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बेसिक इंग्लिश का दूसरा सत्र (कक्षा प्रारंभ : 22 अक्तूबर, शाम 3:30 से 5:30)
The structure of Atmosphere and Heat Budget
Apr 24, 2015

The Atmosphere of Earth
The atmosphere is made up of gases and vapour, and receives incoming solar energy from the sun giving rise to what we call climate. We actually live at the bottom of this indefinite layer of atmosphere where the air is densest. With altitude the air thins out and it is still a matter of conjecture where the atmosphere ends. One estimate puts this limit at about 600 miles above sea level.

Composition Of The Atmosphere

Compound

Distribution

Nitrogen

78%

Oxygen

21% 

Argon 

0.9%

Water vapour 

0.4% (around 1% at the surface) 

Carbon dioxide 

0.03% 


1. Structure Of The Atmosphere 
The structure of the atmosphere can be broadly divided into five layers viz. Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere. The drawing below shows the different layers of the atmosphere. The ground is shown in brown at the bottom of the picture. The altitude in kilometres is given on the left hand scale and is approximate.



 


1.Troposphere

1. Begins at the surface and extends to between 7 km (at the poles) and 20 km (at the equator)

2. Temperature in the troposphere decreases with altitude i.e. the lowest parts are the warmest

3. The troposphere contains roughly 75% of the mass of the atmosphere and 99% of its water vapour

4. The lowest part of the troposphere, where friction with the Earth’s surface influences air flow is called the planetary boundary layer. Usually extends from a few hundred metres to about 2 km.

5. The tropopause is the boundary between the troposphere and the stratosphere

2. Stratosphere

1. Extends from the troposphere to about 51 km

2. Temperature increases with height

3. Restricts turbulence and mixing

4. Commercial airliners usually fly within the stratosphere (10 km) to optimize jet fuel burn and to avoid atmospheric turbulence.

5. The stratopause is the boundary between the stratosphere and the mesosphere.

3. Mesosphere

1. Extends from stratosphere to about 80 km

2. Upon entering the earth’s atmosphere, most meteors burn up in the mesosphere

3. Temperature decreases with height

4. The mesopause, the end of the mesosphere, is the coldest place on Earth with an average temperature of -100 C

4. Thermosphere

1. Biggest layer of the atmosphere

2. Extends from the mesosphere to about 500-1000 km

3. Thermopause is a temperature boundary contained within the thermosphere

4. Temperature increases up to the thermopause, then remains constant

5. The temperature can reach 1500 C. However, despite the high temperature one would not feel warm because the atmospheric density is too low to enable heat transfer

6. The International Space Station orbits in the thermosphere (320 – 380 km)

7. The ionosphere is formed in this layer as a result of ionization caused by ultraviolet radiation

        8. The boundary between the thermosphere and the exosphere is called exobase.

5. Exosphere

1. Uppermost layer of the atmosphere

2. It is a transitional zone between the Earth’s atmosphere and interplanetary space and does not fully fall within the atmosphere

3. Extends to about 190,000 km. This is half the distance to the Moon, at which the influence of solar radiation becomes greater than the Earth’s gravitational pull

4. The density is so low that molecules can travel hundreds of km without colliding with each other

5. Composed mainly of the lightest gases such as hydrogen and some helium

2. Other Layers And Boundaries Of The Atmosphere

Within theses broad layers there are sub layers of research significance.

1. Ozone layer

1. It is contained within the stratosphere at about 10 – 50 km above the Earth’s surface

2. About 90% of the ozone layer is present in the stratosphere

3. The ozone layer absorbs 93-99% of harmful ultraviolet light

4. Ozone is formed when UV light strikes oxygen in the stratosphere to split the oxygen atoms, which then reform as ozone

5. The ozone layer was discovered by the French physicists Charles Fabry and Henri Buisson in 1913

6. British meteorologist GMB Dobson established a worldwide network of ozone monitoring stations between 1928 and 1958 that continues to operate today. He also developed a spectrophotometer (called the Dobsonmeter) to measure stratospheric oxygen from the ground. The Dobson unit, a measure of ozone density is named in his honour.

