental polar air masses move out of their source regions, bringing distinct weather patterns to the areas they traverse. Their primary characteristics include cold temperatures, clear skies, and generally dry conditions. As these air masses spread southward, they can cause significant cold snaps, leading to widespread freezing temperatures.
When a continental polar air mass moves over warmer bodies of water, such as the Great Lakes in North America, it can lead to lake-effect snow. The cold, dry air picks up warmth and moisture from the relatively warmer lake surface. This added moisture then freezes and falls as snow on the downwind shores of the lakes, often resulting in heavy, localized snowfall. Beyond lake-effect snow, the arrival of a cP air mass typically brings stable weather with minimal cloud cover and reduced chances of precipitation.