All the physical properties of air masses which determine the weather characteristics produced by them are acquired from their source regions. When the air masses move out from their source regions, the temperature, humidity and the nature of the underlying surface bring about certain changes in them.
Therefore, a satisfactory classification of air masses must encompass these transitions. No classification scheme should be based on the geographical location of various air mass source regions alone.
Air masses are classified on the basis of (a) the location of their source regions, and (b) the nature of the surface over which they move towards other regions.
Pettersen has classified air masses into the following five major categories on the basis of their source regions: (1) Tropical air masses, (2) Polar air masses, (3) Equatorial air masses, (4) Arctic air masses, and (5) Antarctic air masses. But according to Byers, there are only three major categories of air masses, i.e. Arctic, Polar and Tropical air masses.
Trewartha, on the basis of the geographical location of air masses, classifies them into the following two broad categories:
(1) Polar air mass, for which a capital P is used.
(2) Tropical air mass, for which a capital T is used.
He is of the opinion that Arctic, Antarctic and Equatorial air masses do not have individual identity of their own. They are considered to be the modified forms of polar and tropical air masses.
He further subdivides the polar as well as tropical air masses into two types on the basis of the nature of the surface of their source regions, i.e., land or water:
(a) Continental air mass indicated by a small letter c.
(b) Maritime air mass indicated by a small letter m.
The continental air masses originate over the continents, and the maritime air masses form over the oceans. Maritime air masses have originally large quantity of moisture which they have picked up from the oceans over which they were formed.
They have, therefore, a natural tendency for condensation in them. On the contrary, the continental air masses are originally dry. When they move out to oceans, they acquire a large amount of moisture by the process of evaporation from the sea surface.
In this way the continental air masses develop the characteristics of maritime air masses gradually. Similarly, the maritime air masses while passing over the continents lose there moisture and undergo modification rather slowly. Hence, on the basis of the source regions as well as the nature of their surface the following four principal types of air masses may be considered:
(1) Continental Polar air masses (cP)
(2) Maritime Polar air masses (mP)
(3) Continental Tropical air masses (cT)
(4) Maritime Tropical air masses (mT).
Air Masses and Source Regions:
January and July air masses and their source regions are shown in Figures 33.2 and 33.3. The principal source regions of the earth may be classified according to the nature of surface (land or water) and the latitude of the region. Thus, the source regions are classified as under:
The Arctic source regions are located in the high latitudes, where the surface is permanently covered with snow and ice. Thus, they are the coldest regions on earth. The polar source regions do not mean the regions around the geographic poles.
They are situated between the Arctic source regions and the sub-tropical highs. Note that the Arctic source regions are colder than the polar source regions. The tropical source regions occupy the subtropical high pressure belt. The equatorial source regions are located around the equator between the trade winds of the northern and southern hemispheres.
The following discussion relates to the major air masses that form during the winter and summer months in different regions of the world:
(1) Wintertime continental polar (cP) air masses:
These air masses have their source regions in the central Canada and Siberia. They are extremely cold, dry and stable. Since the surface is completely frozen and is snow-or-ice-covered, these air masses are the coldest wintertime air masses.
They produce intense cold waves when they move out to some other regions. Because of extreme dryness of the air, there are no clouds in these air masses.
However, after these air masses move out of their source regions, they are modified while they pass over a warm surface. When cP air mass becomes cPK after modification, cumulus or stratocumulus is not uncommon.
(2) Summertime continental polar (cP) air masses:
These air masses have their source regions in the central parts of high-latitude continents. Central Canada offers a typical example of such a source region. Because of the surface heating, snow cover disappears.
Summer time cP air masses in their source regions are cool and dry, but not necessarily stable. Actually these are the modified forms of the winter-time cP air masses which have been heated in the lower layers.
Their lapse rates are comparatively less steep. When cPK air mass moves out to oceanic surface, it is modified into cPW air mass with haze, fog, and low stratus clouds.
(3) Wintertime maritime polar (mP) air masses:
These air masses are cool and moist and form over the open oceans in the higher latitudes. They are originally cP air masses which have undergone extensive modification over the open oceans. These air masses contain few clouds in their source regions.
But when they are dragged into cyclones or are forced to ascend mountain barriers, extensive precipitation is produced by them.
Their lower layers are moist and unstable, but they are dry and cold in their upper parts. The convective instability in the lower layers of these air masses produces showery, squally weather.
(4) Summertime maritime polar (mP) air masses:
These air masses originate in the source regions of mP air masses. They are cool and moist in the lower parts, but dry aloft. A temperature inversion is produced sometimes with moisture discontinuity. They are stable upto moisture discontinuity. The general temperature is slightly higher than that in the mP air masses.
(5) Wintertime tropical maritime (mT) air masses:
They have their source regions over the warm oceans in both the hemispheres. They are warm, moist and unstable. They release abundant precipitation whenever they occur.
The lapse rates in the lower levels often approach the dry-adiabatic rate, and the lapse rates are steep up to tropopause. Moisture is well distributed to high levels. When these air masses are lifted over fronts or high mountains, they produce heavy precipitation.
(6) Summertime tropical maritime (mT) air masses:
The source regions of these air masses are located in the belt of the great semi-permanent highs of the tropical oceans including the Caribbean Sea. The mT air masses are very warm and moist and highly unstable. These air masses have convective instability.
(7) Continental tropical (cT) air masses:
These air masses have their source regions in the subtropical high pressure land areas. They have high temperatures and low moisture content. The tropical continental air does not spread extensively beyond its source regions.
In the United States these air masses are important only in the summer season. They are dry both in winter and summer. In summer they are very hot. Subsidence and stability are found in the upper parts of these air masses in their source regions.
When cT air is found aloft over warm, moist air at the surface, the atmosphere becomes convectively unstable, and violent thunderstorms or tornadoes are produced.