Air Masses of North America | Climatology | Geography

In this article we will discuss about the wintertime and summertime air masses of North America.

1. Wintertime Air Masses:

i. Continental Polar Air Mass (cP):

Continental polar air mass is cold (K), dry and stable (s), and originates over the snow-covered cen­tral Canada to the north of 50°-60°N latitude, and Alaska while continental arctic air masses originate over arctic basin and Greenland ice cap. These air masses move out of their source regions and enter the USA between the Rocky Mountains and the Great Lakes.

Extensive land surface with topographic homo­geneity and slow anticyclonic circulation provide most ideal conditions for the origin and development of continental polar air mass. This air mass moves in southerly and south-easterly directions and brings ex­treme cold conditions. In fact, the arrival of this cP air mass produces intense cold waves in the vast area of the USA, with the result most of the places record temperature below freezing point.

After reaching the southern and south-eastern shores of the Great Lakes the continental polar cold air mass (cPK) is modified and thus becomes moist and unstable and yields heavy precipitation in the form of snowfall locally known as lake-effect snow. The modi­fied air mass, when moves eastward, is forced to move upward along the Appalachians, becomes unstable, clouds are formed and the western slopes of the Appa­lachian mountain receive heavy snowfall.

It may be pointed out that so long as the cold continental polar air mass moves over the snow-covered surface, it is least modified but as it crosses middle Illinois and enters the snow-free surface it is warmed from below and thus the cold air mass is modified to warm continental polar air mass (cPW) and the stability of the air mass decreases.

The continental polar warm air mass (cPW) while moving through east-central USA meets maritime tropi­cal (mT) air mass and polar front is formed which induces cyclonic conditions and winter precipitation occurs to the east of the Rocky Mountains.

The cPW air masses are modified in the south-eastern USA due to:

(i) Mechanical turbulence produced by corrugated ter­rains of the southern Appalachians,

(ii) Subsidence of air from above and resultant stability, and

(iii) Instabil­ity in the lower layer of the air masses caused due to addition of heat and moisture.

ii. Maritime Polar Pacific Air Masses (mP):

These air masses are called maritime polar pa­cific air masses because these originate in the northern parts of the North Pacific, mainly near Aleutian Islands where winter low pressure is formed. This region is surrounded by continental polar air mass from all sides except in the south. The water surface is warmer than the air lying over it. Consequently, when the cold maritime polar air mass comes over this area it is warmed from below and thus becomes unstable.

This mPKu air mass (maritime polar cold unstable air mass) picks up moisture throughout its journey south-east­ward to the west coast of North America. This moist air mass gives sufficient precipitation while rising along the western slopes of the Coast Ranges. During summer season polar Pacific air mass becomes stable.

When polar Pacific air mass reaches the Great Plains of the USA after crossing over the coastal ranges and the Rocky mountains, it undergoes the process of thermo­dynamic modifications and thus is transformed into cold, stable and dry continental polar air mass (cPWs) which induces anticyclonic conditions in the central states of the USA, with the result sky becomes clear and air circulation becomes slow and temperature returns to normal.

iii. Maritime Polar North Atlantic Air Mass (mP):

This air mass originates over the North Atlantic Ocean mainly in the region between Greenland, New­foundland, and Labrador where winter temperature ranges between 5°F and 40°F while during summer season it becomes 50° to 60°F. Since the general air circulation in this region is from west to east and hence this air mass has little influence on North America.

Sometimes cyclonic winds draw maritime polar North Atlantic air mass in the north-eastern parts of the USA mainly to the east of the Applachians and to the north of Cape Hatteras. This air mass is dry and stable in its upper layers while it is moist and unstable in the lower parts.

This air mass brings in bad weather which is locally known as ‘north-eastern’ characterized by strong northeast cold winds, exceedingly low temperature well below freezing point, high moisture content in the air and possible precipitation mainly in solid forms such as snowfall, sleet and hailstorms.

iv. Tropical Maritime Atlantic Air Masses:

These air masses (mT) originate over Gulf of Mexico, Caribbean Sea and subtropical western por­tion of the North Atlantic Ocean. They are hot, moist and unstable and are capable of unleasing heavy show­ers. These air masses affect most parts of the USA east of the Rocky Mountains. Temperature ranges between 21°C and 26°C and remains almost uniform in the source regions, with the result tropical maritime Atlan­tic air mass becomes warm and moist air mass.

It may be pointed out that it becomes very difficult to this air mass to enter the southern and central USA in winters because there is complete dominance of continental polar air mass over these areas. Whenever tropical maritime Atlantic air mass enters the USA it is cooled from below because it is much warmer than the ground surface and thus becomes dry and stable.

