Geography (Optional)Handout by Md.
Rizwan
AIR MASSES
Defination: Large volume of air having homogeneity with respect to temperature &
humidity- AIR MASS.
Horizontal homogeneity- Imp. Characteristic.
Air masses are born if air is static over a region for a long duration &Characteristic of the
surface.
Primary source Region (x) – Birth region of Airmass.
Secondary source Region (y)- movement of 10 sources region
The region of origin of Airmass known as PSR, when move out & reside over a surface for a
considerable duration, there surfaces are called secondary source region.
The direction of airmasses is guided by the prevailing winds.
Airmasses results in the following:
i. Wether conditions of the region of travel (stability instability)
ii. Frontogenesis (formation of front due to contrasting air masses)
iii. formation of temperate cyclones
iv. effect on upper toposphereic circulation i.e.,
Note:Airmasses can cause stability & instability of atmosphere.
Movement of air mass
Hp& LP
Variation in Temperatures.
Origin/ Birth of airmass& criteria
Characteristic of primary source region
i. Significantly large region.
ii. Uniform surface (homogeneity) i.e., topography/terrain
iii. Uniformity of climatic condition
iv. Weather/ atmosphere should be stable.
Hp regions suitable for airmass origin as Lp- unstable.
Locations of PSR:
i. Hp/polar Hp
ii. Tropical Hp
Majority of Airmasses in S. hemisphere are of maritime dominance nature due to absence of
continent
VAJIRAM & RAVI 1
N. Hemisphere: Polar air masses exhibit continentality in
Specific areas for airmass development:
i. Artic sea
ii Green land
iii. Siberia steppe
iv Tibet
v. Tibet
vi. Arabian peninsula
vi. Sahara desert.
Movement of airmasses:
S-stability
u- instability
If the air mass is heated from belwo the air inside the airmass will ascend & will result in
atmosphereic disturbances (or) UNSTABLE WEATHER CONDITIONS (U)- COLD AIR
MASS.
When the airmass over a cold surface, i.e., colled from below, the air settles & creates
stability airmass on 2 source region.
When the contrasting air masses try to converge, there is a formation of an abstract boundary
between then which is inclined with the surface. This abstract boundary is known as front &
its process is known as FRONTOGENESIS.
Fronts cannot be formed within the tropics due to absence of the contrast thus, frontal rainfall
occurs between 45-600 (s/w the easterlies polar hp&westerlies tropical Hp)- their
convergence in sub tropical –Lp- frontal rainfall.
CHARACTERISTICS OF PRIMARY SOURCE REGION OF AIR MASSES
Air masses: Large volume of air having homogeneity with respect to the adj. temperature
and humidity. Primary source region is the region where the airmasses are born. The
movement of such Air masses by wind movement forms 2 source regions.
Characteristics of Primary Source Region:
● The area of origin for Air masses should be significantly larger.
● Homogeneity of the region Topography / Terrain / Temp / Humidity.
● Uniform climatic conditions.
● Stable atmosphere i.e.; HP regions.
VAJIRAM & RAVI 2
Major regions of Air masses:The air masses are classified based on the source regions and
their features. Similarly, they are of 4 types: (i) Continental Air masses, (ii) Maritime Air
masses.
Based on geographical locations, the air mass is classified as
(i) Polar air mass originating is polar regions and
(ii) Tropical air masses originating in the tropical regions.
NOTE:Continental air mass modifies in to maritime air mass while passing through outer
surface.
● Where, W – warm air mass and K = cold air mass
● Majority of airmasses in the S. Hence are of maritime nature due to the absence of land
and the N-Hemisphere represents the continental air masses due to the land dominance
(or) continentality.
● Specific areas / regions for air development :
● Arctic sea
● Green land
● Arial and semi Arial regions.
● Tibetan peninsula.
GLOBAL DISTRIBUTION OF AIR MASSES:
Note on Frontogenesis and frontolysis. Discuss its impact on weather.
Frontogenesis :The development of a front over warm and cold air masses is known as
frontagenesis. Frontogenesis can occur in the following conditions:
● Temperature difference – contrasting temperatures of cold / warm air masses.
● Direction of air mass: of the movement of the primary source region is in horizontal
manner, there are no much temp! Variations for the formation. The fronts can be of the
following types :
● War front : (ii) Cold front; (iii) Stationary front and (iv) Occluded front.
Frontolysis :The death of the front over the existing temp! differences leads to frontolysis. This
results in the end of the disturbances / instabilities existing in the atmosphere.
● The movement of warm air above cold air marks the lessened intensity of front
development / cyclconic disturbances.
VAJIRAM & RAVI 3
Impact of front on weather:
● Western disturbances: The formation of front over areas results in the formation of
temperature cyclones which are said to be disastrous.
● Formation of cyclones: Formation of temperature cyclones lying between the tropical
regions are due to the same impact.
● Thunderstorms: The heavy downpour associated with thunderstorms called cloud bursts
in the hilly regions.
CONVERGENCE OF CONTRASTING AIR MASSES WILL CAUSE:
i. induction of Lp on the surface.
ii. formation of front
iii. frontal rainfall
iv. jet stream- in the absence of coriolis force. (in the mid latitudes. Absent in extreme poles/
extremely cold & warm regions)
Frontogenesis may get converted into a temperate cyclone because of differential coriolis
force acting on airmasses or front (polar front theory)
When the front becomes parallel to the surface, it is known as frontolysis (death of front)-
causes inversion of temperatures.
Note Imp:
Lp – instability
Cold airmass- instability
Cold front – instability
Hp- stable
Warm a.m : stable
Warm front : stable.
VAJIRAM & RAVI 4
