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

Nitrogen

• 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.

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

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 up to 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