The atmosphere of the earth is made up of a mixture of gases. This pool of gases behaves like a ‘filter’ and saves us from harmful radiation of the sun. In a group, these atmospheric gases behave as one unit which we label it as air. Since gravitational force of the earth attracts everything downward, the atmosphere carries its own weight as a result of this pull of gravity. Molecules in the air move randomly in all directions and this motion exerts pressure on all objects that come into contact with it. This pressure is known as atmospheric pressure. Near the earth’s surface, molecules of air are packed together and so the spaces between them are reduced, becoming denser. At the same time, the density of air decreases with the increase in altitude which is why a person feels difficulty in respiration as soon as he goes up.
Our atmosphere is divided into a number of layers based on chemical composition, temperature and function. Let’s discuss each of them.
Almost 99 percent of the weight of atmosphere lies within an altitude of about 30 km (20 miles).
Layers of the Atmosphere in relation to Chemical Composition
The atmosphere is divided into two regions i.e.,
- It is the outer region of the atmosphere.
- Starting from exosphere (also known as outer sphere), it comes down to about 80 km (50 miles) above the earth’s surface. An exosphere is almost a vacuum.
- This layer holds less than 0.001% of the entire mass of atmosphere.
- The upper part of this layer is within the orbit of International Space Station (ISS). Majority of the Space Shuttles also revolve inside heterosphere.
- The mixture of gases is unevenly distributed in the heterosphere.
- The upper heterosphere contains lightest elements like hydrogen and helium. Likewise, the lower part has heavier elements like nitrogen and oxygen.
- It lies beneath the heterosphere.
- It extends from 80 km (50 miles) downwards up to the surface of the earth.
- The mixture of gases is almost uniformly distributed in the homosphere.
- The density of gases varies quickly inside this layer.
Gases in the Homosphere (% age by volume)
Nitrogen (N2) | 78 %
Oxygen (O2) | 21 %
Argon (Ar) | 1 %
Carbon Dioxide (CO2) | 0.04%
Layers of the Atmosphere in relation to Temperature
The earth’s atmosphere is in the form of layers and each layer has distinctive temperature properties. Starting from lowest to highest altitude, these layers are:
- It is the lowermost layer of the earth.
- The boundary of troposphere varies. At the equator, this layer extends up to 18 km as a result of high temperature whereas at the poles, it ends at about 8 km (5 miles) above the surface of the earth.
- It is the principal layer where all life (layer of biosphere) exists.
- About 90 percent of the mass of atmosphere lie in this region.
- Almost all the water vapors and clouds exist here.
- It is this layer of the atmosphere where weather exists.
- The temperature of air and density of oxygen molecules decreases with the increase in altitude within troposphere.
- The uppermost limit of troposphere is called tropopause (the suffix ‘pause’ meaning ‘to change’). Here the temperature is about -57oC (-70oF)
- This layer starts from 18 km above the earth’s surface and goes up to 50 km (11 to 31 miles).
- The uppermost boundary of stratosphere is called stratopause. Here the temperature is about 0oC (32oF).
- This layer starts from 50 km and ends at 80 km (30 to 50 miles).
- It lies inside the homosphere.
- The outermost boundary of mesosphere is known as mesopause.
- Mesopause is the most freezing region of atmosphere. The temperature here is about -90oC (-130oF).
- This layer is also known as heat sphere.
- The outermost boundary of thermosphere is called thermopause.
- The temperature of this layer is very high, extending up to about 1200oC (2200oF).
- In reality, thermosphere is not that hot as it appears.
Layers of the Atmosphere in relation to Function
- Ozonosphere (ozone layer)
- It is the outer layer.
- It is present all over thermosphere layer and also goes toward the layer beneath it i.e. mesosphere.
- It receives and absorbs quite a number of rays from the sun. The names of these rays are:
- Ultra violet radiation (shorter wavelengths)
- Gamma rays
This layer absorbs these rays and then changes the atoms to the ions that are charged positively.
- It is part of stratosphere layer.
- This layer has high level of ozone. Ozone consists of three atoms of oxygen (O3) and absorbs ultraviolet rays. The ultraviolet radiation is very harmful for living organisms and ozone protects life on earth by absorbing these rays.