In this article we will discuss about:- 1. Meaning of Air Mass 2. Source Regions of Air Mass 3. Classification.
Meaning of Air Mass:
“An air mass may be defined as a large body of air whose physical properties, especially temperature, moisture content, and lapse rate, are more or less uniform horizontally for hundreds of kilometres”. According to A.N. Strahler and A.H. Strahler (1978) “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.” An air mass may be so extensive that it may cover a large portion of a continent and it may be so thick in vertical dimension that it may vertically extend through the troposphere.
It may be pointed out that since a single air mass is so large that it may cover hundreds of thousands to millions of square kilometres of the earth’s surface, and hence horizontal homogeneity of an air mass in terms of its physical properties may not be practically possible because the nature and degree of uniformity of air mass properties are determined by, (i) the properties of the source area and the direction of its movement, (ii) changes introduced in the air mass during its journey away from the source area, and (iii) the age of the air mass.
The vertical distribution of temperature in an air mass, and moisture content of the air are two basic properties of an air mass which control the weather conditions of the area affected by that air mass. An air mass is designated as cold air mass when its temperature is lower than the underlying surface while an air mass is termed warm air mass when its temperature is higher than the underlying surface.
The boundary between two different air masses is called front. The physical properties of an air mass are determined on the basis of the characteristic features of the surface through which it travels. An air mass also affects and modifies temperature and moisture conditions of the areas visited by it and in turn it is also modified by the local conditions of the visited areas.
Source Regions of Air Mass:
The extensive areas over which air masses originate or form are called source regions whose nature and properties largely determine the temperature and moisture characteristics of air masses.
An air mass originates when atmospheric conditions remain stable and uniform over an extensive area for fairly long period so that the air lying over that area attains the temperature and moisture characteristics of the ground surface. Once formed, an air mass is seldom stationary over the source region, rather it moves to other areas.
An ideal source region of air mass must possess the following essential conditions:
(i) There must be extensive and homogeneous earth’s surface so that it may possess uniform temperature and moisture conditions. The source region should be either land surface or ocean surface because irregular topography and surface comprised of both land and water cannot have uniform temperature and moisture conditions.
(ii) There should not be convergence of air, rather there should be divergence of air flow so that the air may stay over the region for longer period of time and thus the air may attain the physical properties of the region. It is thus, apparent that anticyclonic areas characterised by high barometric pressure and low pressure gradients are most ideal regions for the development of air masses.
(iii) Atmospheric conditions should be stable for considerable long period of time so that the air may attain the characteristics of the surface.
There are 6 major source regions of air masses on the earth’s surface e.g.:
(1) Polar oceanic areas (North Atlantic Ocean between Canada and Northern Europe, and North Pacific Ocean between Siberia and Canada-during winter season),
(2) Polar and arctic continental areas (snow-converted areas of Eurasia and North America, and Arctic region-during winter season),
(3) Tropical oceanic areas (anticyclonic areas – throughout the year),
(4) Tropical continental areas (North Africa-Sahara, Asia, Mississippi Valley zone of the USA – most developed in summers),
(5) Equatorial regions (zone located between trade winds – active throughout the year), and
(6) Monsoon lands of S.E. Asia.
Classification of Air Masses:
Any classification of air masses must consider the fact that all of their weather characteristics (mainly temperature, humidity and lapse rate) are properly represented and incorporated. Thus, the weather conditions of air masses at their source regions and thermodynamic and mechanical modification introduced in them during their journey away from their respective source regions must be taken into consideration while classifying them into definite categories.
There are two approaches to the classification of air masses e.g.:
(1) Geographical classification, and
(2) Thermodynamic classification.
(1) Geographical Classification:
The geographical classification of air masses is based on the characteristic features of the source regions.
Trewartha has classified air masses on the basis of their geographical locations into two broad categories viz.:
(i) Polar air mass (P), which originates in polar areas. Arctic air masses are also included in this category.
(ii) Tropical air mass (T), which originates in tropical areas. Equatorial air masses are also included in this category.
These two air masses have been further divided into two types on the basis of the nature of the surface of the source regions (whether continental or oceanic areas) e.g.:
(a) Continental air masses (indicated by a small letter c), and
(b) Maritime air masses (indicated by a small letter m).
It may be pointed out that a continental air mas§ gets modified and is transformed into maritime type while passing through ocean surface but maritime air mass is seldom transformed into continental type while passing through land surface.
Based on above facts air masses are classified into the following four principal types according to their geographical locations:
(i) Continental polar air mass (cP).
(ii) Maritime polar air mass (mP).
(iii) Continental tropical air mass (cT).
(iv) Maritime tropical air mass (m t).
(2) Thermodynamic Modifications and Classification of Air Masses:
Thermodynamic modification of an air mass involves its heating or cooling from below while passing through different surfaces away from the source region. Heating of an air mass causes decrease in the vertical stability of the atmosphere.
After being originated the air masses move out of their source regions to other regions and in the process they modify the weather conditions of the areas travelled by them and in turn they also get modified by the surface conditions over which they move. The thermodynamic modifications of air masses, besides heating from below, also include evaporation of water into the air from below or into intermediate layer by precipitation from moist air aloft.
