About air masses and it's major types with necessary image .Related course Climatology.Air mass is a volume of air defined by its temperature and water vapor content. Air masses cover mant thousands of miles and adapt to the characteristics of the surface below them. They are classified according to latitude and their continental or maritime source regions. Cold air masses are termed polar or artic,while warmet air masses are deemed tropical. Continental and superior air masses are dry while martime and monsoon air masses are moist.Weather fronts separate air masses with different density (temperature or moisture) characteristics.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". 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 modifications introduced in them during their journey away from their respective source regions must be taken into definite categories. There are two approaches to the classification of air masses.

1. An assignment on Classification of Air Masses
(7.3)
Course Name :Fundamental of Climatology
Course Code:(GEO-2102)
Submitted To:
Nahrin Jannat Hossain
Assistant Professor
Department of Geography and Environment
Jagannath University ,Dhaka
Submitted by:
Sadia Tasmin Isha
Student ID: B180602049
Session 2018-19
14th
batch
Department of Geography and Environment
JAGANNATH UNIVERSITY, DHAKA

2. Date of submission : 19-06-2021
Definition of Air mass:
In Metrology, an Air mass is a volume of air defined by its temperature and water vapor content. Air
masses cover mant thousands of miles and adapt to the characteristics of the surface below them.
They are classified according to latitude and their continental or maritime source regions.
Cold air masses are termed polar or artic,while warmet air masses are deemed tropical. Continental and
superior air masses are dry while martime and monsoon air masses are moist.Weather fronts separate air
masses with different density (temperature or moisture) characteristics.
An air mass is a large valume of air in the atmosphere that is mostly uniform in temperature and moisture.
Air masses can extend thousands of kilometers across the surface of earth and can reach from ground level
to tha stratosphere 16kilometers(10miles) into the atmosphere.
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".
It maybe pointed out that since a single air mass is so large that it may cover hundreds of thousands to
millions of squire kilometres of the earth's surface and hence horizontal homogeneity of an air mass in
terms of its physical properties may not be parctically possible because the nature and degree of uniformity
of air mass properties are determined by-
❏ The properties of the source area and the direction of its movement.
❏ Changes introduced in the air mass during its journey away from the source area and
❏ The age of the aur mass.
Classification of air mass:
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 modifications
introduced in them during their journey away from their respective source regions must be taken into
definite categories.
There are two approaches to the classification of air masses. They are-

3. I. Geographical classification and
II. Thermodynamic classification
1.Geographical classification :
The geographical classification of air masses is based on the characteristic features of the source regions.
Basically air masses are classified into
1. warm air mass and
2. Cold air mass
on the basis of temperature of the source regions. Trewartha has classified air masses on the basis of
temperature of the source regions.Trewartha has classified air masses on the basis of their geographical
locations into two broad categories viz.(i) Polar area mass (P) which originates in polar areas.(ii)Arctic air
masser are 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 (weather continental or Oceania areas) e.g.
(a)Continental air masses (indicated by a small letter c)
(b)Maritime air masses (indicated by a small letter m).
It may be pointed out that a continental air mass gets modified and is transformed into 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.
1) Continental polar air mass (cP)
2) Maritime polar air mass (mP)
3) Continental tropical air mass(cT)
4) Maritime tropical air mass (mT)
Figure (1):Types of Air masses
2.Thermodynamic Classification :
Based on thermodynamic an mechanical (dynamic) modifications air masses are divided into
1. Cold air mass and
2. 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 :
1. Temperature is very low because of loss of heat through out going longwave terrestrial radiation
2. Specific humidity is extremely low

4. 3. 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 area where cold air masses reach starts decreasing
2. The air mass is warmed form below and thus normal lapes rate increases and tge air becomes
unstable. This mechanism causes convective currents.
3. If the cold air mass lies over warm ocean surface, then its specific humidity increase an cumulo-
nimbus clouds are formed.
4. The usual visibility in the air mass is maintained.
5. Precipitation occurs only when the air mass lies over warm ocean surface but if it lies over warm
continent, there is clear weather.
6. 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 then 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 conditionsThey are further divided into
(a)Continental warm air mass
(b)Maritime warm air mass
3.Composite Classification :
Based on thermodynamic and mechanical (dynamic) modifications and some other considerations air
massea are divided into 16 types as follows :
(A)Continental Polar Air Masses (cP):
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):
5.Maritime Polar Cold Stable Air Mass(mPKs)
6.Maritime Polar Cold Unstable Air Mass(mPKu)
7.Maritime Polar Warm Stable Air Mass(mPWs)
8.Maritime Polar warm unstable Air Mass(mPWu)
(C)Continental Tropical Air Masses (cT):
9.Continental Tropical Cold Stable Air Mass (cTKs)
10.Continental Tropical Cold Unstable Air Mass (cTKu)
11.Continental Tropical Warm Stable Air Mass (cTWs)
12.Continental Tropical Warm Unstable Air Mass (cTWs)
(D) Maritime Tropical Air Masses (mT):
13.Maritime Tropical Cold Stable Air Mass (mTKs)

