AIR MASSES

WHAT IS AN AIR MASS?

The concept of the air mass is perhaps the most fundamental in modern meteorology. A body of air in which the upward gradients of temperature and moisture are fairly uniform over a large area is known as an Air Mass. In terms of area, a single air mass may be of sub-continental proportions; in vertical dimensions it may extend through the Troposphere. A given air mass is characterized by a distinctive combination of temperature, environmental lapse rate, and specific humidity. Thus we find air mass differing widely in temperature – from very warm to very cold, and in moisture content – from very dry to very moist.

AIR MASS SOURCE AREAS

Some areas are much more conducive (favorable) to the formation of homogenous air masses than others and may be considered as “Air Mass Sources Areas”. One type of source area is that in which air stagnates prior to an outward movement. The areas of calms associated with the centers of large, stationary anticyclones are illustrative. A second type of source area includes extensive surface regions of general climatic uniformity, such as the broad expanses of warm oceans in the low latitudes and the snow covered surfaces of continents in the high latitudes during the winter season. Such areas are large enough to produce a certain degree of uniformity within the air streams that pass over them.

MODIFICATIONS

The characteristics of air masses are derived first from the source areas where they originate and second from the areas they cross once they leave their source areas. The modifications in route may take many forms, but generally the lower portions of the invading air masses are the first to be altered. The great air streams that form the circulation systems at the earth’s surface thus are continually undergoing some alterations outside their source areas. The extent of modifications frequently can be observed by examining the vertical profiles of air masses. Nearly all modified air masses exhibit distinctly different properties in their upper and lower portions and some remarkably sharp boundaries can be formed separating the lower modified portion from the upper. One of the most frequent modifications is formed by change in temperature.
The principle modifications that take place in surface air masses may be summarized as follows:

1. Thermodynamic Modifications
i. Heating at the earth’s surface
ii. Cooling at the earth’s surface
iii. Cooling above the earth’s surface by loss of radiation
iv. Addition of water vapor at the surface

2. Mechanical Modification
i. Mixing by passage over rough terrain
ii. Orographic, convergent and convectional lifting
iii. Subsidence (decline) from above

CLASSIFICATION OF AIR MASSES

Air masses can be divided into groups according to their source regions. They are classified according to two categories of generalized source regions:
1. Latitudinal Position of the Globe which primarily determines thermal properties, &
2. Underlying surface – continents or oceans – determining the moisture content.
With respect to “Latitudinal Positions”, five types of air masses are as follows:
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AIR MASS---SYMBOL-------->SOURCE REGIONS
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Arctic--------A------->Arctic Ocean and fringing (bordering) lands
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Antarctic-----AA----->Antarctica
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Polar----------P----->Continents and oceans (50o – 60o North and South)
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Tropical-------T------>Continents and oceans (20o – 35o North and South)
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Equatorial-----E------>Oceans close to equator
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With respect to “Underlying Surface”, two further sub-divisions are imposed on the preceding type as follows:
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AIR MASS------SYMBOL-->SOURCE REGIONS
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Maritime---------M------>Oceans
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Continental------C------>Continents
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By combining types based on “Latitudinal Positions” with those based on “Underlying Surface”, a list of six important air masses results. These are:
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AIR MASS------------SYMBOL--->PROPERTIES
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Continental Arctic-------cA
Continental Antarctic----cAA--->Very cold, very dry (winter)
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Continental Polar--------cP---->Cold, dry (winter)
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Maritime Polar-----------mP--->Cold, moist (winter)
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Continental Tropical-----cT--->Warm, dry
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Maritime Tropical--------mT--->Warm, moist
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Maritime Equatorial------mE---->Warm, very moist
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1. Continental Arctic and Antarctic

Source Areas: Ice and snow covered surface in Greenland, Antarctica, Hudson Bay, Mackenzie Basin (River), Mongolia Lake, (Lake) Baikal Region, Scandinavia.

Typical Characteristics: Low water vapor content; temperature inversions above surface cooling zone (2000-3000 ft.); subsidence (decline) above generally stable.

Major Modifications: Rapid rise in humidity in lower levels, when passing over water creates instability; inversion aloft (uphigh, overhead) destroyed by passing over rough terrain.

Associated Weather: Clear skies except when modified; low temperatures at night; strong winter storms (blizzards) along leading edges (Arctic Front).

2. Continental Polar

Source Areas: In winter, the cold continent surfaces mostly free of ice and snow; in summer cool surfaces in Canada and USSR; rare summer south of USSR.

Typical Characteristics: Found only in North Hemisphere; often modified continental Arctic air mass; clear skies at night; scattered cumulus clouds during the day; some subsidence above.

Major Modifications: Generally becomes highly unstable when away from source areas; persistent (continuos) overcast skies with passage over rough terrain.

Associated Weather: Clear air; frost hazard in late spring and early fall; summer convectional showers near water bodies; involved in most frontal activity in mid latitudes.

3. Maritime Polar

Source Areas: Ocean areas, latitudes 40o – 60o north and south

Typical Characteristics: In winter mainly modified continental Arctic and continental Polar air brought into ocean areas, resulting instability – Maritime Polar Cold Air; surface temperatures rarely sub-freezing; summer air generally stable – Maritime Polar Warm Air.

Major Modifications: Maritime Polar Cold Air on lee side of mountains (as in the Rockies) dry, clear and warm; passage over cold surface produces stability at lower levels.

Associated Weather: Fog and overcast skies, when passing over a cold surface; cyclonic and orographic conditions produce heavy precipitation; fronts with Maritime Polar Warm Air produce drizzly rain.

4. Continental Tropical

Source Areas: High planes with sub-tropical anti-cyclones (mainly in winter); Sahara, South Africa, Central Australia, South-West China, North India, Andean Planes.

Typical Characteristics: Extremely low humidity; hot dusty turbulent air; subsidence of warm air aloft.

Major Modifications: Becomes highly unstable in lower levels when passing over water (as over the Mediterranean Sea).

Associated Weather: Hot, dry weather; much haze; sometimes associated aloft with tornadoes.

5. Maritime Tropical

Source Areas: Warm tropical and sub-tropical ocean areas beneath sub-tropical anti-cyclones.

Typical Characteristics: Usually subsidence and low lapse rate above; generally fairly stable; some low level instability; often an inversion aloft.

Major Modifications: Rising humidity and increasing instability away from source areas; passage over cold land in winter increases instability – Maritime Tropical Warm Air; in summer, air sometimes unstable over warm land.

Associated Weather: Hazy air; scattered cumulus clouds; passage over cold land results in broad overcasts; produces most continental snow fall in mid latitudes; convectional rain with Maritime Tropical Cold Air in summer.

6. Maritime Equatorial

The name maritime equatorial is sometimes applied to very moist and warm air masses, originating within a few degrees of equator. These are a pronounced form of maritime tropical air masses.
This air mass holds about 200 times as much water vapor as the extremely cold Arctic and Antarctic air mass.