An air mass is a vast body of air with uniform characteristics of temperature and moisture across its horizontal expanse. These large atmospheric bodies acquire their traits from the region over which they form, known as the source region. Their movement drives day-to-day weather changes over large portions of the globe. This article focuses on the Continental Polar (cP) air mass, detailing its origin, properties, and the weather patterns it delivers.
Defining Continental Polar Air Masses
Continental Polar air masses develop over large, high-latitude land areas, typically the interior of Canada and Alaska, or Siberia. The “Continental” designation means the mass forms over land, resulting in dry air due to the lack of significant water sources. The “Polar” designation indicates formation in cold regions, making the air mass cold.
The coldness of this air mass results from long winter nights and rapid radiational cooling over snow-covered ground in its source region. This process forms a shallow layer of surface high pressure, often called an “Arctic High,” characterized by low temperatures and low dew points. Since cold air holds little water vapor, cP air is both cold and very dry. The frigid air near the surface also makes the air mass highly stable, meaning it resists vertical movement.
Winter Weather Phenomena
The southward movement of a Continental Polar air mass during winter is responsible for significant weather changes in the mid-latitudes. When this dense, cold air displaces warmer air, it results in a rapid drop in temperature known as an arctic outbreak or cold wave. This cold wave can lead to hard freezes that affect agriculture and infrastructure far south of the source region.
Continental Polar air is associated with high pressure, which brings fair weather and clear skies. The air mass is so dry that it prevents extensive cloud cover, allowing for strong solar heating during the day but intense radiational cooling at night. Regions under the influence of a cP air mass experience bright, sunny, but frigid days.
A specific weather phenomenon occurs when this cold, dry air mass moves across the Great Lakes. As the cP air moves over the warmer lake water, it rapidly absorbs heat and moisture from the surface. This causes the lower layers of the air mass to become unstable and rise, leading to narrow bands of clouds and precipitation. This results in highly localized snowfall, known as lake effect snow, on the downwind shores of the Great Lakes.
Summer Weather Phenomena
While the Continental Polar air mass is known for its winter characteristics, its summer influence is important. As the air mass moves southward over warmer land, it is modified, losing its cold nature. However, it maintains its low moisture content, which defines its summer weather.
When a cP air mass pushes into regions dominated by warm, humid air, it provides relief from muggy conditions. This air mass reduces both temperature and humidity. The air arriving from the north is cool and dry, displacing the moisture-laden air.
The stability and low moisture content of the summer cP air mass result in clear, sunny days. The dry air prevents daytime heating from forming deep, moisture-filled clouds and thunderstorms. The skies are bright and the air feels crisp, creating comfortable, low-humidity conditions.