Atmosphere: The present day composition of atmosphere is the product of a lengthy evolutionary process that began more than four billion years ago. It is composed of many different gases and suspended particles. As you can see from the table there are about 12 gases in the atmosphere. However, most of them are present in trace amounts.


Nitrogen and oxygen are the major constituents while CO2 is only 0.03%. Because of continual mixing of atmospheric gases, this composition is almost constant for approximately 15 km height. We may travel anywhere on the earth and be confident that we are breathing essentially the same type of air.

Atmosphere also contains minute liquid or solid particles in a suspended form which are known as aerosols. Most of these are found in the lower atmosphere near the earth’s surface. The originate as a result of forest fires, wind erosion of soil, as sea salt crystals from ocean sprays as well as, from industrial and agricultural activities.




On the basis of the variation in air temperature, the atmosphere has been divided vertically into four layers: the troposphere, stratosphere, mesosphere and thermosphere.


Troposphere or the lowermost layer extends up to an altitude of 15 km at the equator to 8 km at the poles. The temperature in this zone decreases with increasing altitude or height.


The text layer, the stratosphere, extends up to an altitude of approximately 50 km. Here a gradual increase in temperature occurs with altitude. Pilots of jet aircraft prefer to fly in the stratosphere as it is relatively stable and free from weather fluctuations from providing excellent visibility. Scientists are worried about the growing pollution in the stratosphere. As pollutants enter into this zone, they are likely to remain here for a long time. We shall be elaborating this in a later unit.


In mesosphere, the next layer, temperature is constant in the lower portion but then rapidly decreases with altitude. At 80 km the temperature is the lowest in the atmosphere. Above this the thermosphere in which the temperature again increases at higher altitude. Human activity at present seems to have little direct impact on the outermost atmospheric layer.


Pressure Gradient


You know that air pressure is the weight of the atmosphere over a unit area of the earth’s surface. The average air pressure at sea level is approximately 1 kilogram per square centimeter. At any point at sea level the air pressure is the same. Gravity compresses the atmosphere on the earth’s surface so that air pressure decreases with increasing altitude.


Weather forecasters on the TV and Radio usually report air pressure in mm i.e., in unit of length. This in fact refers to the height to which a column of mercury can rise in the barometer at a specific station at a given time. However, it is more appropriate to express pressure in millibars. The average pressure at sea level is 1013.25 mb.


Let us see what happens to the air pressure and density as we go higher up in the atmosphere. Air density which is the mass per unit volume also diminishes with altitude. Ninety-nine per cent of the atmosphere’s mass lies between the earth’s surface and an altitude of approximately 32km. Approximately half of the atmosphere’s mass lies between the surface of the earth and an altitude of 5.5 km. At this altitude the air pressure too remains only one-half of the pressure at sea level.


In most cases the reduction of pressure is not a limiting factor for the distribution of plants and animals at high altitudes. There are other adverse conditions like low temperature, lack of food, unsuitable soil etc. Many species of beetles have been found in the highest meadows of Himalayas. While earthworms have been found up to the snowline in the Andes mountains. However, for warm blooded vertebrates reduced air pressure and density at high altitudes causes impairment in respiration.


 The expansion and thinning of air accompanying the lower air pressure at high altitudes triggers physiological changes in human beings. For example, a person at high altitude may experience dizziness, headaches and shortness of breath, but gradually adjusts or acclimatizes to the low oxygen levels. However, people cannot adjust to pressure at altitudes higher than approximately 5.5 km.


Relatively a slight change in the air pressure can trigger important changes in the weather. A large volume of air which is relatively uniform in terms of its temperature and water vapour content is called an air mass. As air masses move from one place to another, surface air pressure falls or rises causing in weather. As a general rule low pressure causes stormy weather and when air pressure rises the weather improves.


Global Air Circulation


The major wind systems of the earth result because large masses of air around the earth’s equator are forced to rise from the bottom due to surface heating. Cold air from high altitudes rushes in to replace the void thus created. The warm air from the equator travels toward the poles where it descends and returns towards the equator along the surface of the earth.


1)            The heat retention of air masses is not the same as for oceans. You already know that land cools as well as heats up more rapidly, and land masses are not distributed uniformly.


2)            A force associated with the earth’s rotation deflects the air flow in the northern hemisphere to the right and in the southern hemisphere to the left.


Let us now explain air movements at tropical latitudes. The surface air that rushes to fill the equatorial void from the

North is deflected to the right and becomes the north-east trade wind. It meets similar wind the south-east trade

Wind coming from the south that was deflected to the left. But less land is present in the southern hemisphere to

Obstruct the path of the south-east trade wind so the trade winds do not meet at the equatorial plane but some

What to the north. This is called the inter-tropical convergence zone, a zone of heavy rainfall.




Here strong high pressure regions over Siberia in winter cause winds to blow out of the continent towards the coast. But we in India are protected from these cold winds by the Himalaya range of mountains. For this reason we do not experience a severe winter. In summer, however, strong low pressure regions over Siberia draw great quantities to moist air over the land from the oceans producing the great summer monsoon characteristic of south-eastern region countries. Again because of the Himalaya mountain range we get the heavy monsoon rains as the moister –laden winds strike against them.

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