The continent of North America is triangular in shape with it base towards the north in the higher latitudes, and apex towards the south in the low latitudes. Thus, because of its peculiar shape and geographical location, the continent is considered to be ideal for the formation of air masses with different physical properties.
The following section attempts to examine in brief the physical properties of the principal air masses of North America season-wise.
Winter Air Masses
(1) Continental polar (cP) air masses:
These air masses originate over the snow-covered interior regions of Canada and Alaska. Continental arctic air masses form over the Arctic basin and the Greenland ice cap. They are extremely cold and dry. Some writers distinguish the continental arctic air from cP air by its lower temperatures.
But some meteorologists make no distinction between the two. The cP air masses are generally stable in their source regions. Because of the prolonged earth radiation the surface temperature is very low. This results in a strong and persistent temperature inversion in the atmosphere. Clouds are almost non-existent.
These air masses enter the United States of America between the Rocky Mountains and the Great Lakes in the form of extremely cold polar anticyclones. The onslaught of these rapidly moving air masses makes the east-central United States abnormally cold. Willett and Byers refer to these as continental Arctic (cA) air or maritime Arctic (mAK) air. But according to the modern classification they are designated as polar air masses.
After entering the southern and eastern United States, the winter cP gets modified by coming into contact with the warm surface. Addition of moisture along with mechanical turbulence renders the continental polar air rather unstable in the lower-layers.
By the time the cP reaches the south and eastern shore of the Great Lakes, the cPK becomes very moist and unstable which results in abundant lake-effect snow.
As the modified cP air masses move towards the rugged terrain of the Appalachians, they are forced to rise; so that the sky becomes overcast and heavy snowfall occurs on the western side of the mountains.
On the other side of the Appalachians, the descending air masses are warmed adiabatically. The latent heat released during condensation evaporates the clouds, and the snowfall ends.
In the east-central United States the cPW air masses meet the mT air masses and a polar front comes into existence. At the polar front thus formed, the mTK overrides the cPW along the warm front which results in winter precipitation in the areas east of the Rocky Mountains.
Because of the evaporation of precipitation falling from the warm front clouds, stratus clouds form beneath the inversion layer in the cPW. Ground is sometimes covered with dense fogs. By the time the cP air masses reach southeastern United States, they are modified on account of the following:-
(a) Mechanical turbulence produced by the rugged terrain.
(b) Stability produced by the subsidence of the air aloft.
(c) Instability produced in the lower part of the air masses because of addition of heat and moisture from the surface.
As the cP air masses reach the latitude of central Illinois, the lower layers get additional heat and moisture from the surface. However, these factors have little modifying influence on them. Their upper-level stability continues to persist.
The sky becomes cloudless and the lower temperature of the season is recorded. On reaching the Gulf of Mexico, the mTKs air masses are involved in cyclonic circulation and produce heavy precipitation locally.
Between the great Lakes and the Atlantic coastal areas the continental polar air has different characteristics. Because of cyclonic circulation, the upper-level stability diminishes and the clouds yield snowfall. The Great Lakes and the rugged terrain of the Appalachians are the main factors in modifying the cP air.
Since the general circulation in the Great Basin and the Pacific coastal regions is from west to east, the cP air masses seldom enter this region.
Besides, the Rockies present great physical barrier in their path. Whenever the cP air enters into this region, the mechanical turbulence and mixing because a slight increase in its temperature. Even these air masses make the winters of the Pacific coast very severe.
When mP air masses from the Pacific Ocean cross the Rocky Mountains, they descend and become a little dry. Byers have referred to these modified air masses as cP. On moving above the cold surface of the continent they become cPW.
(2) Maritime polar (mP) air masses:
Maritime polar air masses of the United States originate over the North pacific and the northwestern Atlantic from Newfoundland to Cape Cod. Because the general circulation in the middle latitudes is from west to east, the North pacific air masses have greater influence on North America than the northwestern Atlantic air masses.
Sometimes the cP air masses that originate in Siberia invade this continent. As these air masses move toward the continent, they pick up moisture from the relatively warm waters of the Pacific.
