Indian geography 3

THE ATMOSPHERE OF EARTH

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%

Keywords: ias, study material, general studies, geography

STRUCTURE OF THE ATMOSPHERE

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
   

   
   

   Layers of the atmosphere
   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

Keywords: ias, study material, general studies, geography

OTHER LAYERS AND BOUNDARIES OF THE ATMOSPHERE

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

Keywords: ias, study material, general studies, geography

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

Keywords: ias, study material, general studies, geography

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

Keywords: ias, study material, general studies, geography

WATER VAPOUR IN THE ATMOSPHERE

• 99.9% of water vapour is contained in the troposphere
• Condensation of water vapour into liquid or ice is responsible for rain, snow etc
• The latent heat released during condensation is responsible for cyclones and thunderstorms
• Water vapour is also a potent greenhouse gas
• Water vapour is most common gas in volcanic emissions (around 60%)

Keywords: ias, study material, general studies, geography

CARBON DIOXDE IN THE ATMOSPHERE

• It is an important greenhouse gas
• Natural sources of carbon dioxide in the atmosphere include volcanic activity, combustion of organic matter, respiration, decay of forests etc
• Current carbon dioxide levels (0.0384%) are around 35% higher than the levels in 1832
• The concentration of carbon dioxide is higher in the northern hemisphere because it has greater land mass and plant mass than the southern hemisphere
• 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)