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Chapter 13. Atmosphere (Geography Notes)


Nitrogen (N2) 78.084 %
Neon 00.0018 %
Oxygen (O2) 20.947 %
Helium 00.0005 %
Argon (Ar) 00.934 %
Methane 00.0002 %
Carbon Dioxide (CO2) 0.0314 %
Krypton 00.0001 %
Hydrogen (H2) 00.00005 %
Structure Of Atmosphere

➤ The atmosphere has a layered structure.
➤ This is because of density stratification as a result of this lighter gases move up and denser one settles down.
➤ The atmosphere can broadly be divided into 5 layers, there are :
(1) Troposphere
➤ The troposphere is the lowest layer of the atmosphere.
➤ It extends roughly to a height of 8 km. near the Poles and about 18km. at the equator.
➤ Temperature decreases with height, roughly at the rate of 1°C for 165 meters of ascent. This is known as normal lapse rate.
➤ This layer contains dust particles and over 90 per cent of the earth’s water vapour.
➤ All vital atmospheric processes leading to various climate and weather conditions take place in this layer.
(2) Stratosphere
➤ It lies beyond the Troposphere.
The zone separating the two layers is known as the tropopause.
➤ The stratosphere extends upto a height of 50 km.
➤ In the lower part of this layer
(upto 20 km) temperature remains constant. Afterwards it gradually increases upto a height of 50 km because of the presence of Ozone layer.
➤ Clouds are almost absent and there is very little dust or watervapour.
(3) Mesosphere
➤ It extends upto a height of 80 km.
➤ Temperature decreases with height again and reaches upto –100°C at the height of 80 km.
➤ Bulk of the meteors are destroyed in this region.
➤ It is also called ‘Chemosphere’.
➤ In upper part of the Mesosphere (60-80 km) oxygen exists in its atomic form.
(4) Thermosphere
➤ It is located between 80 to 400 kms.
➤ In its lower part, there is an electrically charged layer called the Ionosphere.
➤ Radio waves transmitted from the earth are reflected back to the earth by this layer.
➤ Temperature again starts increasing with height because of radiation from the Sun.
➤ The aurora borealis and aurora australis sometimes occur in the upper part of thermosphere and lower part of the exosphere, where they overlap into the thermosphere.
➤ Thermosphere is a region of the atmosphere that is ionized by solar radiation.
➤ It is responsible for auroras.
➤ During daytime hours, it stretches from 50 to 1000 km and includes the mesosphere, thermosphere and part of the xosphere.
➤ Ionization in the mesosphere largely ceases during the night, so auroras are normally seen only in the thermosphere and lower exosphere.
➤ The ionosphere forms the inner edge of the magnetosphere.
(5) Exosphere
➤ It is the uppermost layer of the atmosphere extending beyond the Thermosphere above a height of 400 kms.
➤ Here the atmospheric gases are very thin.
➤ This part is extremely rarefied and gradually merges with the outer space.
➤ Here the temperature may reach to astonishing high value of more than 5000°C.
Atmospheric Pressure
➤ Atmospheric Pressure is the weight of the column of air at any given place and time.
➤ It is measured by means of an instrument called a Barometer.
➤ It is measured as a force per unit area. The units used by meteorologists for this purpose are called Millibars (mb).
➤ The distribution of atmospheric pressure is shown on a map by Isobars.
➤ An Isobar is an imaginary line drawn through places having equal atmospheric pressure reduced
➤ Horizontal movement of the air is called as wind. Due to horizontal difference in air pressure, air flows from areas of high pressure to areas of low pressure.
➤ The vertical or nearly vertical movement of air is referred to as air current.
➤ Winds and air currents together comprise a system of circulation in the atmosphere.
Types of Wind

