Weather patterns are governed by immense, uniform bodies of air known as air masses. These air masses acquire specific characteristics, such as temperature and moisture, from their regions of origin. Meteorologists use a standardized, two-letter classification system to quickly identify these air masses, allowing for concise communication about current and expected weather conditions.
Defining Continental Polar (cP)
The abbreviation cP stands for Continental Polar, identifying an air mass that is cold and dry. The first, lowercase letter, ‘c’, signifies a continental origin, meaning the air mass formed over a large landmass, which results in very low moisture content and a dry atmosphere. The second, capital letter, ‘P’, denotes a polar origin, indicating that the air mass developed in high-latitude regions, generally poleward of 60 degrees latitude.
The cP classification is distinct from other types, such as Maritime Polar (mP), which is cool and moist, or Continental Tropical (cT), which is hot and dry. A cP air mass is also generally stable, meaning it resists vertical air movement, which contributes to its clear weather patterns.
Source Regions and Inherent Properties
Continental Polar air masses primarily originate over the interior landmasses of North America and Eurasia, specifically northern Canada, Alaska, and Siberia. These regions are ideal source areas because they are large, flat, and often snow-covered, allowing the air to sit stagnant and consistently take on surface characteristics. The air becomes intensely cold primarily through radiational cooling, which is effective during the long winter nights at these high latitudes. This cooling effect is amplified by the lack of large bodies of water, which prevents the air from picking up significant moisture. The resulting air mass is characterized by high surface pressure, very low temperatures, and extremely low dew points.
Weather Effects of Continental Polar Air
The arrival of a cP air mass over populated mid-latitude regions dramatically changes the weather, with the effects varying significantly between seasons. In the winter months, cP air masses are responsible for severe cold snaps and bone-chilling temperatures that can plunge 30 to 40 degrees Fahrenheit in a matter of hours. This air is generally associated with clear skies and high pressure, leading to fair but frigid weather.
As this extremely cold and dry air mass moves southward across the relatively warmer, unfrozen Great Lakes, it undergoes a transformation known as air mass modification. The air rapidly absorbs heat and moisture from the water surface, causing the air to become unstable and produce localized, heavy snowfall known as “lake-effect” snow. This phenomenon occurs when the air mass encounters the downwind shore, dumping the accumulated moisture as snow.
In the summer, the influence of a cP air mass is far less extreme but still noticeable and often welcomed. When it pushes south, the air mass brings a temporary reprieve from hot and humid conditions. The air, though still originating from high latitudes, has warmed considerably as it travels over the continent, but it remains significantly cooler and much drier than the warm, moist air it replaces, resulting in a period of comfortable, low-humidity weather with clear skies and reduced chance of precipitation.