The field of meteorology relies on a standardized system of shorthand notation to communicate the characteristics of large, moving bodies of air known as air masses. These masses acquire specific properties of temperature and humidity from their point of origin, which are summarized in a simple two-letter code. This classification system allows scientists and forecasters to quickly convey the nature and potential impact of a weather system.
What Continental Polar (CP) Means
The abbreviation CP stands for Continental Polar, one of the four principal types of air masses affecting North America. This designation uses a two-part classification system. The first letter, “C,” indicates the air mass forms over land (Continental), meaning it is dry because little moisture is available for evaporation.
The second letter, “P,” denotes the air mass’s temperature characteristics (Polar). This signifies that the air originated in a high-latitude region, making it cold as it develops far from the equator.
Defining Characteristics of CP Air
A Continental Polar air mass is defined by its dry, cold, and stable physical properties. The low moisture content, resulting from its formation over land, means the air has a very low dew point. This dryness is a major factor in the clear, cloudless skies often observed when a CP air mass is dominant.
The air within a CP mass is cold, sometimes extremely so, and is dense due to this lower temperature. This density contributes to high atmospheric pressure at the surface, which is why CP air masses are frequently associated with strong, stable high-pressure systems. These high-pressure conditions tend to suppress vertical air movement, making the air mass highly stable. The combination of stability and low moisture limits the formation of significant clouds or widespread precipitation.
The stability of the air mass can lead to a temperature inversion near the ground, where temperatures increase with altitude instead of decreasing. This inversion traps pollutants and moisture near the surface, potentially leading to fog or poor air quality in urban areas. When CP air moves over a warmer surface, the lowest layer warms, but the air mass as a whole remains cold and dry.
Source Regions and Trajectories
Continental Polar air masses originate over vast, high-latitude continental landmasses, primarily interior Canada, Alaska, and Siberia. These regions are characterized by long nights, extensive snow cover, and intense radiative cooling of the ground, which cools the air above it. Snow cover is important because it reflects incoming solar radiation, further chilling the air mass.
Once formed, a CP air mass is pushed across the continent by upper-level winds, often plunging southward into the lower latitudes of the United States. This movement is responsible for the most intense cold snaps and freezing weather experienced across the central and eastern parts of the continent during winter. The trajectory of the air mass dictates the severity and location of the cold weather.
As the cold, dry air travels southward, it begins to modify by warming from the ground beneath it. If the air mass moves across a body of relatively warm water, such as the Great Lakes, it can pick up warmth and significant moisture. This process, known as air mass modification, results in the intense, localized weather phenomenon called lake-effect snow downwind of the water body. The air mass continues its journey, bringing clear and frosty conditions until it is eventually transformed.
How CP Air Masses Compare to Others
Continental Polar air is distinct due to its unique combination of temperature and moisture compared with other major air mass types. Maritime Polar (MP) air masses are also cold but are significantly wetter because they form over high-latitude oceans. MP air often brings cool, damp, and cloudy conditions, contrasting sharply with the dry, clear weather of CP air.
Maritime Tropical (MT) air masses, which form over warm ocean waters like the Gulf of Mexico, represent the opposite extreme. MT air is warm and very moist, making it the primary source of humidity, summer thunderstorms, and heavy precipitation. When a cold, dry CP air mass meets a warm, moist MT air mass, their boundary forms a cold front, which is a major mechanism for creating stormy weather.
Continental Tropical (CT) air, another continental type, is hot and dry, forming over desert regions in the summer, such as the southwestern United States. While CT air shares the dryness of CP air, its temperature is significantly higher. CT air typically brings stable, clear, and scorching conditions.