(1) Planetary winds : These winds blow throughout the year from one latitude to the other in response to the latitudinal differences in air pressure, for example, the trade winds and the westerly winds.
Trade Winds : The winds which blow from the sub-tropical high pressure towards the equatorial region of low pressure regularly throughout the year in many areas especially the oceans and the hot deserts from north-east in the northern hemisphere. It brings little rain except on the line of convergence of the two trade wind systems.
Westerlies : The Westerly winds are those which blow with great frequency from the Horse Latitudes towards the polar region throughout the year with varying intensity and cause rain near the polar regions.
Westerlies are stronger in the Southern Hemisphere because of the vast expanse of ocean water. Owing to their ferocious nature, they are also described as “Roaring Forties”. “Furious Fifties” and “Shrieking Sixties”
which were dreaded terms for navigation.
Doldrums : Also known as inter-tropical convergence Zone (ITCZ). It is the equatorial belt of low atmospheric pressure where the north-east and south east Trade winds converge, it is a region of calmness. The calmness periodically broken by storms, accompanied by heavy rains.
Horse Latitude : They are the sub-tropical belts of high atmospheric pressure over the oceans (near 30° latitude) between the regions of Trade winds and Westerlies. They are regions of calm, light variable winds and
Coriolis Force : The phenomena of deflection of all winds to the right, anti-clockwise in the northern hemisphere, while they deflects to the left, clockwise in the Southern hemisphere with respect to the rotating Earth is called Coriolis Force. The Coriolis force is zero at the equator and maximum at the poles as it depends on the sine of latitude. With the increasing wind velocity, coriolis force also increases. Since, this phenomenon was firstly proved by a French Scientist Ferrel, it is called Ferrel’s Law.
(2) Periodic winds : The Winds changing their direction periodically with change in season are called periodic winds. Monsoons are the best examples of large scale modification of the planetary wind system.
Monsoon : The word monsoon has been derived from the Arabic word “Mausam” which means season. The monsoon winds thus refers to the wind systems that have a pronounced seasonal reversal of direction.
➤ The monsoon winds blow over India, Pakistan, Bangladesh, Myanmar, Sri Lanka, Arabian Sea, Bay of Bengal, South-Eastern Asia, Northern Australia, China and Japan.
(3) Local Winds : Local winds develop as a result of local differences in temperature and pressure.
They affect small areas and are restricted to the lowest levels of the troposphere.
Loo : A very hot and dry wind
(hot wave) in the North Western India and Pakistan which blows from the west in afternoon of May and June and may cause sunstroke.
Mistral : The cold wind which originates over the snow-covered mountains of Alps and blows towards the Mediterranean Sea.
Chinook and Foehn : Warm and dry local winds blowing on the leeward sides of the mountains are called Chinook in the USA and Foehn in Switzerland.
Tornado : An extremely violent whirl-wind, covering a small area, sometimes wind velocity exceeds 300 km per hour. It occurs frequently in the Mississippi basin of the U.S.A. and Sahara.
Harmattan : The warm and dry winds blowing from north-east and east to west in the eastern parts of Sahara Desert are called “Harmattan”. Similar winds are called ‘brickfielder’ in Australia, ‘blackroller’ in USA, ‘Shamal’ in Mesopotamia and Persian Gulf and ‘Norwester’ in New Zealand.
Sirrocco : It is a warm, dry and dusty wind which blows in northerly direction from Sahara desert and after crossing Mediterranean Sea reaches Italy, Spain etc. Similar winds are known as ‘Khamsin in Egypt, ‘Gibli’ in Libya, ‘Chilli’ in Tunisia,


An air mass is defined as a large part of atmosphere, having similar physical properties (especially temperature and humidity) spreading over hundreds of kilometers. In order to acquire the similar physical properties air mass must be stationary for a longer period of time on the source regions. The notable centre for the development is anticyclone area which is characterised by high pressure and low pressure regions.
Classification of Air mass Generally the air mass is classified into 4 types:
(1) Polar Air mass (P) – It originates in polar region between 60° N and S. Polar air mass is cold.
(2) Tropical Air mass (T) – It originates between 25°N and S upto equator. Tropical air mass is warm.
(3) Continental Air mass (C) – It originates over continents (land mass) and is dry in nature.
(4) Marine Air mass (M) – It originates over the oceans and in moist in nature.
These four air masses are combined with one other to form four principal air masses such as:
(1) Continental polar mass air (CP) – Cold, air and stable.
(2) Maritime polar mass air (MP) – Cold, air and unstable.
(3) Continental Tropical air mass air (CT) – hot, dry, stable (at height) and at surface Instable.
(4) Maritime Tropical air mass air (MT) – warm, moist and unstable.

