Read this essay to learn about air mass. After reading this essay you will learn about:- 1. Introduction to Air Masses 2. Definition of Air Mass 3. Genesis and Transformation 4. Modification 5. Identification and Analysis 6. Classification 7. Air Masses in Different Regions of the World.
Essay Contents:
- Essay on Introduction to Air Masses
- Essay on the Definition of Air Mass
- Essay on the Genesis and Transformation of Air Mass
- Essay on the Modification of Air Masses
- Essay on the Identification and Analysis of Air Mass
- Essay on the Classification of Air Mass
- Essay on the Air Masses in Different Regions of the World
Essay # 1. Introduction to Air Masses:
Based on the distribution of solar radiation on the surface of the earth, three different thermal regions can be identified. These are tropical, temperate and cold or high latitude regions. There is an uneven distribution of solar radiation on the earth’s surface. The amount of insolation is more in the tropical regions as compared to temperate and polar regions.
Because of this variation, the weather systems in the temperate and cold regions are quite different from that of tropical weather systems. Sharp changes in temperature and wind are found in the temperate and high latitude regions whereas such type of changes in temperature and wind are rarely seen in the tropical regions.
These zones have different types of air masses with different thermal characters. Thus, there is latitudinal imbalance in the heat budget of the earth.
The weather of temperate regions is characterised by cyclones and anticyclones. These cyclonic and anticyclone systems are often associated with the motion of air masses. Most of the weather changes occurring in different regions of the world are due to the interaction of different air masses. The air mass acquires the properties of the underlying surface.
If the air mass is lying on the land surface for a longer period, then it will become warm or cold depending upon the temperature of the underlying surface. Similarly air mass lying on ocean absorbs moisture and transports this moisture to the land surface.
It causes precipitation over the land and releases enormous amount of latent heat. At the same time it can transport lot of heat energy from one region to other. Clouds and rainfall occur at the zone of contact of air masses of different thermal characteristics.
Essay # 2. Definition of Air Mass:
An air mass is defined as a large body of air with fairly uniform temperature and moisture characteristics. It can be several kilometres across and extend upward to the top of the troposphere. A given airmass is characterised by a combination of surface temperature, environmental temperature lapse rate and surface specific humidity from extreme hot to icy cold as well as moisture content.
Essay # 3. Genesis and Transformation of Air Masses:
An air mass is often approximately homogeneous in its horizontal extent with reference to temperature, humidity and air pressure. Although temperature, humidity and pressure change vertically across the length and breadth of the air mass, there is very little change in horizontal direction and air mass gains its uniform character in a source region.
Source region is the place where the genesis of air mass takes place. This may be a region where air is still for long enough to acquire the properties of the surface beneath.
Such as in the anti-cyclonic systems of northern parts of Canada, Siberia and elsewhere or where the fairly constant winds such as north-eastern winds produce the uniform conditions of the atmosphere for a longer period of time. Air masses tend to develop either over oceans or over land.
As far as land is concerned, there should be large area, where there are no high mountain ranges to produce large changes in temperature, pressure and humidity, e.g. no genesis region of air masses over Himalayas, over eddies and over rocks etc.
Beginning of Air Mass:
The idea of air mass was first of all introduced into the subject of meteorology by Norwegian scientist, meteorologist Bergeron in 1928. Favourable conditions for the development of air masses are the areas of subsiding and divergent air and these areas lie in the semi-permanent high pressure belts.
In the belt of low pressure along the equator, however, there is a weak equatorial convergence and stagnated air produce equatorial air masses, the development of the high pressure is seasonal.
Source Regions:
Air masses originate in the areas where conditions promote the development of vast bodies of uniform and horizontal air. Such areas are homogeneous and physically large. The source region should have uniform and flat topography with calm conditions. Also in such areas there must be a sufficient stagnation of the atmospheric circulation so that upper air gets the properties of the underlying surface.
Areas of uneven topography or where the land and water are existing, are unfavourable for the formation of air masses. Similarly, areas with convergent winds can not serve as the source region.