The modification of air masses depends on 4 factors e.g.:
(i) Initial characteristics of air mass in terms of temperature and moisture content,
(ii) Nature of land or water surface over which a particular air mass moves,
(iii) Path followed by the air mass from the source region to the affected area, and
(iv) Time taken by the air mass to reach a particular destination.
An air mass while moving over the surface whose temperature is greater than the lower layer of the moving air mass, is heated from below and becomes unstable due to resultant steepened lapse rate and upward movement of air. This mechanism causes condensation, cloud formation and precipitation if the moving air mass contains sufficient amount of moisture content.
On the other hand, if the moving air mass is warmer than the surface over which it travels, it is cooled from below resulting into atmospheric stability which restricts upward movement of the air and thus there is no chance for condensation, cloud formation and precipitation. It is, thus, obvious that cold polar air masses while moving from their source regions to relatively warmer surfaces become unstable because they are warmed from below.
On the other hand, warm tropical air masses, when move out of their source areas and reach colder surfaces, are cooled from below, causing atmospheric stability and dry weather.
A warm air mass (w) is that whose temperature is greater than the surface temperature of the region visited while if the air mass is colder than the surface temperature it is called cold air mass (k). It is apparent that the warmness or coldness of an air mass is determined by the temperature of the underlying surface. Air mass also undergoes thermodynamic modification when evaporation is added to it from outside.
An air mass is termed stable air mass when air descends while an air mass becomes unstable when upward movement of air is operative. Such mechanical modifications in an air mass are introduced due to cyclonic and anticyclonic conditions.
Besides, mechanical modifications are also introduced due to:
(i) Turbulent mixing caused by eddies or convection,
(ii) Divergence and convergence of air masses and their effects on lapse rate of temperature,
(iii) Subsidence of air and lateral expansion on the ground surface (anticyclonic condition),
(iv) Lifting of air and convergence of air at the ground surface (cycolnic condition), and
Based on thermodynamic and mechanical (dynamic) modifications air masses are divided into:
(i) cold air mass and
(ii) warm air mass, each of which is further divided into:
(a) stable air mass, and
(b) unstable air mass.
Cold air masses originate in the polar and arctic regions.
They are characterized by the following properties in their source regions:
(i) Temperature is very low because of loss of heat through outgoing long-wave terrestrial radiation.
(ii) Specific humidity is extremely low.
(iii) Stability increases and normal lapse rate of temperature is low.
Cold air masses after moving out from their source regions and reaching other areas have the following properties:
(1) The temperature of the areas where cold air masses reach starts decreasing.
(ii) The air mass is warmed from below and thus normal lapse rate increases and the air becomes unstable. This mechanism causes convective currents.
(iii) If the cold air mass lies over warm ocean surface, then its specific humidity increases and cumulonimbus clouds are formed.
(iv) The usual visibility in the air mass is maintained.
(v) Precipitation occurs only when the air mass lies over warm ocean surface but if it lies over warm continent, there is clear weather.
(vi) If the cold air mass lies partly over warm ocean surface and partly over adjoining cold land surface, then cyclonic conditions are induced.
Cold air masses are further divided into (a) continental cold air mass, and (b) maritime cold air mass.
(2) Warm air mass is that whose temperature is greater than the surface temperature of the areas over which it moves. Such air mass is cooled from below and thus its lower layer becomes stable due to which its vertical movement stops. Warm air masses generally originate in the subtropical regions characterized by anticyclonic conditions. They are further divided into (a) continental warm air mass, and (b) maritime warm air mass.
Based on thermodynamic and mechanical (dynamic) modifications and some other considerations air masses are divided into 16 types as follows:
(A) Continental Polar Air Masses:
(1) Continental Polar Cold Stable Air mass (cPKs)
(2) Continental Polar Cold Unstable Air Mass (cPKu)
(3) Continental Polar Warm Stable Air Mass (cPWs)
(4) Continental Polar Warm Unstable Air Mass (cpWu)
(B) Maritime Polar Air Masses (mp):
(1) Maritime Polar Cold Stable Air Mass (mPKs)
(2) Maritime Polar Cold Unstable Air Mass (mPKu)
(3) Maritime Polar Warm Stable Air Mass (mPWs)
(4) Maritime Polar Warm Unstable Air Mass (mPWu))
(C) Continental Tropical Air Masses (cT):
(1) Continental Tropical Cold Stable Air Mass (cTKs)
(2) Continental Tropical Cold Unstable Air Mass (cTKu)
(3) Continental Tropical Warm Stable Air Mass (cTWs)
(4) Continental Tropical Warm Unstable Air Mass (cTWu)
(D) Maritime Tropical Air Masses (mT):
(1) Maritime Tropical Cold Stable Air Mass (mTKs)
(2) Maritime Tropical Cold Unstable Air Mass (mTKu)
(3) Maritime Tropical Warm Stable Air Mass (cTWs)
(4) Maritime Tropical Warm Unstable Air Mass (cTWu)
c = continental, T = tropical, m = maritime, K = cold, W = warm, u = unstable, s = stable