5.3: Air Masses

Air Masses and Global Weather Patterns

An air mass is a vast pool of air spanning thousands of square miles that exhibits relatively uniform temperature and moisture characteristics throughout its horizontal extent. These masses develop within source regions, which typically feature flat topography and calm winds that minimize turbulent mixing. Within three to five days, radiation and vertical heat mixing establish an equilibrium between the surface conditions and the overlying air. Systems dominated by high pressure serve as excellent source regions because atmospheric subsidence pushes the air downward and drives it outward.

Air masses are classified based on the latitude and surface type of their source region. Latitude dictates temperature, dividing air masses into extremely cold Arctic air (60⁰ to 90⁰  latitude), cold Polar air (40⁰ to 60⁰ latitude), warm Tropical air (15⁰ to 35⁰ latitude), and exceedingly warm Equatorial air. The surface type determines humidity; maritime air masses forming over oceans are moist, whereas continental air masses forming over land are dry. Temperature also influences moisture capacity, meaning that cooler maritime polar air holds less moisture than warm maritime tropical air.

As air masses leave their source regions, their properties are modified by the terrain they traverse. For instance, when a cold continental polar air mass travels south over warmer land, heat transfers from the ground into the air, creating unstable conditions. This is designated by adding a k to the abbreviation, as seen in cPk. A notable example occurs in late autumn and early winter when cP air absorbs heat and moisture from the open Great Lakes; when this air encounters land, topographic barriers force it upward, generating lake-effect snow.

Conversely, an air mass can become stabilized if it passes over a colder surface. When an unstable maritime tropical air mass moves over the cold California Current, heat is stripped from the lower layer of the air mass. At the same time, upper-level subsidence from a subtropical high causes adiabatic warming aloft, which decreases or inverts the environmental lapse rate. This makes the air stable, designated with an s (mTs), which suppresses vertical uplift and diminishes the probability of precipitation. On the other hand, moving over a warm current like the Gulf Stream increases the instability of a maritime air mass, making precipitation more likely and adding a u to its classification (mTu).

Global and Regional Distribution of Air Masses

The classification of air masses based on their geographic source regions reveals a highly organized global pattern. By analyzing the temperature and moisture characteristics of the troposphere at a macro scale, scientists can map out where these massive air reservoirs form and how they interact on a regional level.

Global Distribution and Classification

The image Figure \(\PageIndex{1}\) illustrates the worldwide distribution of primary air mass source regions. This global view emphasizes that air masses are bounded by latitudinal zones and the underlying surface type, whether land or ocean.

Polar and Arctic Regions

In the high latitudes of the Northern Hemisphere, shown in blue shades on Figure \(\PageIndex{1}\), vast continental zones across northern North America and Eurasia form the source regions for continental Polar and continental Arctic air masses. These are labeled as cP + cA on the map. Because these landmasses are extremely cold and relatively flat, the air aloft loses heat rapidly to the ground, creating dense, dry, and freezing air reservoirs. In the extreme southern latitudes, the frozen continent of Antarctica serves as an isolated source region for continental Antarctic air, labeled as cAA, which represents the coldest and driest air mass type on Earth.

Flanking these cold continental masses are the high-latitude oceans, where maritime Polar air masses, labeled as mP, develop. These regions appear as light blue patches over the North Pacific, North Atlantic, and the southern oceans. While mP air masses are still cold, their contact with open ocean water makes them significantly more humid than their continental counterparts.

Figure \(\PageIndex{1}\): A global map displaying the geographic source regions and latitudinal distribution of planetary air masses. (CC BY SA 4.0; Michael E Ritter via The Physical Environment)

Figure \(\PageIndex{1}\) is a global map illustrating the distribution of air mass types by latitude and surface type, using color shading and abbreviations. The map uses blue tones for cold air masses and red tones for warm air masses, with labels placed over continents and oceans.

• High Latitude Regions (Cold Air Masses – Blue Shades) - At the top of the map in the Northern Hemisphere and at the bottom in Antarctica is labeled as “cP + cA” appears over northern North America and Eurasia, indicating continental Polar and continental Arctic air masses Label “mP” appears over the North Pacific and North Atlantic oceans, indicating maritime Polar air masses At the bottom over Antarctica, label “cAA” identifies continental Antarctic air, the coldest air mass
• Summary of Verbatim Labels Present - The map includes the following air mass abbreviations: cP, cA, cAA, mP, mT, cT, mE is labeled as “cT” appears over land areas such as northern Africa and Australia, indicating continental Tropical air masses Label “mT” appears over oceans surrounding these landmasses Along the equator, label “mE” marks maritime Equatorial air masses, forming a continuous band of warm, moist air
• Low Latitude Regions (Warm Air Masses – Red Shades) - Near the equator and subtropics:
  • Mid-Latitudes to Subtropics (Mixed Zones) - Transition zones between cold and warm air masses labeled “mP” continues over southern ocean regions “mT” appears along subtropical ocean regions, indicating maritime Tropical air masses.