The major source regions of the world air masses are not only homogeneous in nature, but also characterised by anti-cyclonic circulations, which favour the development of horizontal temperature uniformity required in the air masses.
The important source regions of the world are:
1. The snow covered arctic plains of N. America, Europe, Asia and Antarctica,
2. The sub-tropical and tropical oceans,
3. The Sahara desert of Africa, and
4. The continental interiors, Asia, Europe and N. America.
Essay # 4. Modification of Air Masses:
Air masses can move from one region to another under the influence of pressure gradient. When an air mass moves to a new area, its properties begin to change because it is influenced by the new surface environment.
At any particular place or locality, the properties of any air mass depend not only on the nature of source region but also on the transformation or changes of some properties which the air mass has undergone when moving from source region to the observation point e.g. An air mass lies in polar region, when it reaches the tropical region, most of its properties related to temperature, relative humidity etc. will be modified.
These modifications are of two types:
1. Thermodynamic.
2. Mechanical.
1. Thermodynamic Modifications:
These are changes in the properties of air masses due to the thermal characteristics of the underlying surface. These modifications result from the transfer of heat between the bottom of air mass and the surface over which it is moving.
The degree of change or modification depends upon the:
i. Temperature of the underlying surface.
ii. Path of air mass as it leaves the source region.
iii. No. of days it has taken in reaching the observation point.
Unstable Conditions:
If an air mass moves over a surface which is warmer than its bottom temperature, then there will be a consequent warming of the lower air layer, which will result in increased lapse rate and associated instability. This will favour the convection current or ascent in lower layers with the possibility of cooling, condensation and precipitation.
Stable Conditions:
Contrary to it, when an air mass moves over a surface which is colder than its bottom temperature, then there is chilling of the surface air and the development of surface inversion, with a consequent increase in the stability of air mass. Due to this stability, convection currents are absent and consequently there is no condensation and precipitation.
It is clear that the polar air masses will experience most frequently the modifications of first type (unstable conditions) and the tropical air masses will normally undergo the changes of the second type (stable conditions) because tropical air mass moves towards poles and polar air mass moves towards tropics.
Thermal character of an air mass is identified by using letters ‘ W’ and ‘K’ for ‘warm’ and ‘cold’, respectively. The warm air mass is indicated by ‘W’ and cold by ‘K’. If the warm air mass is lying over the cold land surface then the bottom of warm air mass becomes cold and tends to produce stability. Vertical turbulence and convective activity is completely absent.
The chilling of the air is limited to a very shallow layer, usually the first few thousand feet. On the other hand, if cold air mass is lying over warm land surface then the bottom of cold air mass becomes warm. Due to warming, air mass becomes unstable. The warming at the surface associated by its convective nature usually extends to a much greater height.
In an air mass which is moderately unstable aloft, there is much more convective activity and water vapours can extend up to greater heights of the troposphere.
Further thermodynamic modifications are caused by the addition of moisture due to evaporation from water bodies (rivers, lakes, oceans etc.) or from rain drops, which fall through the air mass from an over-running air current. This way heat and moisture is added into the lower atmosphere, which results in increased instability.
2. Mechanical Modifications:
These are of many types and are also responsible for causing the stability or instability. Turbulence caused by the friction of earth surface (hills, mountains, uneven topography etc.) or from thermal differences cause the vertical mixing of the atmosphere. This way the heat and moisture may be carried upwards and a thick layer of air is thereby transformed or modified.
More important modifications, however, are, from large scale horizontal convergence or divergence, which occur well above the surface of earth (approximately 1km). Since circulations cause ascending and descending movements of air and affect the air mass layers.
Horizontal divergence is associated with subsidence. It causes stability, which results into no condensation or precipitation. Horizontal convergence is associated with ascent i.e. convective activity which steepens the lapse rate and this increases the instability and also increases the cloud formation and precipitation.
Consequently, an air mass, which is a part of an anti-cyclonic circulation associated with upper air subsidence is rendered more stable. Similarly, air which descends on the leeward sides of the mountain barriers is made more stable by subsidence.
