Composition and Structure of Atmosphere
This article deals with Composition and Structure of Atmosphere’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here
- Atmosphere is combination of two words ‘Atmo’ and ‘sphere’. It means that region of Earth which has ‘air’.
- Atmosphere is present as life saving layer between outer space and land surface. It is the source of important gases which are important for the existence and continuity of life. It also filters the harmful rays travelling towards Earth
Composition of Atmosphere
- Earth’s atmosphere is composed of a mixture of various gases .
- It is held to earth by gravitational forces
- Atmosphere is denser at sea level & thins or gets rarefied rapidly upward . It should be noted that, 99% of the mass of atmosphere is confined to height of 32 km
- Percentage of different gases (by volume) in atmosphere is as follows :-
- Atmospheric gases don’t interact with each other chemically & don’t lose their own property.
- These gases can be divided into two groups based on their distribution horizontally
|Permanent Gases||– Nitrogen , Oxygen, Hydrogen & Argon. |
– Their quantity remain same on all places .
|Variable Gases||– Water Vapour , Carbon dioxide & Ozone |
– Their quantity vary from region to region. Eg : In coastal areas, there will be more water vapours and in cities, there will be more Carbon dioxide.
– They can absorb heat & hence known as Green House Gases.
- Based on vertical distribution, they can also be grouped into two groups. Heavy gases like Nitrogen , Oxygen and Methane have high composition near earths surface. While going up, composition of lighter gases keep on increasing but since there is high turbulence, no effective separation occurs in most of gases except for two gases.
|Water Vapour||Near surface of earth, they are upto 2% by volume but no trace present above 10-12km.|
|Ozone||Found mainly between 10-50 km in stratosphere.|
Side Note : Important gases in Atmosphere ( not on basis of percentage but function )
1 . Nitrogen
- Present in atmosphere in highest proportion (78%)
- It is very important for living organisms because it is an important element of Amino acids which form protein
2 . Oxygen
- Second most abundant gas in atmosphere (21%)
- All the living organisms use it for breathing
- Meteorologically very important gas .
- It is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation . Hence, it is mainly responsible for Green House effect.
- Ozone gas is found between 10-50 km
- It act as filter and absorb UV rays .
- But scientists are very concerned about the depletion of ozone layer due to action of chlorofloro carbons on Ozone
5. Water Vapour
- Variable gas
- Can be upto 4% by volume in wet tropics
- In dry & cold areas of desert & polar deserts it can be less than 1% of air .
- It also absorbs parts of the insolation from the sun and preserves the earth’s radiated heat.
- May originate from different sources & include sea salts, fine soil, smoke-soot, ash, pollen, dust & disintegrated particles of meteors.
- It is concentrated in lower parts , yet convectional air currents can take them to great heights .
- They perform two very
- It provides Hygroscopic nuclei around which water vapour condenses to produce clouds .
- They absorb and reflect small amount of radiation rays of sun.
Structure of Atmosphere
Atmosphere can be divided into five distinct layers based on the thermal characteristics and temperature variations (note : these divisions are based on thermal characteristics)
1 . Troposphere
- Troposphere is the lowest layer of atmosphere and it is very important for all the living organisms
- Name has been derived from Greek word ‘Tropos’ which means mixing and ‘sphere’ which means ‘region’. Hence, ‘troposphere’ means ‘region of mixing’
- Troposphere is zone of air turbulence because in this zone, convectional air currents rise due to heating of earth surface
- Thermal Characteristic of Troposphere
- Temperature decreases with increase in height (reaches – 60 degree Celsius at tropopause).
- In normal conditions, the rate of decrease of temperature is (ie lapse rate) is 6.5 degree Celsius per kilometre . This happens because of decrease in gases with increase in height
- However, due to local reasons, at some places this phenomena reverses also (called Temperature Inversion).
- Height of Troposphere
- on Equator, it is 18 km (gases are heated up and rises upward from strong convectional currents)
- on poles, it is 8 km (gases are cold and settles down)
- At average its height is upto 12 kilometre from ground.
- All weather phenomena occur in this layer as it has dust particles and water vapour. This layer has clouds which produce precipitation on the earth.
- It is the region between Troposphere and Stratosphere which is 1.5 kilometre high
- The fall in temperature comes to an end in this region
- Turbulent mixing of gases, winds, and radiation etc. none of the weather activities take place in this region
- Stratosphere extends from end of Tropopause up to a height of 50 km from the earth’s surface.