Weather Front

➤ A weather front is a boundary separating two masses of air of different densities and is the principal cause of meteorological phenomenon.
➤ The air masses separated by a front usually differ in temperature and humidity.
➤ Cold fronts may feature now bonds of thunders forms and severe weather and may on occasion be preceded by squall lines or drylines.
➤ Warm fronts are usually preceded by stratiform preciptiation and fog.
➤ The weather clears quickly after a front’s passage.
➤ Some fronts produce no precipitation and little cloudiness, although there is invariability in a wind shift.
➤ Cold fronts and occluded fronts generally move from the west to east while warm fronts move poleward.
Types of Front : There are four types of Fronts —
1. Cold Fronts
2. Warm Fronts
3. Occluded Fronts
4. Stationary Fronts
1. Cold Fronts : A cold front is located at the leading edge of the temperature drop off which in an isotherm analysis shows up as the leading edge of the isotherm gradient and it normally lies within a sharp surface trough.
➤ Cold fronts often bring heavy thunder forms rain and hail.
➤ Cold fronts can produce sharper changes in weather and move up to twice as quickly as warm fronts since cold air is denser than warm air.
2. Warm Fronts : Warm fronts are the leading edge of a homogeneous warm air mass, which is located on the equatorward edge of the gradient in isotherms.
➤ These fronts lie within broader trough of low pressure than cold fronts.
➤ A warm front moves slowly than the cold front which usually follows because cold air is denser and lander to remove from the earth’s surface.
➤ Clouds ahead of the warm front are mostly stratiform and rain form gradually increases as the front approaches.
➤ Fog can also occur preceding a warm frontal passage.
3. Occluded Front : The overtaking of a warm front by a cold front in an atmospheric depression.
Which ultimately lifts the warm sector off the surface of the Earth, is called an Occluded front. This front shows the characteristics of both cold and warm front.
4. Stationary Front : This front is formed when two contrasting air masses converge in such a way that they become parallel to each other and there is no ascent of air.
Cyclone in centre of low pressure surrounded by elliptical arrangement isobars. The pressure increases outward and the air move inward as the centre has low pressure. In the northern hemisphere it has anticlockwise movement and clockwise in the southern hemisphere. It can be circular, elliptical or ‘V’ shape.
It is of two types
– Extratropical/Temperature cyclone
– Tropical cyclone
➤ An extratropical cyclone is a synoptic scale low pressure weather system that does not have tropical characteristics, being connected with fronts and horizontal gradient in temperature
(1) Extratropical/Temperate cyclone
It is a simple depression or atmospheric disturbance having low pressure in the centre and pressure increases outward. It is found in the middle latitude (35°-65°) in both opposing air masses (warm, moist and light tropical air mass, front is created where these two air masses converge and lead to the development of temperate cyclone. The size of cyclones varies from 150 km to 3000 km. On an average vertical extent is 10-12 km. Its average summers velocity is 32 km per hours for winter it is 48 km per hour.
(2) Tropical Cyclone The development of the cyclone over the tropical region lying between tropic of cancer to capricorn are called as Tropical Cyclones. Generally the tropical cyclones are formed along the zone of confluence of south-east and north-east trade winds. The confluence zone of these two winds is also known as the Inter Tropical Convergence Zone (ITCZ). •• Its average diameter varies between 80 km to 300 km. At times it is restricted to 50 km or even less in diameter. •• Velocity varies between 32 km/hr to 180 km/hr or more when it is coverted into a hurricane. •• Tropical cyclone becomes more vigorous and move with high velocity over ocean but over land it becomes feeble as it reaches interior portion of the continents. •• There are less number of isobars and are more or less in circular shape. This results in rapid rush of wind towards the centre. •• Every year it occurs at a particular period of a year, mainly during summer season. •• It is not generated near equator as the coriolis force is negligible at equator.
Tropical Cyclones are known by different names :
Hurricanes – Carbbean and Pacific coast of Mexico.
Typhoons – Sea of China and Japan
Cyclones – India / Australia
Willy-Willies – North Australia
Tornado – South and Eastern USA.
Bagguio – Philippines

Comparison Between Temperature And Tropical .

Tropical CycloneExtra-tropical cyclone
The tropical cyclone have a thermal origin, exclusively over the tropical seas.Formed in middle or high latitudes, due to the development of front (35°-65° N and S)
The size of the tropical cyclone is 1/3 of temperate cycloneIt is much extensive size.
Strongest winds of tropical cyclones take place at surface.Strongest winds of mid-latitude cyclones are higher up in atmosphere.
It is associated with single eyeThere is more than one place where wind and rain is active.
Wind velocity is very highIt is low in comparison to tropical cyclone.
Its relation with upper level air is not clearIt has a distinct relation with of air.

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