Lecture 15 & 16 : Geography

Topics Covered in this Lecture

  1. Evolution of Atmosphere
  2. Composition of Present Atmosphere
  3. Structure of Present Atmospheres

Composition Atmosphere

The gaseous envelope that surrounds the earth is known as the atmosphere. It is estimated that the height of the atmosphere is around 1600 km from the sea level. This atmosphere is divided into the outer atmosphere and inner atmosphere at a height of 80 – 90 kilometers. This boundary between the inner and outer atmosphere is the Karman line.

It is made up of three things-

  1. Gases
  2. Water Vapor
  3. Particulate matter


Nitrogen 78%
oxygen 21%
argon 0.093%
co2 0.03%
neon o.o2%
helium, krypton, xenon, hydrogen [trace gases]

nitrogen and co2 constitute 99% of atmospheric gases and that is not easily mixed. co2 is the most important gas because life won’t be possible without it because-

1. Plant utilise co2 to manufacture food by the process of photosynthesis and give out o2 as a biproduct.

2. co2 is an efficient greenhouse gas.

greenhouse gases are those gases which are transparent to incoming shortwave solar radiations but they are opaque or they keep outgoing long wave terrestrial radiations. Thus increasing the temperature of earth’s surface atmosphere. This effect is also known as global warming. The amount  of co2 has drastically increased after industrial revolution based on coal and hence today, the amount of co2 in atmosphere is 0.045%

Water Vapour

  • It is the most important variable gas in the atmosphere. Its percentage varies from o-5%. It is an important greenhouse gas.

  • variation of water as per latitudes the amount of water vapor goes on decreasing from the equator towards the poles but the lowest water vapor is found in the subtropical high-pressure belts.

  • water vapor varies as per the time of a day humidity is maximum afternoon while it is least in the early morning.

  • water vapor varies as per season it is maximum during the dry summer season [April may] while it is minimum during the winter season.

  • It varies as per altitudes 90% of water vapor found below the height of 5km more than 50% of water vapor is below the height of 2kms.

Particulate matter

  • It consists of dust particles, pollen grains, crop residues, volcanic ash, etc. which is present in the atmosphere.

  • the particulate matter acts as condensation nuclei [resting place] and thus helps in the formation of clouds and precipitation.

  • they help in absorption, reflection, and scattering of sunrays [insolation]. Thus giving different shades to the sky.


On the basis of the composition of the gases, the atmosphere is divided into two different layers:

  1. Homosphere
  2. Heterosphere

The homosphere is the region where the composition of gases is the same and the Heterosphere has a different composition of gases.

On the basis of change of temperature from the ground to the upper atmosphere, we divided the atmosphere into- troposphere, stratosphere, mesosphere, ionosphere, and exosphere.


  • It is the lowermost layer means- a region where there’s the mixing of gases. This is the most important layer because it is the only layer that supports life.

  • All weathered phenomena such as winds, clouds, rains, occur in this layer.

  • In the troposphere, the temperature decreases with increase in height at 6.5 degree celcius  kilometres over the equator while it is around 8 kilometres over the poles.

  • The zone where the mixing of the air stops is known as tropopause.


  • There are calm conditions in the tropopause.

  • Tropopause and stratosphere is ideal for flying aircrafts.

  • The height of the troposphere is greater over the equator than at the poles.

  • Over the equator tropopause is located at the height  of 18 kilometres.

  • While over the poles the tropopause located  at the height of 8 kilometres.

  • Hence, as the temperature decreases with the increases in height due to greater height of troposphere over the equator, temperature is lesser at tropopause over the equator (-80?c as compared to poles -50?c to -60?c)


Above the troposphere upto the height of 50 kilometres is the stratosphere.

  • Stratosphere is important because the lower stratosphere has the concentration of ozone gas and it is known as an ozonosphere.

  • This ozone gas blocks harmful ultra violet radiations and thus protects the earth from global warming as well as human beings from cancer.

  • In the recent years the thickness of ozone layer has reduced. This reduction is maximum over the poles which is known as ozone hole.

  • All chlorinated and brominated hydrocarbon are ozone depleting substances. Hence in 1985, there was a convention known as VIENNA CONVENTION for the protection of OZONE LAYER.

