Air Masses of North America

We have understood the theory of air masses; now we’ll see how it plays out in the real world — specifically in North America and Asia. In this section we will discuss North America.

A. Winter Air Masses

In this season, the continent experiences interactions between polar, tropical, continental, and maritime air masses — creating a wide variety of weather patterns.

1. Continental Polar Air Mass (cP)

Origin:

• Snow-covered central Canada (north of 50°–60°N) and Alaska.
• Arctic variant forms over the Arctic Basin and Greenland ice cap.

Source Region Conditions:

• Flat, extensive land; slow-moving high pressure (anticyclone) → ideal for cP formation.
• Extremely cold, dry, and stable.

Movement and Effects:

• Moves south and southeast into USA between Rocky Mountains and Great Lakes.
• Brings extreme cold waves — many areas record temperatures below freezing.

Lake-Effect Snow:

• When cP air crosses warmer southern & southeastern shores of the Great Lakes → picks up moisture → becomes moist, unstable → produces heavy local snowfall (lake-effect snow).

Appalachian Impact:

• Moving eastward, cP air is lifted along the Appalachians → instability → heavy snowfall on western slopes.

Modification Over Land:

• Over snow-covered land → little change.
• Over snow-free areas like middle Illinois → warmed from below → becomes warm continental polar air mass (cPW) → less stable.
• Meets maritime tropical (mT) air mass → polar front forms → cyclonic activity → winter precipitation east of Rockies.

Further Modification in SE USA:

• Turbulence from Appalachian terrain.
• Subsidence from above → stability.
• Heat and moisture addition → instability in lower layers.

2. Maritime Polar Pacific Air Mass (mP)

Origin:

• Northern North Pacific, near Aleutian Islands (winter low-pressure zone).

Properties at Source:

• Cold, moist; water warmer than air → heated from below → unstable (mPKu).

Path and Effects:

• Travels southeast toward US west coast → gains moisture.
• Rising along western slopes of Coast Ranges → heavy precipitation.

Summer: Becomes stable.

Modification Over Interior:

• After crossing Coastal Ranges & Rockies → dries out, becomes cold, stable, dry continental polar warm stable (cPWs).
• Induces anticyclonic conditions in central USA → clear skies, slow winds, normalised temperatures.

3. Maritime Polar North Atlantic Air Mass (mP)

Origin:

• North Atlantic between Greenland, Newfoundland, and Labrador.
• Winter temps: -15°C to -9°C; summer: 10°C–16°C.

Impact on North America:

• Limited influence due to prevailing westerlies.
• Occasionally pushed into NE USA east of Appalachians & north of Cape Hatteras by cyclonic systems.

Properties:

• Upper layers → dry, stable.
• Lower layers → moist, unstable.

Local Name: Nor’easter

• Strong NE winds, sub-freezing temperatures, high moisture, and precipitation (snow, sleet, hail).

4. Maritime Tropical Atlantic Air Mass (mT)

Origin:

• Gulf of Mexico, Caribbean Sea, subtropical western North Atlantic.

Properties at Source:

• Hot, moist, unstable; temps 21°C–26°C, nearly uniform.

Winter Entry:

• Often blocked by dominant cP air in southern/central USA.
• If it enters → warmer than land surface → cooled from below → maritime tropical warm stable (mTWs) → generally dry and stable.

Modification:

• Contact with cP air increases upper-air instability → maritime tropical warm unstable (mTWu) → heavy showers, especially on windward mountain slopes.

Summer Impact:

• Brings hot, humid, sultry weather in southern & southeastern USA.
• Frequent thunderstorms.

5. Maritime Tropical Pacific Air Mass (mT)

Origin:

• Subtropical East Pacific, west of USA & Mexico; high-pressure zone SW of California.

Properties:

• Subsiding air from above → stable (mTks), dry, cool near Pacific coast.

Rain Potential:

• Associated with cyclonic activity → instability → rainfall.
• Rarely crosses Rockies.

✅ In summary:
In winter, North America is like a meteorological battleground — with cold, dry cP air masses pushing south, moist unstable mP air masses feeding precipitation on west coasts, and warm mT air masses from the Gulf occasionally breaking through to bring rain or storms. The outcome depends on the paths, modifications, and interactions of these air masses.

B. Summer Air Masses

In summer, the patterns of air mass movement over North America shift. The contrasts between warm tropical and cooler polar air masses remain important, but the temperature differences are less extreme than in winter, and moisture becomes a bigger player in shaping weather.

1. Polar Continental Air Mass (cP)

Origin:

• Snow-covered central Canada and Alaska.

Source Region Conditions:

• Warmer than winter but still cooler than tropical areas.
• Cold, dry, unstable in source; Arctic-origin variant in summer is cold and stable initially.

Modification:

• Moving south over warm ground → heated from below → instability increases.
• Moving over ocean → becomes warm due to thermodynamic modification.

Movement & Impact:

• Limited southward reach, but extends far east.
• Brings chilly, pleasant weather to eastern and central USA, reducing summer heat waves.
• When linked with cyclonic systems → sporadic rainfall in north-central and eastern USA.

2. Maritime Polar Atlantic Air Mass (mP)

Origin:

• Region between Cape Cod and Newfoundland.

Characteristics:

• Initially cold and stable.
• Can reach as far south as northern Florida → lowers temperature by 15–25°F.

Weather Effects:

• Clear skies, full visibility, low humidity → fine, pleasant summer weather from Newfoundland to Cape Hatteras.
• No ground fog (dry lower layers).

3. Maritime Tropical Atlantic Air Mass (mT)

Origin:

• Near Bermuda (subtropical high-pressure zone).

Path & Spread:

• Moves northwest, dominates much of USA east of Rockies in summer.
• Drawn inland by thermal low pressure over southern & central USA.
• Polar front near Great Lakes blocks entry into Canada.

Effects:

• Increases temperature & humidity → oppressive, sultry weather in central/eastern USA.
• Crossing Gulf of Mexico warms the lower layers → instability → becomes maritime tropical unstable (mTKu).

Weather Outcomes:

• Produces thunderstorms and cyclones with heavy showers.
• Moving north → loses moisture over Upper Mississippi Valley.
• Rising over Rockies → heavy downpours, sometimes cloudbursts.
• Crossing Appalachians → thunderstorm showers.

4. Maritime Polar Pacific Air Mass (mP)

Origin:

• Near Aleutian Islands, North Pacific.

Summer Properties:

• Stable due to subsiding air → cold, stable (mPs).

Modification Over Land:

• Continental surfaces warmer than Pacific waters → warmed from below → more turbulence in lower layers, reduced humidity, clouds disappear → dry weather.

Impact:

• Lowers coastal California temperatures in summer.
• Crossing Rockies → resembles cP air mass in properties.

5. Maritime Tropical Pacific Air Mass (mT)

Origin:

• Tropical North Pacific, off west coast of Central America.

Impact:

• Minor role in summer due to dominance of mP Pacific air masses along west coast.

6. Continental Tropical Air Mass (cT)

Origin:

• Mexico, western Texas, eastern New Mexico.

Characteristics:

• Very hot daytime temperatures, low humidity, minimal rainfall.

Impact:

• Moves into Great Plains → causes extreme arid conditions.
• Long duration presence → drought development.

✅ In summary:
Summer air masses in North America bring more varied weather than winter, with moisture-laden mT air masses often clashing with cooler cP or mP masses to produce thunderstorms, while dry cT air masses can trigger droughts. The balance between heat, moisture, and stability determines whether regions experience oppressive humidity, pleasant cool spells, or destructive storms.