On the other hand, air mass which enters the cyclonic circulation (i.e. which is dominated by convergence and lifting) is made more unstable. These changes mainly affect those parts of the air mass well above the surface or friction layer and are likely to be independent of other modifications resulting from surface heating or cooling.
Essay # 5. Identification and Analysis of Air Mass:
As an air mass leaves its source region, it experiences many modifications in its way and ultimately it becomes difficult to identify it e.g. if it is formed over a colder region and subsequently passes over a warm ocean surface, its temperature and moisture content increases.
Local surface conditions, ocean currents, topography and minor seas and lakes or night time radiation produce quite different values of temperature and humidity at the bottom of the air mass. Therefore, it is necessary to analyse the conditions in the upper air and to understand the processes that cause changes in air mass properties.
Identification of air mass is based upon three kinds of information:
1. The history of the change in air mass since it left the source region.
2. Horizontal characteristics at certain levels in the upper layers of the air mass.
3. The vertical distribution of temperature, wind and humidity.
Because the Raobs (Radio Sonde Observations) reporting stations are limited and because the major changes in air masses may occur between the times of Raobs, therefore the historical approach alone is inadequate.
Maps plotted with wind, humidity and temperature values at one or more levels show a minimum influence of local surface conditions and reveal moisture and energy components of the upper part of the air mass. Constant level charts of 850, 700, 500, 300 and 100 mb may be plotted and analysed.
The charts showing the horizontal distribution of important weather elements in the upper troposphere help to identify the air masses and their boundaries. The vertical cross-section based upon Raobs or upper wind reports make it possible to do the vertical analysis in the air, especially those that affect the vertical motion.
Wind direction and wind velocity in particular are useful in judging the air mass movement and in flight planning. Temperature and humidity values from Raobs are analysed as a basis for forecasting temperature changes, cloudiness, storms and precipitation.
Upper air observations provide information on the vertical distribution of meteorological elements and these are carried out by means of Raobs. The raobs consist of lightweight box fitted with radio transmitter and sensing devices for pressure, temperature and relative humidity.
Meteorologists commonly use sylphon cells, thermisters and carbon coated hygristers for recording air pressure, temperature and relative humidity, respectively.
This instrument is sent with a gas filled balloon aloft and it bursts at an altitude of 25-30 km. It sends the signals of all the above-mentioned parameters of various heights to the ground stations where these are recorded and translated into usable data. Analysis of the Raobs from a number of ground stations helps the forecasters to determine the properties of air masses.
At some stations, radio sonde is followed by radar for recording the direction and speed of wind at various levels. The technique is known as Rawin and combined technique is known as Rawin Sonde.
Another type of instrument used for this purpose is trans-sonde. It is a balloon born radio meteorological device, the altitude of which can be controlled by releasing blast or gas from the balloons as it floats for several days at a constant pressure level. It can obtain the data over the oceanic surfaces.
Similar large scale techniques known as global horizontal sounding techniques (ghost) have provided valuable data over the southern hemisphere westerly winds.
Essay # 6. Classification of Air Mass:
Classification of air mass is based upon the source region and the temperature & moisture conditions.
The two main categories are:
a. Tropical or Sub-tropical.
b. Polar or Sub-polar.
Because the great source regions are located at high and middle latitudes. Sub-divisions are made depending upon their continental or oceanic origin and further classification is made according to what modifications these air masses have experienced as they have moved from their source regions.
Finally air masses become so much modified that they need special designations for climatological purposes. This classification is very important as the air masses dominate the different regions and determine the climate of these regions.
In practice, letter symbols are used to designate air masses and these small letters are placed first following the source regions like tropical (T) and polar (P), equatorial (E), arctic (A) and Antarctic (AA) are used.
Trewartha (1967) conducted a study of air masses based on geographical position.
The air masses can be classified into two main categories:
(1) Tropical air mass (T)
(2) Polar air mass (P)
He emphasised that the Equatorial, Arctic and Antarctic air masses may be taken as the modified form of tropical and polar air masses. These air masses can be further sub-divided on the basis of their source regions. Small letters are used to indicate continental and maritime air masses by ‘c’ and ‘m’, respectively.
