An air mass is a large volume of air defined by its relatively uniform temperature and moisture characteristics across a wide horizontal area. Air masses adapt to the surface properties of the region where they form. Meteorologists classify these air masses based on their latitude of origin and whether they form over land or water. The Maritime Polar air mass, often abbreviated as mP, is one of the distinct types that significantly influences the weather of coastal and continental regions.
What Defines a Maritime Polar Air Mass
The Maritime Polar air mass is primarily characterized as being cool and moist. Its designation uses the standard three-letter meteorological notation, where the lowercase “m” indicates a maritime, or moist, origin over the ocean. The capital “P” signifies its polar origin, meaning it develops over high latitudes, typically poleward of 40 degrees. This combination results in an air mass that is inherently colder than tropical air masses but carries considerably more moisture than air masses formed over land.
These air masses commonly originate over the northern parts of the Pacific and Atlantic Oceans. For North America, significant source regions include the North Pacific, often involving air masses that begin as continental polar air over Asia before moving eastward. The North Atlantic also serves as a source.
Maritime Polar air masses generally form over oceans between 40 and 60 degrees latitude. It often exhibits characteristics of being unstable.
The Mechanism of Formation
The Maritime Polar air mass is frequently a modified version of a Continental Polar (cP) air mass, illustrating a process called thermodynamic modification. Continental Polar air is initially very cold, dry, and stable, having formed over frozen landmasses in northern Canada or Siberia. The formation of mP air begins when this cold, dry air mass moves away from the continent and flows out over a relatively warmer ocean surface.
As the air mass travels over the warmer water, it undergoes two primary processes that change its characteristics. The first is heating from below, where heat is transferred from the relatively warm ocean surface into the cold air. This warming causes the lower layers of the air mass to become less dense and rise, creating atmospheric instability. This instability is a defining feature of the mP air mass, as it promotes vertical air movement.
The second process is the addition of moisture through evaporation from the ocean surface. The cold air passing over the water readily absorbs water vapor, significantly increasing the air mass’s humidity. This moisture, combined with the instability caused by the heating, means the air mass is primed for cloud formation and precipitation. The transition from a stable, dry cP air mass to an unstable, moist mP air mass is therefore fundamentally driven by the transfer of heat and moisture from the ocean.
Typical Weather Impact on Coastal Regions
When a Maritime Polar air mass reaches a coastline, the instability acquired during its formation dictates the resulting weather conditions. Because the air has been warmed from below, it is unstable and conducive to convection, leading to the development of cumulus and cumulonimbus clouds. This instability results in weather characterized by frequent showers, which can include rain, hail, or snow, particularly on the western and northern sides of continents.
The moisture-laden air is often near saturation, meaning that any forced lifting can easily result in precipitation. When the mP air encounters coastal mountain ranges, a phenomenon known as orographic lift occurs, forcing the air upward and causing it to cool, which leads to heavy rain or snow on the windward slopes. This effect is particularly noticeable in regions like the Pacific Northwest of the United States.
The mP air mass is also notorious for bringing widespread cloudiness and, in some cases, fog and drizzle. If the air mass moves over increasingly cold water or surfaces, the lower layers cool and become more stable, which can suppress the showery activity. This change results in weather conditions dominated by low clouds, extensive fog, and light, persistent precipitation like drizzle, instead of the heavy, convective showers associated with strong instability.