  • In 1987 under the Vienna convention MONTREAL PROTOCOL was signed which banned all chlorinated and brominated hydro carbons because of their ozone depleting property.

  • They were replaced by fluorinated gases or hydrocarbons because fluorine does not deplete the ozone layer. example- HFC

  • In the year 2016, the KIGALI AGREEEMENT are also known as Kigali amendment to the Montreal protocol, even HFC or fluorine were banned by Montreal protocol.

  • HFC or R23 written on AC’S is a super GHG and hence all the countries in the world will phase out R23 or HFC by 2048.

  • In the lower part of stratosphere, very high clouds known as CIRRUS clouds are found.

  • Stratosphere is also ideal for flying air craft but due to extremely high height, supersonic gets generally flow through stratosphere.

MESOSPHERE (above the stratosphere)

In this layer the temperature goes on decreasing with increase in height and it becomes -80?c to -100?c). this is the coldest layer of atmosphere.

Mesopause is a boundary between inner and outer atmosphere. As the density increases in the inner atmosphere, meteorites burn after entering the mesosphere.


  • The outer atmosphere experiences very rapid rise of temperature and hence it is known as thermosphere.

  • The thermosphere is further divided into ionosphere and exosphere. Ionosphere from 90 to 400km.

  • It is layer which has presence of charged particles known as ions. Then ions reflect radio waves back to the earth making satellite communication possible.

  • In this layer there is a mixing of solar radiation as well as magnetic field of the earth giving rise to AURORA LIGHTS known as aurora borealis in northern hemisphere and aurora Australis in southern hemisphere.


The uppermost part of atmosphere above 400 kilometres. It has dominance of hydrogen and helium because of extremely low density. Though there is high temperature. It cannot be measured by normal thermometer International space station (ISS) is located in the exosphere.


In coming solar radiation

Insolation is the amount of solar radiation received by the earth. Earth receives only one part out of 2 billion parts of solar radiation. This is known as solar constant.

The average value of solar constant 2 cal/gm/

Or 2 langley//cm^2. Insolation is the most important source of heating of the earth. This solar radiation comes in the form of short waves which heats up the earth during the day time. The earth on other hand gives back the heat in the form of long waves terrestrial radiation. These long waves example- infrared (IR) are trapped by the GHG.

Which heats up the atmosphere .

The solar radiations comes in the form electro magnetic spectrum which from short wavelength

To long wavelength are as follows:-

As the blue light has the shortest wavelength therefore it scattered by atmospheric dust oe particulate matter and hence sky looks blue in colour. During the down and dusk, as the sun is farthest away from us only red and orange colour is seen by the human eyes and therefore sky looks reddish orange during sunrise and sunset.


  • Angle of sun’s rays:

Vertical sun rays (afternoon) will have more heating potential because they fall on lesser area. On the other hand inclined sunrays causes less heating due to :

They fall on larger surface area.

They travel through longer distance and much thicker layer of atmosphere which causes absorption, scattering and reflection of sunlight.

  • Rotation and revolution:

Rotation of the earth is responsible for day and night.

Revolution of earth as well as rotation of earth on an inclined is responsible for variation in the length of day and night and seasons. The earth is inclined at an angle of 22.5 to its axis and at an angel of 66.5 to 23.5 plane.

Revolution of the earth on an inclined axis and the subsequent rotation causes equinox and solstice. Summer solstice occurs on 21st of June. The sun rays are vertical in the northern hemisphere, especially on the tropic of cancer.


When the sun’s rays fall on the equator, there’s equal day and equal night on all the places of the earth and hence known as equinox. There are two kinds of equinox-

  1. Spring equinox
  2. Autumnal equinox

The earth continues its revolution in such a way on 22nd of December, the sun rays vertically fall over the tropic of Capricorn causing winters and shortest days in Northern Hemisphere while summers in Southern hemisphere.

Transparency of atmosphere

The dust particles, aerosols absorb, reflect, and scatter the insolation. Thus, decreasing the insolation. For example, Volcanic eruptions.

