Air masses are expansive volumes of air that exhibit consistent temperature and moisture properties throughout their extent. They can stretch for thousands of miles horizontally and vertically. The characteristics an air mass possesses are primarily determined by its source region. These large bodies of air transport warmth, cold, dryness, or moisture from their source areas to other regions, influencing local weather patterns.
What Defines a Polar Air Mass
A polar air mass is a large body of air that forms over high-latitude regions, typically between 40° and 60° north or south latitude. These air masses are inherently cold from less direct solar radiation. The air within a polar air mass adopts the temperature conditions of its source region, acquiring uniformly low temperatures from extended contact with cold surfaces.
Polar air masses are colder than tropical air masses but not as intensely cold as arctic air masses. The moisture content of a polar air mass depends on whether it forms over land or water. Air masses originating over continents are usually dry, while those forming over oceans tend to be moist. This influences precipitation as they move.
Types of Polar Air Masses
Polar air masses are categorized into two types: continental polar (cP) and maritime polar (mP). Continental polar air masses develop over large, high-latitude landmasses like northern Canada, Siberia, or central Asia. These air masses are cold and dry, forming over vast, often snow-covered land areas with limited moisture. In contrast, maritime polar (mP) air masses originate over the cold ocean waters in high latitudes, like the North Atlantic and North Pacific Oceans.
While still cold, these air masses are more moist than their continental counterparts. Maritime polar air can sometimes originate as continental polar air that moves over warmer ocean waters, picking up heat and moisture, becoming modified.
How Polar Air Masses Form and Move
The formation of polar air masses involves atmospheric processes, with radiative cooling playing a role. Over snow and ice-covered surfaces in high latitudes, the ground radiates heat away efficiently, cooling the overlying air. This prolonged cooling leads to the development of a stable layer of cold, dense air near the surface. High-pressure systems often coincide with these source regions, with light winds and sinking air, which allows the air mass to stagnate and acquire surface temperature and moisture properties.
Once formed, polar air masses move away from their source regions, driven by global wind patterns and pressure gradients. Their motion is influenced by upper atmosphere airflow, including the jet stream. As these cold, dense air masses move into mid-latitude regions, they can influence local weather. When encountering warmer air masses, they form fronts, leading to weather phenomena like precipitation and temperature drops.