An air mass is a vast body of air possessing relatively uniform properties of temperature and moisture content. These characteristics are acquired from the source region where the air mass forms. Meteorologists classify these masses using a two-letter code based on moisture content—continental (c) or maritime (m)—and temperature—Polar (P) or Tropical (T). This yields four primary types: Continental Polar (cP), Continental Tropical (cT), Maritime Polar (mP), and Maritime Tropical (mT). This article focuses on the Continental Polar (cP) air mass.
The Far North Source Region
The “Continental” part of the name indicates that this air mass originates over large landmasses, while “Polar” signifies its formation at high latitudes, generally between \(40^{\circ}\)N and \(60^{\circ}\)N. The primary source regions for the Continental Polar air mass are the vast, interior areas of northern Canada, Alaska, and Siberia. These high-latitude regions lack significant water bodies and are often snow-covered for extended periods in winter.
The formation of a cP air mass is driven by radiational cooling, which is maximized during long winter nights when little incoming solar radiation is present. The land surface rapidly loses heat, cooling the air directly above it. This intense surface cooling makes the air dense and heavy, contributing to the formation of a large, semi-permanent area of high pressure. Light surface winds and flat terrain prevent the cold air from mixing with warmer air above, allowing the air mass to acquire uniform, cold characteristics.
Defining Temperature and Moisture Levels
The air that characterizes a Continental Polar air mass is defined by three properties: low temperature, low moisture content, and high atmospheric stability. The temperature is profoundly cold, particularly in winter, because the air has rested over a frigid, snow-covered land surface. While cP air is cold, it is not as cold as Continental Arctic (cA) air, which originates even further north.
The cP air mass is exceptionally dry, characterized by very low moisture levels. This dryness is a consequence of two factors: the continental source region and the properties of cold air. Since the air forms over land, it has no large body of water from which to draw moisture. Cold air has a significantly lower capacity to hold water vapor than warm air, so any existing moisture condenses out quickly in the source region. The result is an air mass with very low dew points.
The air mass is also characterized by high stability, meaning the air tends to resist vertical movement or convection. This stability is caused by intense cooling from below, which results in a temperature inversion—a layer where temperature increases with height near the surface. The denser, cold air remains trapped near the ground, making it difficult for the air to rise and produce clouds or precipitation.
Typical Weather Impact
When a Continental Polar air mass moves out of its source region, its properties translate directly into specific weather effects across mid-latitudes. The high stability and low moisture content often lead to clear skies and excellent visibility. The lack of cloud cover allows for maximum heat loss at night, resulting in severe frosts and the coldest temperatures experienced during the winter season.
The migration of a cP air mass is responsible for rapid temperature drops and persistent cold snaps that affect large areas. As the dense, cold air pushes southward, it displaces warmer air ahead of it, sometimes dropping temperatures by \(30\) to \(40\) degrees within a few hours. However, the air mass can be modified as it travels over a warmer surface.
A notable example of this modification occurs when the cold, dry cP air moves across the relatively warmer Great Lakes. The air quickly absorbs heat and moisture from the water surface, becoming less stable near the ground. As this now-moistened air hits the downwind shores, it rises, cools, and drops heavy, localized snowfall known as lake-effect snow. Though the air mass is initially dry, this interaction with a large water body can generate significant, localized weather.