Fundamental Of Physical Geography Chapter 8 Composition And Structure Of Atmosphere

Atmosphere is a mixture of many discrete gases, water vapours, and dust particles.

i. Gases: Nitrogen (78%), Oxygen (21%) are the two major gases. The remaining 1% is composed of a number of gases like hydrogen-argon, ozone, helium, carbon dioxide, etc. oxygen is the most vital gas for sustaining life.

The various gases present in the atmosphere are as:

ii. Water Vapour: The amount of water vapour varies from place to place and time to time. It represents 2% of the air by volume. It is mostly found in the lower layers.

iii. Dust Particles: These are also found in the atmosphere in different sizes. The major sources of dust particle are deserts, lakes and dry rivers etc. These are found in the lower layers of the atmosphere. These are responsible for the formation of clouds, fog and smog.

The troposphere is the most important of all the layers of the atmosphere for the following reasons:

1. All the weather-making processes take place this layer.

2. Great contrast in climate and weather patterns are observed at different altitudes due to changes of temperature in this layer.

3. The presence of the large amount of water vapour in this layer is responsible for condensation, clouds and precipitation.

4. Dust particles give rise to fog, clouds smog, etc. in this layer.

5. Convection currents are formed to this layer for the heating and cooling of the air.

6. The air is unstable in this layer and gives rise to atmospheric disturbances like stormy cyclones.

The characteristics are:
i. The atmosphere is a vast envelope of gases.

ii. The heavier gases are found in lower layers.

iii. The atmospheric pressure decreases with altitude.

i. Atmosphere makes the earth habitable.

ii. Life supporting gases are found in the atmosphere. These are oxygen and nitrogen for animal and plant life. It is the store house of water vapour.

iii. It protects us from the falling meteors.

iv. It absorbs ultra-violet rays.

v. It acts as a greenhouse by moderating the extremes of hot and cold.

The features are mentioned as under: 

1. It is the lowest layer of the atmosphere.

2. Its average height is about 16 km.

3. In this layer the temperature decreases at the rate of 1°C per 165 m.

4. It is a zone of convection currents.

5. All the weather phenomena take place in this layer.

6. This zone has about 90% of the total water vapour of the atmosphere.

Many gases are found in the atmoshere but it can be considered to be composed of mainly nitrogen and oxygen.
An estimate of the composition of the atmosphere is given in the following table:

Other rare gases are hydrogen, helium and ozone. Besides water vapour, dust particles, smoke, salts and other impurities are also present in the air, in varying quantities. As a result, the composition of air is never constant. It varies from time to time and place to place. However, if these variable elements are removed from the atmosphere, its make-up would be fairly constant all over the earth, at least in the lower atmosphere.

Of the many constituents, carbon dioxide, dust particles, water vapour and ozone are of great importance for the earth’s climatic conditions.

The nitrogen does not easily enters into chemical union with other substances, but gets fixed into the soil. It serves mainly as diluent or dissolver. It regulates combustion. Oxygen, on the other hand, combines with all the elements and is most combustible. Carbon dioxide is an important gas in the atmospheric processes. It can absorb heat and thus allows the lower atmosphere to be warmed up by the heat radiation coming from the sun and from the earth’s surface. Green plants in the process of photosynthesis utilise carbon dioxide from the atmosphere.

The atmosphere consists of almost concentric layers of air. These layers are of varying temperature and density. Density is highest near the earth’s surface and goes on decreasing upwards.

The atmosphere can be divided into five layers of air.
They are described below:

1. Troposphere: It is the lowest layer of the atmosphere. It extends roughly up to a height of 9-10 km near the poles and about 18 km at the equator. The temperature decreases with the height in this layer. The normal lapse rate of temperature is 1°C for 165 metres. It is a vast physical laboratory of the air in which nature is always showing some experiment, on a grand scale. Formation of clouds and various forms of precipitation; heating and cooling and development of pressure system; blowing of winds and the acts of respiration by all types of life, are all taking place in this layer of the atmosphere. It is an unstable layer of the atmosphere. The vertical currents moving throughout the year make it unstable. All weather changes occur in this layer which are useful for the life on earth.

Tropopause: The upper layer of troposphere is called tropopause. It is a thin layer and its height changes according to latitudes. In fact, this is a transitional zone which connects the lower troposphere with the upper stratosphere.