2. Ionosphere

1. Stretches from the thermosphere to the exosphere (100 km – 700 km)

2. This is caused due to ionization by solar UV radiation

3. Responsible for radio propagation by reflecting radio waves back to the Earth’s surface thereby enabling long-distance communication

4. Plays an important part in atmospheric electricity (like lightning)

5. Responsible for auroras

3. Homosphere and Heterosphere

        1. Homosphere is the part of the atmosphere where gases are well mixed due to turbulence

2. This includes the troposphere, stratosphere and mesosphere

3. Heterosphere is the part of the atmosphere where gases are not well mixed

4. This usually happens above the turbopause (100 km) where distance between particles is large due to low density

5. This causes the atmosphere to stratify with heavier gases like oxygen and nitrogen present in the lower layers and lighter gases like hydrogen and helium in the upper layers.

4. Planetary boundary layer

1. Part of the troposphere closest to the Earth’s surface and most influenced by it

2. Friction with the earth’s surface causes turbulent diffusion

3. Ranges from 100 m to about 2 km

5. Magnetosphere

1. A mix of free ions and electrons from solar wind and the Earth’s atmosphere

2. It is non-spherical and extends to more than 70,000 km

3. It protects the Earth from harmful solar winds

4. Mars is thought to have lost most of its former oceans and atmosphere to space due to the direct impact of solar winds. Similarly Venus is thought to have lost its water due to solar winds as well

6. Karman line

        1. Defines the boundary between the Earth’s atmosphere and outer space

2. Lies at an altitude of 100 km above mean sea level

3. At this altitude the atmosphere becomes too thin for aeronautical purposes

4. However, there is no legal demarcation between a country’s air space and outer space.

7. Van Allen Belt

1. It is a region of energetic charged particles (plasma) around the Earth held in place by the Earth’s magnetic field

2. Extends from about 200 km to 1000 km

3. Has important implications for space travel because it causes radiation damage to solar cells, integrated circuits, sensors and other electronics.

3. Physical Properties Of The Atmosphere

1. Pressure and thickness

1. Atmospheric pressure at sea level is 1 atmosphere (around 14.7 psi)

2. 50% of atmospheric mass is below an altitude of 5.6 km

3. 90% of atmospheric mass is below 16 km

        4. 99.99% of atmospheric mass is below 100 km

2. Density and mass

        1. Atmospheric density decreases with height

        2. Density at sea level is about 1.2 kg/cu.m

4. Optical Properties Of The Atmosphere

1. Scattering

1. When sun’s rays pass through the atmosphere, photons in light interact with the atmosphere to produce scattering

2. Eg: on overcast days there are no shadows because light reaching the surface is only scattered, indirect radiation, with no direct radiation reaching the earth

3. Scattering is responsible for blue appearance of the sky, and for red appearance of sunset

2. Absorption

1. The atmosphere absorbs radiation of different wavelengths, allowing only certain ranges (UV to IR) to pass on to the earth’s surface

3. Emission

1. The atmosphere absorbs and emits IR radiation

2. Earth cools down faster on clear nights than on cloudy nights because clouds absorb IR radiation from the Sun during the day and emit IR radiation towards the Earth at night

3. Greenhouse effect is directly related to emission, where certain greenhouse gases (carbon dioxide) prevent IR radiation from the earth’s surface to exit back to space

4. Water Vapour in the Atmosphere

1. 99.9% of water vapour is contained in the troposphere

2. Condensation of water vapour into liquid or ice is responsible for rain, snow etc

3. The latent heat released during condensation is responsible for cyclones and thunderstorms

4. Water vapour is also a potent greenhouse gas

5. Water vapour is most common gas in volcanic emissions (around 60%)

5. Carbondioxde in the Atmosphere

        1. It is an important greenhouse gas

2. Natural sources of carbon dioxide in the atmosphere include volcanic activity, combustion of organic matter, respiration, decay of frests etc