In this way, the cold, dry and stable air masses are modified into mild and humid air masses near the surface. An element of instability is introduced in their lower portions. cPWs air masses are thus modified into mPKu.
On reaching British Columbia and north coast of the United States continental polar air is changed to maritime polar (mPK) air. In the coastal areas of the United States the air masses develop a steep temperature gradient which results in instability in them. Low clouds form in these air masses which produce winter rainfall in the plains and snowfall in the mountains.
Along with the extra-tropical cyclones coming from the North pacific the mPK air enters into North America. With their advent the winds become gusty and surface visibility fine. Cumulo-nimbus clouds yield abundant precipitation.
During the winter, anticyclonic conditions prevail in the Great Basin between the Sierrra- Cascade Range and the Rockies. Since there is subsidence of air in these anticyclones, the maritime winds become cold and dry in the coastal valleys. Because of upper-level subsidence, these air masses are stable aloft which results in dense fogs in the coastal valleys.
When the maritime polar air masses from the Pacific reach the eastern slopes of the Rocky Mountains, they are modified into cPWs. But these modified air masses are less cold and less dry than the polar continental (cP) air masses.
The weather produced by the modified air masses is fine with moderate temperatures. However, where they join the mT and cP air masses, weather becomes stormy. The mP air masses after entering into the source region of the cP air masses are modified into cP, thereby completing the cP-mP-cP cycle.
The mP air masses from the north-western Atlantic seldom affect the weather of North America. But sometimes the winter cyclonic winds draw mP in the north-eastern United States.
The weather produced by the invasion of Atlantic air masses is locally known as a northeaster which is characterized by strong northeast winds, freezing temperatures, high relative humidity and the possibility of precipitation. The influence of north-western Atlantic air masses is restricted to the area east of the Appalachians and north of Cape Hatteras only.
(3) Maritime tropical (mT) air masses:
These air masses are essentially hot, humid and unstable. They are also capable of releasing heavy precipitation. During winter the south-eastern part of the United States are dominated by them. They originate over the Gulf of Mexico, the Caribbean Sea and the tropical western Atlantic Ocean. The tropical Pacific is another source region for mT air masses which affect only a small area.
Since the eastern and central United States during the winter is dominated by anticyclones and cP air masses, the mT air masses do not enter into these regions. Whenever they get an opportunity to move out to these regions, the lower portions become very cold and stable.
In these regions mT air masses are modified into mTW. When they are caught into the cyclonic circulation of the region and forced to rise upward along the fronts, most of the precipitation over the eastern and central States is produced by them.
Otherwise they produce dense advectional fog because of their chilling by the cold ground. The winter heavy precipitation of the Appalachian region is produced by these air masses. They are marked by convective instability aloft.
Summer Air Masses:
(1) cP air masses:
During summer the continental polar air masses originate in the northern high latitude regions of the continent. The central part of Canada presents ideal conditions for their origin. In their source region they are cold, dry and unstable.
The summer air mass which originates on the cold Arctic Ocean is initially cold and stable. When the Arctic air mass moves southward over the warmer land surface, it is heated from below as a result of which the stability gradually disappears.
On the continental surface this air mass is modified into cPK, which becomes cPW on reaching the ocean. It then produces mist, fog and low-stratus clouds. In their source region the weather remains cloudless.
During the day the surface heating causes convective turbulence in the lower layer of these air masses. But the condensation level being high, cumulous clouds are seen scattered in the sky. With the advent of the continental polar air masses the eastern and the central areas of the United States experience chilly weather.
The summer air masses seldom move towards the southern part of the United States. These air masses generally move towards the eastern states. While moving towards the east and south the air masses are heated from below, even then their initial stability persists.
However, when these air masses are involved into the cyclones visiting the region, they produce scattered rainfall. In the northern parts of the eastern and central United States the southward advance of continental polar air masses puts an end to the summer heat waves and produces fine weather.
(2) Maritime polar (mP) Pacific air masses:
During the summer month the mP Pacific air masses originate in the same source region as the winter mP air, but they have different physical properties. In the lower parts they are cool and humid, but are dry aloft.
