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Air Masses- concept, classification and properties
Air Mass
An air mass is a vast body of air (a macro-scale phenomenon that can have global
implications) in which the condition of temperature and moisture are much the same at all
points in a horizontal direction. In other words, an air mass is any large body of the lower
atmosphere that has fairly uniform conditions of temperature and moisture. Air masses form
an integral part of the global planetary wind system. An air mass takes on the temperature
and moisture characteristics of the surface over which it forms. The processes of radiation,
convection, condensation, and evaporation condition the air in an air mass as it travels. The
region where the air mass develops is known as a source region of an air mass. The source
region of an air mass is any large body of land or water from which the air derives the
characteristics. Two conditions govern the occurrence of the principal source regions:
1. They occur in the regions of relative calm in the general circulation, where air
motion is sufficiently slow for homogenous air mass characteristics to develop.
2. Source regions are usually the areas where the earth’s surface itself is fairly
uniform, such as oceans, deserts or large ice and snow-covered areas.
Classification of Air Masses
Typically, air masses are classified according to the characteristics of their source
region or area of formation. Air masses are also classified as being either continental or
maritime in terms of moisture characteristics. They are also sub-categorised on the basis of
thermodynamic and mechanical changes. The classification of air masses is shown in figure
2.3.
Polar Air-masses (P)
CP
CPK
CPKS
Lecture prepared by:
MP
CPW
CPKU CPWS
CPWU MPKS
MPW
MPK
MPKU
MPWS
MPWU
Mr Manzoor Ahmad Bhat
Air Masses- concept, classification and properties
Tropical Air-masses (T)
CT
CTK
CTKS
CTKU CTWS
MT
CTW
MTW
MTK
CTWU MTKS
MTKU
MTWS
MTWU
Where; C= Continental, P = Polar, K = heated from below, W= cooled from above,S
= stable in upper side, U = unstable from upper side.
Fig. 2.3: Classification of Air masses
Continental polar air masses: These are characterised by dry, cold and stable conditions
with very cold and dense air during winter at the source region. The weather during winter is
frigid, clear and stable with very little cloudiness and precipitation. However, the weather is
less stable during summer. The source regions for these air masses are the Arctic basin,
northern parts of North America, Eurasia and Antarctica.
Maritime polar air masses: These are actually those continental polar air masses which
have moved over the warmer oceans, got heated up and have collected moisture. The
conditions over the source region are cool, moist and unstable. The weather during winter is
characterised by mild temperatures, high humidity, overcast skies and occasional fog and
precipitation. However, the weather is fair, clear and stable during summer. The source
regions are the oceans between 400 and 600 latitudes.
Continental tropical air masses: These air masses are hot, dry, stable and localised and thus
do not expand beyond the source. They remain dry throughout the year and very hot during
summer with scanty rainfall. The source regions for these air masses are the tropical and subtropical deserts of Sahara, west Asia and Australia.
Maritime tropical air masses: These air masses are warm, humid and unstable at the source
regions. The weather conditions during summer are characterised by high temperature, high
Lecture prepared by:
Mr Manzoor Ahmad Bhat
Air Masses- concept, classification and properties
humidity and conventional rainfall. However, the weather during winter has mild
temperatures, overcast skies with fog and drizzle. The source regions for these air masses
include the tropical and sub-tropical oceans.
Fronts
The air masses with different characteristics do not mix readily and try to retain their identity
as far as temperature and moisture are concerned. The zone of convergence between two air
masses is called a front. When two different air masses collide, a front is formed. A front is
not a sharp wall but rather a zone of transition that is often several kilometres wide. The ideal
conditions for a front to form are temperature differences and converging air which should be
strong enough to move one air mass towards another. The process of the formation of a front
is called frontogenesis. Four types of fronts—warm, cold, occluded, and stationary—can be
formed by air of different temperatures.