3. Current carbon dioxide levels (0.0384%) are around 35% higher than the levels in 1832

4. The concentration of carbon dioxide is higher in the northern hemisphere because it has greater land mass and plant mass than the southern hemisphere

5. Carbon dioxide concentrations peak in May (just after the end of winter in the Northern Hemisphere) and reach a minimum in October (at the end of summer in Northern Hemisphere, when the quantity of plants undergoing photosynthesis is greatest)

5. Insolation

1. The only source of energy for the earth’s atmosphere comes from the sun which has surface temperature of more than 10,800° F. 

2. This radiation from the sun is made up of three parts, the visible ‘white’ light that we see when the sun shines and the less visible ultra-violet and infra-red rays.

3. The visible ‘white’ light is the most intense and has the greatest influence on our climate. 

4. The ultra –violet rays affect our skin and cause sun-burn when our bare  body  is  exposed  to  them  for  too  long  a  period.  

5. The infra-red rays  can penetrate even dust and fog and are widely used in photography. 

6. Only that part of the sun’s radiation which reaches the earth is called insolation.

6  Heat Budget

  • It is estimated that of the total radiation coming to us, 35percent reaches the atmosphere and is directly reflected back to space by dust, clouds and air molecules. It plays practically no part in heating the earth and its atmosphere.

  • Another 14 percent is absorbed by the water vapour, carbon dioxide and other gases. Its interception by the air causes it to be ‘scattered’ and ‘diffused’ so that the visible rays of the spectrum between the ultra-violet and infra-red give rise to the characteristic blue sky that we see above us.

  • The remaining 51 percent reaches the earth and warms the surface.

  • In turn the earth warms the layers of air above it by direct contact or conduction, and through the transmission of heat by upward movement of air currents or convection.

  • This radiation of heat by the earth continues during the night, when insolation from the sun cannot replace it. The earth-surface therefore cools at night.

  • The rate of heating differs between land and water surfaces.

  • Land gets heated up much more quickly than the water. Because water is transparent, heat is absorbed more slowly and because it is always in motion, its absorbed heat is distributed over a greater depth and area. Thus any appreciable rise in temperature takes a much longer time.

  • On the other hand the opaque nature of land allows greater absorption but all the radiant heat is concentrated at the surface, and temperature rises rapidly. Because of these differences between land and water surfaces, land also cools more quickly than water.

Variations in the Heat Budget Across the Globe

⇒ There is an excess of incoming shortwave radiation between 35° S and 40° N and a deficit at higher latitudes compared with the outgoing longwave radiation budget. If equilibrium were to be maintained at every latitude the short and longwave radiation should balance locally and the two curves in the figure would be identical.




⇒ The fact that they are not, and as local mean temperatures close to the equator are not increasing with time and those close to the poles are not decreasing, heat energy must be transported from low latitudes poleward. This is achieved by circulation within both the ocean and the atmosphere, transporting heat away from the equator towards the pole and maintaining a higher temperature at latitudes greater than 50° than would be possible from a system in radiative equilibrium, illustrated by the thin broken curve in the figure.

Heat Transfer in the Ocean and Atmosphere
♦ Much of the heat transport polewards takes place by atmospheric circulation

♦ However, a significant fraction, especially near the equator, the Hadley Cell only weakly transfers heat polewards, most transfers takes place through the surface waters of the ocean.

♦ The ocean surface heat transport is largely by wind blowing across the sea surface driving surface water currents

♦ The oceans are capable of storing heat for a wide range of time scales and subsequently transporting it to other locations.The thermohaline circulation (can store heat for 1000s of years.

♦ The strongest thermohaline circulationis in the Atlantic Ocean, whereas the Pacific Ocean is much fresher and features shallower circulations.This is largely due to differences in salinity. The atmosphere transports water vapour across the isthmus in central America from the Atlantic to the Pacific, leaving the former saltier than the latter.

 


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