Tropical and Equatorial Regions

Moving toward the lower latitudes, the color scheme on Figure \(\PageIndex{1}\) shifts to red tones, designating warm source regions. Continental Tropical air masses, labeled as cT, are restricted to large, arid landmasses. The most prominent cT source region is the Sahara Desert in northern Africa, with smaller regions located in the southwestern United States, southern Africa, and the interior of Australia. These air masses are exceptionally hot and dry due to intense solar radiation and minimal surface moisture.

Surrounding the subtropical landmasses are the vast warm oceans that produce maritime Tropical air masses, labeled as mT. These are found throughout the subtropical Pacific, Atlantic, and Indian Oceans. Because the water temperatures are high, mT air masses absorb massive amounts of moisture through evaporation, making them warm and highly humid.

Directly along the equator, the map displays a continuous band of maritime Equatorial air masses, labeled as mE. This zone experiences uniform, intense solar heating and contains exceptionally high moisture content, fueling persistent convective thunderstorm activity around the globe.

Air Mass Dynamics of North America

While the global map provides a broad overview, the image Figure \(\PageIndex{2}\) provides a regional look at how these distinct systems migrate and interact over the North American continent. The map illustrates the specific trajectories, source regions, and boundaries that dictate daily weather variations.

Figure \(\PageIndex{2}\): A regional map of North America showing air mass trajectories, the Arctic Front, and the Polar Front.  (CC BY SA 4.0; Michael E Ritter via The Physical Environment).

Figure \(\PageIndex{2}\) is titled “Air Masses of North America” and shows a map of North America with labeled air masses, directional arrows, and frontal boundaries.

1. The following air masses are labeled on the map:
   • “cA” (continental Arctic) across northern Canada, with red arrows pointing southward
   • “cP” (continental Polar) across central Canada extending into the northern United States
   • “mP” (maritime Polar) along both the Pacific Northwest and the North Atlantic, with arrows curving inland
   • “mT” (maritime Tropical) shown entering from the Gulf of Mexico, the western Atlantic, and the eastern Pacific
   • “cT” (continental Tropical) located over northern Mexico and the southwestern United States
2. Two dashed lines mark major fronts:
   • “Arctic Front” across northern Canada, separating cA from cP
   • “Polar Front” across the central United States, separating colder northern air (cP, mP) from warmer southern air (mT, cT)
3. Arrows indicate the movement of air masses:
   • Arrows from the north show cold air masses (cA and cP) moving southward
   • Arrows from oceans show mP and mT moving inland toward the continent
   • Arrows from the south show warm air masses (mT and cT) moving northward

Northern Cold Incursions

As depicted by the downward-pointing red arrows in Figure \(\PageIndex{2}\), northern Canada and the Arctic Ocean generate the continental Arctic (cA) and continental Polar (cP) air masses that frequently sweep southward into the central United States.

• The cA air mass originates at the highest polar latitudes and brings severe, bitter cold waves during the winter months.
• The cP air mass develops slightly further south over the Canadian subarctic forest, bringing cold, dry conditions in winter and cool, pleasant weather during summer.

The boundary separating the ultra-cold cA air from the slightly milder cP air to the south is indicated by the dashed blue line labeled as the Arctic Front.

Coastal Oceanic Influences

The maritime Polar (mP) air masses influence both the Atlantic and Pacific coasts of North America, as shown by the curved arrows on the upper left and right sides of Figure \(\PageIndex{2}\).

• The Pacific mP air mass originates over the North Pacific Ocean. As it migrates eastward into the West Coast, it brings cool, damp air and extensive cloudiness, often yielding heavy precipitation when forced over western mountain ranges.
• The Atlantic mP air mass originates over the colder waters of the northwestern Atlantic. Its impact is typically restricted to the northeastern seaboard, where it drives cool northeasterly winds and coastal fog.

Southern Warm Inputs

The lower half of Figure \(\PageIndex{2}\) highlights the warm air masses that counter the northern polar flows.

• The continental Tropical (cT) air mass forms over the desert regions of northern Mexico and the southwestern United States. As indicated by the upward arrow in the southwest, this air mass is strictly a summertime phenomenon, bringing hot, dry, and unstable conditions to the southern plains.
• The maritime Tropical (mT) air masses exert a powerful influence over eastern and western North America. The arrows entering from the Gulf of Mexico and the western Atlantic illustrate how warm, highly humid air continuously pumps moisture into the eastern half of the United States. This mT air is the primary source of fuel for summer thunderstorms, high humidity, and humid heatwaves. A secondary mT source region is visible off the southwestern coast, bringing warm, moist air into the Pacific coast at certain times of the year.

The Polar Front Intersection

The critical zone where these contrasting air masses collide is marked by the lower dashed blue line in Figure \(\PageIndex{2}\), labeled as the Polar Front. This boundary represents the structural clash between cold, dense polar air masses moving south and warm, buoyant tropical air masses moving north. The atmospheric instability generated along the Polar Front serves as the primary breeding ground for mid-latitude cyclones, frontal systems, and severe weather patterns across the continent.