An air mass is an immense body of air maintaining relatively uniform temperature and moisture characteristics. These masses acquire their properties from the surface of their source region, where the air remains stationary for an extended period. Meteorologists classify air masses based on moisture content—continental (\(\text{c}\)) for dry air over land and maritime (\(\text{m}\)) for moist air over water—and temperature: polar (\(\text{P}\)) for cold or tropical (\(\text{T}\)) for warm. The Continental Polar (\(\text{cP}\)) air mass is a significant driver of weather across North America and Eurasia, known for ushering in cold, dry conditions.
Defining the Continental Polar Air Mass
Continental Polar air masses originate over high-latitude interior landmasses, such as central Canada, Alaska, and Siberia. The “Continental” designation means the air mass forms over land, resulting in low humidity and dry air. The “Polar” component indicates the air’s cold temperature, which results from settling over the frigid, often snow-covered surfaces of these northern regions.
During long winter nights, intense radiational cooling further chills the air near the ground in the source region. This process makes the air mass extremely cold and dense, contributing to high surface pressure and a stable atmospheric condition. The combination of dry and stable air ensures few clouds form, maintaining the air mass’s characteristic clear conditions. As a \(\text{cP}\) air mass moves southward, it carries these cold, dry, and stable properties into lower latitudes, dramatically changing the local weather.
Dominant Winter Weather Patterns
The movement of a \(\text{cP}\) air mass is responsible for the most significant cold-weather events across the central and eastern United States. When this body of cold, dense air plunges southward, it often brings a swift and pronounced drop in temperature, sometimes falling by 30 to 40 degrees Fahrenheit within hours. This phenomenon is the classic winter “cold snap,” frequently associated with a surface high-pressure system, sometimes referred to as an “Arctic High.”
The weather within an unmodified \(\text{cP}\) air mass is characteristically cold, dry, and accompanied by clear skies. The stability and low moisture content mean that cloud formation and precipitation are rare, leading to excellent visibility. The high-pressure center often results in relatively calm winds, though the initial intrusion of the cold front can produce strong, gusty conditions.
Despite the clear skies, the extreme cold can pose hazards, with temperatures often dropping far below freezing. The lack of insulating clouds allows for maximum outgoing longwave radiation, which further intensifies the cold, especially overnight. This dry, frigid air can slide to the Gulf Coast, occasionally bringing frost or freezing temperatures as far south as Florida and Texas. The air mass’s stability helps it maintain its properties unless it encounters a surface that forces modification.
Influence of cP Air During Summer Months
While the wintertime \(\text{cP}\) air mass is associated with extreme cold, its influence during the summer is far milder and often welcomed. As the air moves away from its source region in central Canada, it travels over warmer land surfaces. This movement causes the air mass to warm considerably from the ground up, lessening its density and stability.
The weather associated with summer \(\text{cP}\) air masses is typically pleasant, characterized by low humidity, comfortable temperatures, and fair skies. This air provides a temporary reprieve from the heat and humidity brought by Maritime Tropical air masses. The resulting high-pressure system is often referred to as “Canadian high pressure” and is associated with cool, dry conditions. Although the air mass is still relatively cool, the lack of cloud cover and moisture prevents the extreme cold experienced in the winter.
Modification and Lake Effect Phenomena
The dry nature of a \(\text{cP}\) air mass can be drastically altered when it moves over a warmer body of water, leading to the phenomenon known as lake effect. As the frigid, dry air flows across a large, relatively warm lake, such as one of the Great Lakes, the air mass is rapidly modified from below. The warmer water surface transfers both heat and moisture into the overlying cold air.
This process destabilizes the air mass, causing the lower layer to become warmer and saturated with water vapor from the lake. The heated, moisture-laden air rises through the colder air above, a process known as convection, which leads to the formation of towering clouds. Conditions are ideal when the air temperature at about 5,000 feet is significantly colder than the water surface, typically by at least 10 degrees Celsius.
The resulting localized, heavy snowfall is known as lake effect snow, which occurs on the downwind, or leeward, side of the water body. The amount and intensity of the snowfall depend on a long “fetch,” meaning the distance the air travels over the water. They also depend on hills or high terrain on the downwind shore, which forces the air to rise further. This intense modification of the \(\text{cP}\) air mass is responsible for localized snow accumulations in “snowbelt” regions, such as those east of the Great Lakes.