Distance from the sun

As the earth’s orbit is elliptical at perihelion i.e., Jan 3, the earth is closest to the sun and during aphelion on July 4th, the sun is farthest away from the earth and hence there is slightly greater insolation during perihelion as compared to aphelion.


  • The reflective property of the Earth is Albedo of the earth. Albedo of the earth is the amount of solar radiation that is reflected back by the earth into the space and it is expressed as %.

  • As Albedo is reflected by solar radiation, it does not heat the earth neither it heats up the atmosphere. Lighter the substance greater the albedo.  

  • Decreasing order of albedo- Snow>Desert>Grass>Deciduous forest>Evergreen forest>Black soil

  • Greater the albedo larger the heating of the earth. On an average the albedo increases from equator towards the pole.

  • Due to the melting of icecaps and glaciers especially in the polar region, the water now absorbs the sun’s rays which causes heating of the water.

  • This further accelerates the rate of melting of icecaps and glaciers resulting in global warming and a rise in the sea level and associated with climate change.

For example, 2016-17 were the hottest years with severe heat waves in the indo-Gangetic plains.


The amount of sensible heat is known as temperature. The major source of temperature or heating is earth through its long wave.


The atmosphere is heated by-

  1. RADIATION: terrestrial radiation

  2. CONDUCTION: when the two bodies of different temperature touch each other. The hotter body till the time of temperature is equalised.

Air is a bad conductor of heat.

  • Convection: vertical movement of heated air. convention is an important mechanism for the transfer of heat from the lower to upper atmosphere.

  • Advection: horizontal movement of air.


  • Latitudes: On average the temperature decreases from the equator towards the pole. But the highest temperature is not found at the equator.

  • It is found around 20 degree to 30 degree north and south of the equator. This is because the equatorial regions receives rainfall throughout the year. (4’o clock rainfall). On other hand the region between 20 degree to 30 degree north an south are deserts located in sub tropical high pressure belt.
  • Altitude: on average in the troposphere, temperature decreases with the increase in height @6.5C/km. this is known as a normal lapse rate or environmental lapse rate.

  • Water vapor which is an important GHG is mostly concentrated in the lower atmosphere. The maximum amount of water vapor is found below the height of 2 km and hence there is more temperature in the lower atmosphere as compared to upper.

  • As we go in the upper atmosphere the density decreases. Due to a decrease in density, the transfer of heat becomes inefficient. Also, the greenhouse effect is less prominent, and hence temperature decreases.

  • Distance from the coast: in the areas, which are located near the coastline, there is a daily rhythm of land and sea breeze which moderates the coastal climate. This is the maritime effect.

  • On the other hand, in the continental interiors due to the absence of daily winds, summers become very hot and winters are extremely cold. Such kind of climate is an extreme climate. For example, Delhi V/s Chennai, Delhi V/S WB.
  • Difference between nature of land and water: land gets heated and cooled at much faster rate while water takes time to get heated as well as cool.

  • The only superficial layer of the land is heated because the land is opaque. Therefore it heats and cools quickly. Water being transparent the sunlight can penetrate up to the depth of 200m in the water body and therefore slow heating and cooling.

  • As water is mobile and hence there is a redistribution of heat in the form of ocean currents, waves an tides. Such redistribution of heat is absent on land.

  • Specific heat: the specific heat of water is more. In simple words, to raise the temperature of 1 gm of water by that is it requires more heat than land. Similarly to  cool, water needs to give out more quantity of heat than the surrounding land.

  • Insolation is less in the equatorial regions because of cloud cover through out the year. Cloud cover through out the year. Clouds on an average reflect 27% of insolation.
  • Ocean currents: Winds help in the distribution of heat. Warm ocean currents carry warm water from the lower latitudes to the cold polar regions while cold water from polar regions is brought to Equitorial regions by cold ocean currents.

  • The other minor factors affecting the temperature is precipitation, cloudless clear skies or presence of clouds in the sky.

Distribution of temperature

  1. Vertical distribution
  2. Horizontal distribution

Vertical distribution temperature with increase in height 6.5digree/km. This rate of decrease of temperature is known as normal lapse rate environment lapse rate. This experience in troposphere and mesosphere.