Warm Front: A warm front marks the advance of a warm air mass as it rises up over a cold
one. Warm fronts occur in places where warm air advances into relatively cool air. This
interaction causes the warmer air to slowly slide over the top of the underlying cooler air
(Fig. 2.4). This process of gradual overriding of cooler air actually occurs in the upper
atmosphere ahead of the surface warm front and causes the lifting air to cool adiabatically. A
warm front is the transition zone in the atmosphere where an advancing warm subtropical,
moist air mass replaces a retreating cold, dry polar air mass. As the air cools, clouds form,
beginning with high level cirrus clouds at the top of the upwardly moving air mass. As the
warm front approaches, these clouds change to progressively lower stratus clouds,
culminating in rain-producing nimbostratus clouds at the surface front. When these
conditions evolve, the sky may be overcast with a slow but steady rate of precipitation that
may last a day or two. This precipitation can be in the form of drizzle, light rain, and even
snow. The weather conditions of fronts are shown in table 2.1.
Lecture prepared by:
Mr Manzoor Ahmad Bhat
Air Masses- concept, classification and properties
Fig. 2.4: Uplift of air at a warm front.
Cold Fronts: A cold front marks the line of advance of a cold air mass from below, as it
displaces a warm air mass. Cold fronts occur when cool air moves into a region that was
previously dominated by warmer air. The cold air is denser and heavier than the warm air
ahead of it, so the warm air is forced to rise. In this fashion, a cold front is significantly
different from a warm front because the cold air hugs the surface along a cold front and
vigorously drives the warm air ahead of it aloft. Notice in Figure 2.5 that the edge of a cold
front is very steep when compared to the warm front in Figure 2.4. Once the warm air begins
to lift along the cold front, adiabatic cooling starts and vapour condenses, forming clouds.
Because the air rises rapidly, it quickly cools adiabatically, which means that large amounts
of latent heat energy are rapidly released as the water condenses. As a result, rainfall is
intense and of short duration. If sufficient moisture is present and enough latent heat is
quickly released, an intense thunderstorm can form, with the latent heat being the fuel for its
development.
Lecture prepared by:
Mr Manzoor Ahmad Bhat
Air Masses- concept, classification and properties
Fig. 2.5: Uplift of air along a cold front
Table 2.1: Weather conditions associated with warm and cold fronts
Weather
Warm Front
Cold Front
condition
Before the
Passing of
Front
Temperature
Atmospheric
Pressure
Precipitation
Clouds
At the time of
Front
After
Passing
Front
the
of
Before
the
Passing
of Front
Cool
Warming
suddenly
Decreasing
steadily
Levelling off
Warmer
then
levelling off
Slight
rise
followed by a
decrease
Showers,
snow, sleet
or drizzle
Light drizzle
None
Showers
Cirrus,
cirrostratus,
altostratus,
nimbostratus
, and then
stratus
Stratus,
sometimes
cumulonimbus
Clearing
with
scattered stratus,
sometimes
scattered
cumulonimbus
Cirrus
and
cirrostrat
us
Warm
Decreasi
ng
steadily
At the time
of Front
After
Passing
Front
Cooling
suddenly
Levelling
off
then
increasing
Heavy rain
or
snow,
hail
sometimes
Cold
and
getting colder
Cumulus
and
cumulonimb
us
the
of
Increasing
steadily
Showers then
clearing
Cumulus
Occluded fronts: Occluded fronts are formed when cold and warm fronts merge. Occluded
fronts are produced when a fast moving cold front catches and overtakes a slower moving
warm front. Two types of occluded fronts are generally recognized. A cold type occluded
front occurs when the air behind the front is colder than the air ahead of the front. When the
air behind the front is warmer than the air ahead of the front a warm type occluded front is
produced. Warm type occlusions are common on the west coast of continents and generally
form when maritime polar air collides with continental polar or arctic air.
Stationary front: When the surface position of a front does not change, a stationary front is
formed. As the name implies, the air masses around this front are not in motion. A stationary
front can cause bad weather conditions that persist for several days.
Lecture prepared by:
Mr Manzoor Ahmad Bhat