An air mass is a vast volume of air that acquires nearly uniform characteristics of temperature and humidity across its horizontal extent. These large bodies of air develop over homogenous surface regions, such as expansive oceans or continental plains, and can cover hundreds or thousands of square miles. Once formed, air masses migrate, bringing their distinct weather conditions to the mid-latitudes. The Continental Polar (cP) air mass is a primary driver of weather changes in the Northern Hemisphere, particularly during the colder months.
Origin and Properties of Continental Polar Air Masses
Continental Polar air masses originate over high-latitude land surfaces, including the interior plains of Northern Canada, Alaska, and Siberia. The formation process requires the air to remain stagnant for a substantial period over flat, snow-covered terrain. During the long winter nights, the ground radiates heat away, causing the overlying air to become significantly chilled and dense.
The designation “Continental” (c) indicates formation over land, resulting in a very low moisture content. The “Polar” (P) designation signifies that the air is cold, though it is distinctly warmer than the frigid air classified as Continental Arctic (cA).
This air is typically stable, meaning there is little vertical air movement, and is associated with areas of high surface pressure. The combination of low moisture and high stability ensures that the source regions often experience clear skies and calm conditions.
Typical Temperature Ranges and Seasonal Variation
The temperature of a Continental Polar air mass is highly dependent on the season and its proximity to the source region.
Winter Temperatures
Near its origin in winter, the air mass is intensely cold, with temperatures well below freezing being common. During severe outbreaks, they can dip to far below zero degrees Fahrenheit. As the cP air moves southward into the mid-latitudes, it undergoes modification by warming from the underlying surface. This warming causes the overall air mass temperature to gradually increase, though it remains cold relative to the air it displaces. The air mass is considered Polar, rather than Arctic, because it is generally warmer and less extremely cold than the cA air masses.
Summer Temperatures
The summer version of the cP air mass presents a stark temperature contrast. Source regions experience much longer daylight hours and increased solar heating, preventing the air from becoming intensely cold. When summer cP air moves into the mid-latitudes, it brings mild and dry weather. Daytime temperatures often range pleasantly from the 60s to the low 70s Fahrenheit.
Weather Phenomena Associated with cP Air
The migration of a Continental Polar air mass into a warmer region frequently leads to the formation of a cold front. This boundary marks the abrupt arrival of the dense, cold cP air, causing a rapid drop in temperature, often referred to as a cold snap or cold wave. Once the cP air has settled over a region, its stable, high-pressure nature typically results in clear skies and dry conditions.
A well-known phenomenon resulting from cP air movement is lake-effect snow. This occurs when the initially dry, cold air mass passes over a relatively warmer, unfrozen body of water, such as the Great Lakes. The air quickly picks up heat and moisture from the water surface, becoming unstable and saturated. This added moisture is then rapidly released as heavy, localized snowfall when the air cools again upon reaching the downwind shore.
The stable nature of the cP air mass also affects atmospheric mixing, which has implications for air quality. The high pressure and lack of vertical air movement can suppress the dispersion of pollutants near the surface. In urban areas, this can lead to the concentration of atmospheric contaminants, resulting in periods of reduced air quality or haze.