An air mass is a vast volume of air characterized by nearly uniform temperature and humidity. These parcels acquire their traits by remaining stationary over a large surface region for an extended duration. A polar air mass originates in the high latitudes and is defined by its cold temperature. The movement of these cold, dense air masses significantly influences mid-latitude weather, often bringing dramatic temperature changes.
Formation and Core Properties
A polar air mass develops in high-latitude source regions (40 to 60 degrees north or south), where the sun angle is low and daylight hours are limited. To acquire its uniform, cold properties, the air must reside over a large, flat area for several days or weeks with minimal wind movement. This prolonged contact allows the air to cool efficiently, taking on the thermal characteristics of the underlying surface.
The cooling process causes the air to become very dense, leading to the formation of a high-pressure system and atmospheric stability, meaning the air resists vertical movement. The air mass is characterized by clear skies and extremely cold, dry conditions near the surface in its source region.
Categorization of Polar Air Masses
Polar air masses are classified based on the nature of their source region, which determines their moisture content. A Continental Polar (cP) air mass originates over frozen landmasses, such as interior Canada or Siberia. Since it forms over land, it is characterized as cold and dry.
A Maritime Polar (mP) air mass develops over open, high-latitude oceans. Its passage over the ocean allows it to absorb significant moisture through evaporation. Consequently, mP air masses are cool but moist, and they are generally less cold than their continental counterparts.
Influence on Weather Patterns
When a polar air mass moves away from its source region, its properties are modified by the surfaces it travels over. The advance of a cold, dense polar air mass often displaces warmer, less dense air, creating a cold front that results in a sharp temperature drop. Continental Polar air masses, being dry and stable, typically bring clear, sunny skies and bitter cold.
A specific phenomenon occurs when cold, dry cP air moves over the warmer water of large lakes, such as the Great Lakes. The air rapidly picks up moisture and heat, becoming unstable and saturated. This leads to the development of localized, heavy snowfall known as “lake effect” snow on the downwind shores. Maritime Polar air masses, already cool and moist, frequently bring overcast skies, drizzle, or light rain to coastal areas.