2. Stratosphere: It extends up to the height of 50 km. The temperature remains constant up to the height of 20 km, in this layer. Afterwards, it increases upto the height of 50 km because of the presence of ozone. The ozone absorbs the most harmful ultra-violet rays of the sun. Clouds are almost absent in this layer. There is Very little of dust particles and water-vapour in this. The air movements are horizontal. It provides ideal flying condition for jet aeroplanes because, it is free from clouds and other weather disturbances.

3. Mesosphere: The mesosphere lies above the stratosphere, which extends up to a height of 80 km. In this layer, once again, temperature starts decreasing and reaches upto to -100°C at the height of 80 km.

4. Ionosphere: It is located between 80 and 400 km. It is an electrically charged layer. Radio waves transmitted from the earth are reflected back to the earth by this layer. Temperature again starts to increase with height because of sun’s radiation.

5. Exosphere: It is the uppermost layer of the atmosphere extending beyond the ionosphere. This layer is extremely (rarefied) and gradually merges with the outer space.

In fact, we know very little about the atmosphere extending beyond 64 km above the sea-level.

The various geographical factors affecting weather and climate are :

1. Latitude

2. Distribution of land and water

3. The great semi-permanent high and low pressure belts.

4. Winds

5. Altitude

6. Mountain barriers

7. Ocean currents

8. Storms of various kinds.

These control produce changes in temperature and precipitation which in turn give rise to a variety of weather and climates.

ionosphere

1. All the weather-making processes take place this layer.

2. Great contrast in climate and weather patterns are observed at different altitudes due to changes of temperature in this layer.

3. The presence of the large amount of water vapour in this layer is responsible for condensation, clouds and precipitation.

4. Dust particles give rise to fog, clouds smog, etc. in this layer.

5. Convection currents are formed to this layer for the heating and cooling of the air.

6. The air is unstable in this layer and gives rise to atmospheric disturbances like stormy cyclones.

include sea salts, fine soil, smoke-soot, ash, pollen, dust and disintegrated particles of meteors. Most of the solid particlas are kept in suspension in the atmosphere. These particulates help in the scattering of solar radiation which adds varying charming colour of red and orange at dawn and dusk. The selective scattering of solar radiation by dust particles make the sky appear blue. Dust and salt particles act as hygroscopic nuclei around which water vapour condenses to produce clouds. Thus there is a great significance of small solid dust particles (the particulate matter) in the atmosphere.

Other rare gases are hydrogen, helium and ozone. Besides water vapour, dust particles, smoke, salts and other impurities are also present in the air, in varying quantities. As a result, the composition of air is never constant. It varies from time to time and place to place. However, if these variable elements are removed from the atmosphere, its make-up would be fairly constant all over the earth, at least in the lower atmosphere.

Fig. 8.3 : Composition of the air

Of the many constituents, carbon dioxide, dust particles, water vapour and ozone are of great importance for the earth’s climatic conditions.

The nitrogen does not easily enters into chemical union with other substances, but gets fixed into the soil. It serves mainly as diluent or dissolver. It regulates combustion. Oxygen on the other hand combines with all the elements and is most combustible. Carbon dioxide is an important gas in the atmospheric processes. It can absorb heat and thus allows the lower atmosphere to be warmed up by the heat radiation coming from the sun and from the earth’s surface. Green plants in the process of photosynthesis utilise carbon dioxide from the atmosphere.

First : Water vapour absorbs the insolation coming from the sun and thus reduces the amount of insolation reaching the earth’s surface. It also preserves the radiant heat. Thus, it acts as a shield of the earth’s surface. It protects the earth’s surface from the constrasts of very high or very low temperature.

Second : Water vapour causes condensation and precipitation. The water vapour releases and consumes latent heat of the sun through condensation and precipitation processes. The processes of release and consumption cool and warm the air. The condensation process causes the precipitation which falls down on the earth’s surface in various forms.

Third : The amount of water vapour decreases with altitude. Half of the water vapour lies below an altitude of about 2000 metres. It also decreases from the equator towards poles. Its maximum amount lies between 10° and 30° latitudes.

Fourth : Water vapour influences the conditions of stability and instability in the air.

1. Atmosphere makes the earth habitable.

2. Life supporting gases are found in the atmosphere. These are oxygen and nitrogen for animal and plant life.

3. It is the store house of water vapour.

4. It protects us from the falling meteors.

5. It absorbs ultra-violet rays.

6. It acts as a greenhouse by moderating the extremes of hot and cold.