Continental and maritime air masses are found over land and sea, respectively. Continental air masses are dry, whereas maritime air masses contain large amount of moisture.
Based on the source region and nature of the surface, the air masses can be divided into four categories:
(a) Continental Tropical air mass (cT)
(b) Maritime Tropical air mass (mT)
(c) Continental Polar air mass (cP)
(d) Maritime Polar air mass (mP)
Apart from the above classification, thermodynamic and mechanical modifications can also be considered. Capital letter ‘K’ stands for cold and letter ‘ W’ stands for warm. Similarly small letter ‘u’ stands for unstable air mass and letter’s’ stands for stable air mass.
The air masses can be classified into the following categories by using all letters:
(1) Tropical Air Mass (T):
(2) Polar Air Mass (P):
Essay # 7. Air Masses in Different Regions of the World:
1. Maritime Tropical Air Masses (mT) during Winter Season:
The main characteristics of these air masses are given below:
i. These air masses are warm, moist and unstable.
ii. Moisture is well distributed in the troposphere.
iii. Steep lapse rate is prevalent.
iv. Heavy rainfall occurs in the evening hours.
v. These air masses are found over the warm oceans in both the hemispheres.
2. Maritime Tropical Air Masses (mT) during Summer Season:
i. These air masses are very warm, moist and highly unstable.
ii. Convective instability of these air masses is very high.
iii. These air masses are found in the belt of semi-permanent high pressure areas of the tropical oceans including Caribbean sea.
3. Continental Tropical Air Masses (cT):
i. These air masses are warm and dry during summer season.
ii. Sub-tropical high pressure on the land areas are the source regions.
iii. During summer season they are very hot and cause heat waves over the land.
iv. Subsidence is very common.
v. The air mass becomes convectively unstable when cT air is found aloft over warm and moist air at the surface.
vi. The environmental lapse rate exceeds dry adiabatic lapse rate, which leads to the formation of violent thunderstorms or tornadoes.
4. Continental Polar Air Masses during Winter Season (cP):
i. Central Canada and Siberia are the source regions of these air masses.
ii. These air masses are extremely cold, dry and stable.
iii. No clouds occur due to stable conditions.
iv. When these air masses move out of the source region, they generate intense cold waves.
v. When they move across warm surface, they get modified.
vi. Sometimes cumulus and stratocumulus clouds are associated with modified air mass.
5. Continental Polar Air Masses during Summer Season (cP):
i. Central parts of high latitude continents are the source region.
ii. Due to warm surface snow cover disappears.
iii. These air masses are the modified air masses of winter continental polar air masses which have been heated in the lower latitudes.
iv. When continental polar cold air masses move across over oceans, they get modified and become continental polar warm air masses which may cause fog and low clouds.
6. Maritime PolarAair Masses during Winter Season (mP):
i. These air masses are found over the oceans at high latitudes.
ii. These air masses contain some clouds when they are over their source regions.
iii. When these air masses are forced to ascend the mountains, wide spread rainfall occurs.
iv. These air masses are dry and cold in their upper layers and moist and unstable in their lower layers and can produce light rainfall.
7. Maritime Polar Air Mass during Summer Season (mP):
i. Temperature is slightly higher than that in the maritime polar air masses of winter season.
ii. These air masses originate in the source regions of maritime polar air masses.
iii. These air masses are dry in upper layers but cool and moist in the lower layers.
iv. Sometimes temperature inversion appears with moisture discontinuity.
v. These air masses are stable upto moisture discontinuity.
Air Masses of Asia:
Asia is the largest continent extending from the equator to the north pole with hottest air masses to coldest air masses. Therefore, extremes of temperature are found. The upper air data indicates that strong westerlies are found in the upper troposphere during winter season but these westerlies become weak during summer season.
The following air masses are found in this continent:
1. Maritime Tropical Air Masses (mT):
The land mass of southern Asia is confined within the tropics whereas the land mass of eastern Asia is confined to the temperate region. During summer season, these parts are dominated by maritime tropical air mass (mT). Therefore summer monsoon circulation consists of maritime tropical air mass.