This modi­fied air mass is known as maritime tropical stable warm air mass (mTWs). This stable air mass is incapa­ble for precipitation but whenever it comes in contact with continental polar air mass, the upper air instability increases and the air mass is modified to maritime tropical warm unstable air mass (mTWu).

As this air mass rises along the mountain barriers it yields heavy showers. In summer the maritime tropical warm air mass coming from over Mexican Gulf brings hot and sultry weather in the southern and south-eastern states of the USA. This air mass also produces several thun­derstorms.

v. Tropical Maritime Pacific Air Masses:

Pacific air mass originates over the subtropical portions of the East Pacific west of USA and Mexico mainly over the high pressure area located to the southwest of California. The air mass becomes stable because of subsidence of air from above due to anticy­clonic circulation. This maritime tropical stable (mTks) air mass is dry, cold and stable near the Pacific coast of the USA. Whenever this air mass is associated with cyclonic circulation it becomes unstable and brings rains. This air mass seldom crosses the Rockies.

2. Summertime Air Masses:

i. Polar Continental Air Mass (cP):

This air mass originates over the snow-covered central Canada and Alaska. The temperature becomes relatively higher in summers than in winters but ad­joining oceanic areas have relatively low temperature. This air mass is cold, dry and unstable in the source regions but the air mass originating on cold Arctic Ocean during summer season is initially cold and stable.

The stability of the cold and stable arctic air mass disappears when this air mass moves southward over relatively warm ground surface and hence is heated from below. The continental cold air mass (cPK) becomes warm due to thermodynamic modifi­cation when it moves over the ocean. When this air mass moves southward from its source areas, it does not cover long distances but it extends for long dis­tances in the east.

The southward movement of conti­nental polar air mass brings chilly weather in the eastern and central USA due to which the effects of summer heat waves are eliminated and fine weather sets in. Whenever the continental polar air mass is associated with cyclonic circulation, it produces spo­radic rainfall in the north-central and eastern parts of the USA.

ii. Maritime Polar Atlantic Air Masses:

These air masses originate over the area located between Cape Cod and Newfoundland. Initially, these are cold and stable. These reach as far south as northern Florida where temperature is reduced by 15° to 25°F. Low temperature, clear sky and full visibility are the weather characteristics associated with these air masses.

Thus, these air masses produce fine and pleasant weather in the region extending from Newfoundland to Cape Hatteras. There is no ground fog due to dry condition.

iii. Maritime Tropical Atlantic Air Masses:

These air masses originate near Barmuda where high pressure is formed. They move northwestward and control the weather conditions of vast areas of the USA east of the Rocky Mountains during summer months. Thermally induced low pressure over south­ern and central USA draws maritime tropical air masses (mP) far inland but the existence of polar front in the vicinity of the Great Lakes restricts their entry to Canada.

Since temperature and moisture content in the air increases considerably due to arrival of these air masses in the central and eastern USA, the weather becomes oppressive and unpleasant. As these air-masses move out of their source areas and enter the USA after crossing over the Gulf of Mexico, surface temperature increases, and they are modified into maritime tropical unstable air masses (mTKu) because the heating of overlying relatively cold air mass causes atmospheric instability.

Thus, thunderstorms and cyclones are pro­duced which yield heavy showers. As the air mass moves northward it loses its moisture content and become dry in the upper Mississippi valley. When these air masses move westward and rise along the Rocky Mountains they yield heavy downpour with cloud burst. Similarly, when they cross over the Applachians they give heavy showers through thun­derstorms.

iv. Maritime Polar Pacific Air Masses (mP):

Maritime polar Pacific air masses originate in the area near Aleutian Islands in the North Pacific Ocean. The air mass becomes stable because of subsid­ence of air from above during summer season. Thus, this air mass becomes cold and stable (mPs). It may be pointed out that the continental surfaces are warmer than the water surfaces of the Pacific Ocean.

Thus, the maritime airmasses are warmed from below when they reach the continental areas. This causes greater turbu­lence in the lower layer, marked decrease in relative humidity, disappearance of clouds and dry weather. No doubt, the temperature of the west coastal areas of the USA, mainly Californian coasts, is reduced during summer season with the arrival of these air masses. After crossing over the Rocky Mountains maritime polar Pacific air masses are modified and resemble continental polar air masses (cP) in physical characteris­tics.

v. Maritime Tropical Pacific Air Masses (mT):

These air masses originate in the tropical North Pacific Ocean off the west coast of Central America. These air masses are marginalized because of the prevalence of maritime polar Pacific air masses along the west coasts of North America in summers.

vi. Continental Tropical Air Mass (cT):

This air mass originates in the source region comprising Mexico, western Texas (USA) and eastern New Mexico (USA). The daytime characteristics are high temperature, significantly low humidity and scant rainfall. This air mass moves to Great Plains and causes extreme arid conditions. It produces drought conditions if it stays for a long period over an area.