5. 14.Maritime Tropical Cold Unstable Air Mass (mTKu)
15.Maritime Tropical Warm Stable Air Mass (mTKs)
16.Maritime Tropical Warm Unstable Air Mass (mTWu)
c=Continental, T=Tropical, m=Maritime, K=Cold, W=Warm,u=Unstable, s=Stable
Figure (2):Types of air masses
Characteristics of Major Air Masses :
As mentioned above air masses fall in two broad categories on the basis of locational aspect of their cource
regions namely
1. Polar Air Masses and
2. Tropical Air Masses
These are further subdivided into
a.Continental and
b.Maritime air masses
The characteristic features of these major air masses are discussed below:
1.Continental Polar Air Masses (cP):
The polar continental air masses originate over the extensive cold surfaces of central Canada and Siberia
and move outward and are thermodynamically and mechanically (dynamically) modified. These air masses
have different physical characteristics during summer and winter seasons. The polar continental air masses
are generally cold and dry but when these move over warmer surfaces they are heated from below become
unstable and moist to some extent resulting into the formation of limited clouds mainly low
stratocumulus.The cource areas due to their location in high latitudes,are frozen during winter season, the
air mass is cold, dry and stable. Intense cold waves are generated in those areas which are visited by these
extremely cold air masses.
Figure (3):Continental Polar Air Masses (cP):

6. For example, the winter time polar continental air mass after being originated over frozen land surface of
central Canada, bring extreme cold weather in the Mississippi plains of the USA. Even in the summer,
sometimes frost conditions become common as far south as New Orleans, Galveston and Huston of the
USA.In the summer the snow covered surface gets rid off snow and ice because of its melting due to heating
but still summer time continental polar air masses are cool and dry in their source regions of central Canada
and Siberia. When these air masses move over the oceanic surface, they are warmed from below and
become warm and give sone precipitation through cumulus or low stratocumulus clouds.
2.Polar Maritime Air masses (mP):
It may be mentioned that maritime polar air masses also have the same source regions as those of continental
polar air masses. In fact, when continental polar air masses (cP) move out from their source regions and
travel over oceanic surface of high latitudes,their lower parts are heated from below by the relatively warm
surfaces of open oceans and thus become maritime polar air masses (mP) after such modifications.Such
modifications increases temperature lapse rate and causes convective instability in the lower parts.On the
other hand, the upper part is dry and cool.
Figure (4):Maritime Polar (mP)
When such moditied maritime polar air masses strike the mountain barriers. They are mechanically forced
to ascend become unstable and the convective instability results in condensation and much precipitation on
the window slopes of the mountains but while descending on the leeward side of the mountains they are
adiabatically warmed and become stable dry continental air masses. Such situations are found along the
west coasts of North America where the Coast Ranges receive enough precipitation while the eastern slopes
of the Rockies and the Great Plains go dry.
3. Continental Tropical Air masses (cT)
The continental Tropical Air masses have their source regions in the subtropical high pressure areas of hot
deserts located between 20° - 30° latitudes in both the hemispheres which are characterized by vertical
descent and horizontal divergence of wind.
Figure (5):Continental Tropical Air masses (cT)

7. These air masses are characterized by very high temperature (above 40°C), least moisture content, steep
lapse rate, atmospheric stability and dry weather. These air masses seldom move out from their source
regions, but whenever they move out to Ocean surfaces, they are modified to maritime tropical air masses.
4. Maritime tropical air masses (mT):
The maritime tropical air masses have their source regions over worm ocean surfaces of tropical regions
confined between 30°N and 30°S latitudes. These are warm, moist and unstable air masses and more
extensive in areal context. They are associated with convective instability and cumulus and cumulonimbus
clouds who is give abundant rainfall when the air masses is associated with frontal activity or is forced to
ascend buy mountain barriers.
Figure (6):Maritime tropical air masses (mT)
It may be mentioned that the maritime tropical air masses are modified and become stable when they move
towards the the poles and trouble other over colder water (of oceans) or land surfaces, whereas they become
unstable when they move over worm land surfaces.
Air Masses are classified on weather maps using two or three letters.
❏ A lower case letter describes the amount of moisture in the air mass:m for maritime (moist)
and c for continental (dry).
❏ An upper case letter describes the heat of the air mass:E for equatorial, T for tropical, M
for monsoon, P for polar, A for Airtic or Antarctic and S for superior -a unique situation
with dry air formed by a powerful downward motion of the atmosphere.
❏ A lower case letter describes the relationship between the air mass and the earth :K signifies
that the air mass is colder than the ground below it, while W describes an air mass that is
warmer than the ground below it.
The modification of air masses depends on 4 factors e.g.
I. Intial 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 foom 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 for the moving
air mass is heated form below and becomes unstable due to resultant steepeneed lapse rate and upward
movement of air.
Source Regions of Air Mass:
The extensive area over which air mass originate or form are called source regions whose nature and
properties largely determine the temperature and moisture characteristics of air masses.

8. Figure (7):Source regions of global air mass
An air mass originales when atmospheric conditions remain stable and uniform over
an extensive area for fairly long period si that the air laying 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.
There are 6 major source regions of air masses on the earth 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 (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 (zon located between trade winds-active throughout the year)
6) Nonsoon lands of S.E. Asia.
Reference:
Climatology;Savindra Singh.