- The lower part of this layer is highly concentrated with ozone gas which is called as ‘ozonosphere’. It prevents the harmful ultra-violet rays from the Sun to enter into the lower part of the atmosphere
- Thermal Characteristics of
- Temperature increases with height (ie from – 60 degree Celsius at start to 0 degree Celsius at Stratopause) .
- Temperature increases because of absorption of ultra violent rays by ozone gas
- It is turbulence free zone . Hence, it is ideal for flying jet aircraft.(important prelims question)
- Mesosphere lies above the stratosphere, which extends up to a height of 80 km from earth’s surface .
- Thermal characteristics of
- In this layer, once again, temperature starts decreasing with the increase in altitude
- From 0 degree Celsius at start, it reaches up to minus 100°C at the height of 80 km.
- Most of the shooting stars get burned in Mesosphere .Luminous noctilucent clouds form here due to the presence of cosmic dust. ( important prelims question)
- It is the coldest layer of earth .
4. Ionosphere /Thermosphere
- It extends from 80 km to 400 km above Earths Surface ..
- It is called ionosphere due to presence of electrically charged ions that reflect radio waves back and thermosphere because it is at very high temperature.
- Thermal characteristics of
- Temperature increases rapidly with height .
- The temperature increases rapidly up to 1,000 degree Celsius. This is due to absorption of high energy solar radiation and cosmic waves ( which break molecules to ions).
Ionosphere is formed ?
- High energy sun rays and cosmic rays break atoms of gases in this region .
- Molecules become ionised (positive charged ).
- These are highly energised particles & behave as free particle .
- Luminous phenomenon called auroras at higher latitudes when Solar Winds are able to reach ionosphere and collide with ions present in this layer (Aurora Borealis (Arctic Zone ) & Aurora Australis(Antarctic Zone) )
- Use of Ionosphere in radio communication : It is useful in radio communication because ions can reflect radio waves.
- Outermost layer of atmosphere and lies from 400 km to 1000 km from earth’s surface.
- This is the highest layer but very little is known about it.
- It has rarefied contents. It contains mainly oxygen and hydrogen atoms. These atoms can travel hundreds of kilometres without colliding with one another. Hence, matter in exosphere doesn’t behave like gases.
- It gradually merges with outer space.
Magnetosphere/ Van Allen Radiation Belt
- Magnetosphere lies above Atmosphere and extends from 1000 km to 36000 km from earth’s surface
- Although it isn’t part of atmosphere but plays important part in shielding earth from solar & other cosmic winds .
- Magnetosphere is formed due to earth’s magnetic field and it prevents most of solar winds(highly energised particles) from reaching earth .
Polar cusps: regions above geomagnetic poles where solar wind can enter relatively easy to earth’s atmosphere.
- Magnetospheric storms are temporary disturbances in earths magnetic field caused by occurrence of magnetic flares & sunspot. In this process, material from solar coronal mass ejection hits earth
- Major effect in such event is global disruption of radio & telegraphic communication.
- Luminous phenomenon observed in high latitude regions .
- May appear as rolling lights or coloured streaks .
- Produced by entry of charged particles from sun into earth‘s atmosphere & collision of these charged particles with ionised particles in ionosphere . They emit energy on interaction leading to formation of aurora.
- Entry of these charged particles occur at Cusp . Hence, formed at particular places on earth (& not everywhere)
- Occur in Ionosphere.
- Most frequent during intense period / solar minimum of sun spot cycle(sun spots have cycle of 11yr).
Side Topic : Sunspot , Sunspot Cycle & Solar Minimum
- Sun-spots are the regions on the sun where the solar magnetic field is very strong (and as a result, it doesn’t allow solar streams to escape the sun)
- Sun-spot cycle is the solar magnetic activity cycle with the average time period of eleven years.
- Solar minimum is the period of least solar activity in the eleven year solar cycle. During this time, sunspot activity diminishes. According to NASA and other agencies, a solar minimum is about to occur in 2020-21.
Impact of Solar Minimum
- During the solar minimum, coronal holes can last for a longer time. Coronal holes are vast regions in the sun’s atmosphere where the sun’s magnetic field opens up and allows streams of solar particles to escape the sun.
- It could enhance events of geomagnetic storms & auroras, potentially disrupting communications and navigation systems.
- Sun’s magnetic field weakens and provides less shielding from the cosmic rays. This can pose an increased threat to astronauts travelling through space.