The change from moist lower parts to dry upper part is very sharp which results in the formation of an inversion layer, since the water of the adjacent Pacific Ocean is cooler than the continental surfaces. Besides, the Pacific anticyclone is located off the West Coast of the United States, where there is almost continuous southward flow of air having moderate temperatures.
These air masses, even though marked by conditional instability near the surface, are stable aloft because of subsidence. Low stratus clouds and fogs are produced all along the West Coast. As the mP Pacific air masses reach the continental areas, they are heated from below because of their contact with the warm surface.
This leads to greater turbulence in the lower layer with consequent reduction in the relative humidity and hence the clouds disappear. After crossing the Rocky Mountains the mP air masses resemble the cP air in their physical characteristics.
(3) Maritime polar (mP) Atantic air masses:
The air masses originate over the western part of North Atlantic Ocean. Their source region lies between Cape Cod and Newfoundland. They occur during the late spring and early summer. At this time of the year the contrast between the temperatures of adjacent ocean water and the continent is the greatest, so that the air masses move towards the east.
As they move from over the Atlantic Ocean, the temperature of their lower layers is substantially reduced which stabilizes them. Because of the anticyclonic circulation there is upper level subsidence which brings about stability in the upper layers. With these properties, the air masses are designated as mPWs.
The influence of these modified air masses is felt along the coastal regions from New England to Cape Hatteras. They produce a very pleasant and fine weather. The skies are clear, temperatures low and visibility fine. There is complete absence of ground fog.
The thin and scattered clouds floating across the sky do not produce precipitation. However, the mountainous regions do get light precipitation. Since the temperature of these air masses is lower than that of the ground, they are classified as mPKs.
During the summer months the temperature of the northern part of the continent is not allowed to rise because of the combined effect of the mP and cP air masses.
(4) Maritime tropical (mT) Atlantic air masses:
The Bermuda High over the western Atlantic Ocean is the source region of these air masses. They control the weather in the eastern and most of the southern parts of the United States during the summer.
Thermally produced low over the warm continent draws these maritime air masses into Canada. However, the polar front in the vicinity of the Great Lakes does not allow them to move farther into north. These air masses make the summer heat oppressive over most of the central and eastern parts of the United States.
While leaving their source region they are only slightly warmer and more humid than the winter mT, but on entering into the continental landmass an increase of about 11° Celsius are recorded in their lower parts.
In the lower half they are hot and humid so that the surface instability is increased. The upper level instability in these air masses is a common characteristic, so they are designated at mTu. Over the warm continent they are modified into mTKu.
In the Gulf States the strong convective activity produced in these air masses leads to the formation of cumulus and cumulo-nimbus clouds. Warm-season thunderstorms produced locally in the southeastern United States are essentially associated with these air masses.
The air masses from the Gulf-Caribbean-Atlantic source region are the major source of much of the precipitation received in the larger parts of the eastern United States.
(5) Maritime tropical (mT) Pacific air masses:
The mT air masses from the Pacific source region do not have the same impact on the North American weather as their counterparts from the tropical Atlantic Ocean.
However, in summer the Pacific air masses move northward ups the Gulf of California and into the interior of the western United States which is, of course, in response to the thermally maintained low over the hot land areas.
This invasion of the mT Pacific air masses is believed to be monsoonal in character. Contrary to the older belief, it is now widely held that the real source of moisture for the area west of the Continental Divide lies in the tropical North Pacific west of central Mexico.
(6) Continental tropical (cT) air masses:
Since the continent is V-shaped and narrows down towards the south, there is no extensive source region for these air masses in North America. During the summer only northern Mexico and southeastern United States have limited areas where these air masses originate.
They are initially hot and dry. The intense daytime heating at the surface produces a steep lapse rate and turbulence which extend to considerable heights.
Because of dryness there is lack of precipitation. If these air masses occupy certain regions for a long period, drought conditions are produced there. Sometimes these air masses move out to the Southern Great Plains of the United States and produce hot and dry weather.