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Composition and Structure of Atmosphere

  • About 99% of total mass of atmosphere is within the height of 32 km from earth surface, rest above merges with outer space
  • Atmospheric Air Colorless, Tasteless, Odorless, Compressible, Mobile & Expandable
  • Atmospheric air mainly consists of -

 

  • Nitrogen  78.03 %
  • Oxygen     20.99 %
  • Argon        .94 %
  • CO2           .03 %
  • H                .01 %
  • Rest are Neon, Helium, Ozone, Xenon & Krypton in very minute percentage

Composition of atmosphereNitrogen

  • Major cause of atmospheric pressure, Force of wind & reflection of light
  • Regulates combustion & avoids quick burning
  • Dilutes oxygen and prevents rapid burning at the earth’s surface
  • Nitrogen cannot be used directly from the air.
  • The Nitrogen Cycle is nature’s way of supplying the needed nitrogen for living things
  • Get fixed to soil & generates protein in plants (Major source of food)

CO2

  • Heaviest gas in atmosphere, essential for growth of vegetation
  • Transparent to incoming solar radiations but opaque to terrestrial radiations radiated from earth surface which keeps air near the ground warmer
  • Responsible for global warming, as alarming increase in level of CO2 in past few years due to burning of fossil fuels

 

Water Vapour

  • Form in atmosphere due to evaporation from surface water bodies
  • Absorbs part of insolation from the sun & reduces its amount reaching the earth Preserves earth’s radiated heat
  • Hence, acts as a blanket, allowing the earth to neither become too hot or too cold
  • Amount of water vapour decrease with altitude
  • Amount of water vapour decreases from equator to poles

 

Dust Particles

  • Derived by strong winds from different sources on earth surface
  • Acts as hydroscopic nuclei around which water vapour condense & form clouds
  • Intercept & reflect sun rays giving orange colour at sunrise, red colour at sunset & blue color of clear sky
  • Molecules (Dust particles) in air scatter blue light (shorter wavelength) more than red light (longer wavelength) in every direction, hence sky looks blue
  • At sunrise & sunset, sky appears orange & red because blue light has already been scattered out by molecules in the air, out of sight & longer wavelength light reaches out to us
  • Dense Haze & smog (smoke + fog) are also caused due to presence of dust particles in the air

 


Structure of Atmosphere

  • On the basis of chemical composition atmosphere can be divided into 2 parts viz.

 

Homosphere Upto 90 km (Uniformity in chemical composition)

  • Troposphere
  • Stratosphere
  • Mesosphere
HeterosphereAbove 90 Km (non uniformity in chemical composition)

  • Ionsphere
  • Exosphere

 

Each sublayer mentioned in these 2 layers is further divided by a shallow transitional zone ending with term “Pause” 

Structure of atmosphere


Troposphere

  • Average height is 16 Km 10 km at Poles & upto 18 km at Equator
  • Greatest at Equator due to upward transportation of heat by strong conventional currents
  • That is why height of troposphere at a given latitude is greater in summer
  • Temperature decreases with height, roughly at 1*C for every 165 meters Normal Lapse Rate
  • Lowest, Densest & contains 75 % of earth’s atmosphere with 90% of water vapours & dust particles
  • All major atmospheric processes take place in this layer

Tropopause

  • Shallow transitional zone (Unstable zone) b/w Troposphere & Stratosphere (Approx. 1.5 Km)
  • Temperature stops falling in this layer – 80*C over equator & – 45*C over poles

Stratosphere

  • This layer rises upto 50 km (Thicker at Poles than at Equator)
  • Temperature remains constant in its lower portion upto 20 Km & then gradually increases to 0*C till its upper limit i.e. Tropopause
  • increases mainly due to the presence of ozone gas, which absorbs sun’s UV rays
  • Practically no clouds, convection currents, Thundering or lighting, water vapours or dust particles hence airplane fly in this region
  • Few clouds called “Mother of Pearls / Narcreas” can be seen over Antarctica
  • Its lower portion (15 – 35 Km) constitutes Ozone layer which prevents us from harmful UV rays
  • amount of Ozone gas is found at Stratopause i.e. uppermost limit of Stratosphere

Depletion of Ozone Layer

  • Major cause CFCs (mainly from Refrigerator, AC, Spray Cans, Plastic Packaging, Cleaning Fluids, Insulation materials)
  • UV breaks CFCs & release Chlorine atom which reacts with Ozone & convert it into simple oxygen molecule, which is unstable to absorb UV rays
  • Space probes are also responsible for depletion of Ozone layer, as every time a rocket is fired into space, 70 – 150 tons of Chlorine is injected into atmosphere
  • Another cause for ozone layer depletion is oxides of Nitrogen, esp. Nitric oxide, released from supersonic aircrafts, automotive exhaust, in form of nitrates in fertilizers etc.
  • Already a major Ozone hole is formed over Antarctica with risk countries New Zealand, Australia, South Africa, Chile, Argentina etc.

 


Mesosphere

  • Upto 80 ~ 90 Km, Temp. decreases gradually with height until – 100 *C at 80 km
  • Displays wispy clouds at high altitudes due to reflected sunlight from meteoric dust particles
  • Most weather balloons are placed in this region
  • Most meteors burn up in this layer ; Upper limit Mesopause

 


Ionosphere / Thermosphere

  • Extends upto 400 km, contains electrically charged particles (ions) with max. conc. at 250 Km
  • starts rising with increase in height because of ionization by solar radiations
  • Zone of Earths Satellites
  • Auroras Are formed when Earth’s magnetic field traps solar winds in atmosphere, resulting in collision b/w solar wind & atmospheric charged molecules (ions)

 

  • Aurora Borealis Northern light in northern hemisphere (Arctic circle)
  • Aurora Australis Southern light in southern hemisphere (Antarctic Circle)

 

  • All radio waves are reflected in this layer (Radio transmission)

 

  • D layer Reflects signal of low frequency & absorbs of medium & high frequency
  • E layer (Kennedy Heaveside layer) Reflects medium & high frequency radio waves to earth
  • F layer (Appleton layer) Useful for long distance radio transmissions reflects medium & high frequency radio waves to earth
  • G layer Highest layer

 


Auroras

  • Solar flares from Sun (leaving black spots on sun) entering Ionosphere forms Auroras in N & S poles.
  • The auroras appear as sheets of illuminated curtains with streaks or as bright arcs in the sky.
  • These lines or arcs are in the form and directions of the magnetic lines of force of the earth; indicating the involvement of moving charge particles under the influence of concentrated magnetic field of the earth.
  • When they reach the earth, they get concentrated near the poles because of the strong axial magnetic fields there.
  • In other regions of the earth, the magnetic field is weaker and so the aurora does not form although the emission of the solar wind particles takes place uniformly.
  • There is an 11-year solar cycle (on average) that controls the tempo of the aurora. The most recent peak in the cycle occurred in 2000-2001. The next peak is predicted in 2013 – 2014.


Solar Tsunami

  • When solar flares are at peak
  • Can disrupts ions in Ionosphere affecting radio waves, GPS
  • Can burn the satellites
  • Can causes skin cancer in human being
  • Increase the frequency of El Nino – Warming of the Oceans
  • Can burn transformers
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