This air mass is hot and moist, therefore, convective instability is very high. Whenever these air masses move across hot land, strong convection currents are set up leading to towering cumulonimbus clouds, causing heavy rainfall. On the other hand, if maritime tropical air masses ascend mountains, they cause heavy rainfall. South-west monsoon causes heavy rainfall on the windward side of western ghats of India.
South-east trade winds after crossing equator becomes south-westerly monsoon winds due to coriolis force. When these south-westerly reach north China, Korea and Japan, they come across continental Polar cold air masses.
A line of discontinuity is generated between warm moist tropical air masses and continental polar cold air masses. This line of discontinuity is called a polar front. The hot and moist air mass ascends the polar front causing heavy rainfall during summer season. Japan and eastern China receive rainfall from summer monsoon which comes out of an anticyclone over Pacific ocean.
Maritime tropical air masses which originate over the southern parts of the oceans during winter season, fail to enter the vast size of the Asia. But maritime tropical air masses with marked instability are seen in south-west Pacific ocean.
These air masses are not only convectively unstable, but have a very high moisture content as well as high surface temperature. Eastern Indonesia is dominated by unstable tropical maritime air masses originating from continental polar air masses. Winter cyclones moving northward from the southwest pacific ocean are accompanied by maritime tropical air masses.
Sometimes during winter season, south-west Asia is invaded by surface air masses which are very close to maritime tropical air mass. They move across Turkey ahead of eastward moving winter cyclones. These air masses absorb moisture from the eastern Mediterranean sea.
Similarly, when the low pressure area move across the Persian gulf and northern India, it is called western disturbance. They are often associated with maritime tropical air masses.
These air masses differ from typical maritime tropical air masses as they are only modified continental air masses. During winter season, south Asia is dominated by north-east trade winds. These winds acquire maritime characteristics after moving across sea surface and then strike the coastal region. These north-east trade winds are called winter monsoon over Indian Peninsula.
2. Continental Polar Air masses:
During winter season, the central eastern Siberia and Mongolia are the source regions of the continental polar air masses. These air masses are very cold, dry and stable. Temperature inversion is found in the lower layers. This inversion layer is broken when these air masses move towards the mountain barriers of the Pacific.
The temperature and humidity increase in the lower layers when these air masses move away from the source regions. Because of Siberian high pressure and strong anticyclones, the effect of continental polar air masses is experienced over many parts of the continents. These cold air masses are obstructed by the Himalayas to enter Indian sub-continents.
These cold air masses enter into China by land and sea. The temperature of these air masses over China is higher by several degrees than the source region. While crossing Himalayas, the continental polar cold air mass gets heated up due to dry adiabatic warming. These modified continental polar air masses decrease the temperature over North India.
If these air masses continue to blow over north-west India, cold air mass is advected resulting decrease in temperature over vast area. The cold advection leads to cold wave and frosty conditions. Moreover, these air masses produce rainfall in association with extra-tropical cyclones or western disturbances.
3. Maritime Polar Air masses (mP):
The influence of these air masses is confined to the coastal areas of Siberia, Manchuria and Korea. The coastal areas of South Korea remain outside the influence of polar maritime air masses due to the westerly circulation in eastern Asia during winter season. The mainland of China remains completely free from these air masses.
During summer season Manchuria and eastern Siberia remain under the influence of these air masses. Maritime polar air masses form a front with the overlying maritime tropical air masses in the early summer. This front is generally associated with cyclonic circulation and causes light rainfall in Japan.
In winter season, along the western coastal areas of the continents, where the cool air is flowing on shore (coast) towards the interior, is usually moist and warmer than the dry cold air over the land. Then warm air occlusions are specially prevalent.
In its initial stages, this type of occlusion shows most of the characters of the warm front, then follows moderate frontal activity in connection with the upper cold front. As the occlusion process continues, the whole system tends to dissipate, because there is lack of sharp persistent differences in air mass properties.