Horizontal distribution on average temperature from equator towards the pole but this rate of decrease is not uniform because highest temperature is not found at the equator it is found just to the north and south equator which is subtropical high-pressure belt. This is because equatorial regions receive rainfall throughout the year and insolation is also low because they are covered with clouds.

Temperature inversion and its significance

In the normal condition temperature and with increase in height. There is warm air below near the ground surface and there’s a cold air above. But sometimes, there is cool air near the surface and warm air above which means instead of decreasing the temperature increases with increase height such a condition is known as temperature Inversion.

Temperature inversion is characterized by Negative Lapse rate.

Types of Temperature inversion

  1. Advectional inversion
  2. Non-advectional inversion

Advectional inversion is caused due to the movement of air.
 Non-advectional inversion caused due to no movement of air or stationary.
advectional inversion example:

1. sea fogs [due to the mixing of warm and cold]
2. cyclone inversion ocean current
3. Valley inversion
4. Sea smoke

Non-advectional  inversion or Ground/radiation inversion
It occurs due to no movement of air.

The following conditions are necessary for ground inversion:

1. Long cold winter night during the winters, as the night is longer the ground will lose more heat through radiation as compared to gain of heat through insolation.

2. Hence as the winter progresses, the air which comes in immediate contact with cold ground surface becomes colder. As air is a bad conductor of heat, it does not transfer the cold to the upper layers. Hence the upper layer is comparatively warmer causing temperature inversion.

3. Absence of moisture because water vapor is a greenhouse gas and if it is present, it will trap the heat and therefore dry weather conditions will acid temperature inversion.

4. No or less movement of air because the movement of the air will cause mixing of the layer and thus removes the temperature inversion.

5. Cloudless and clear skies because the heat in such a condition is lost to the upper atmosphere. [clouds reflect the back].

Advectional inversion caused due to movement of air when warm and cold air mix cold air being denser settles on the ground while the warm air rises or is thrown upwards.

valley inversion during the night time the upper parts of the mountain lose the heat very quickly to the surrounding and will become colder on other hand heat is happened along with the valley floors due to stones, vegetation and hence the valley floors are warmer. The cold air being denser sinks due to gravity and occupies the valley floors while it pushes the warm air upwards causing temperature inversion.

Importance valley floors are avoided for human settlements and agricultural activities due to extreme cold and frost in the night.

Inversion due to mixing of warm and ocean currents
for example:
most dense fogs are caused due to mixing of warms gulf stream and cold labrodor currents.


1. Fogs reduce visibility and hence hinder transportation and communication. Fog mixes with smoke and forms smog which increases pollution and causes health problems.

2. It promotes atmospheric stability and aridity.

3. Causes the formation of dew and frost:

Dew- when vapors cools and condenses in water droplets [condensation].

Frost- when vapours cools in soli form[ice] [sublimation].

4. Frost is harmful to the crops why? because ice particles if they collected cause frostbite.

Exception frost is beneficial for mocha coffee grown in Yemen.


  • imaginary lines joining the places of equal temperature reduce to the sea level is known as isotherms. As the isotherms do not show the exact temperature of the place, they do not have practical utility [can’t be used by farmer].

  • Isotherms bend when they cross from oceans to continent and vice versa. June isotherms will bent equatorward while passing land to the sea. [because of more heat towards the poles].

  • January isotherms will bend poles wards while passing from land to the sea because the sea will be more cooler towards the poles.

  • Isotherms are irregular and closely spaced in the northern hemisphere. This is because the land dominates the northern hemisphere, the land, and hence temperature changes very fast.

  • On another land in the southern hemisphere, because the water body dominates it, isotherms are widely spaced and more or less parallel to each other because the change of temperature is very slow.

Topics Covered in this Lecture

  1. Insolation : – Factors and Distribution.
  2. Heating and Cooling of Atmosphere
  3. Heat Budget of the Planet Earth
  4. Latitudinal Heat Balance
  5. Temperature : – Factors and Distribution
  6. Inversion of Temperature
  7. Temperature Belts

Topics Covered in this Lecture

  1. Factors responsible for temperature distribution
  2. Temperature Distribution
  3. Temperature Inversion
  4. Thermal Belts

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