We all know that air is all around us. The gaseous envelope covering the earth, is called atmosphere. It is held around the earth by the force of gravity. Different gases are found in different concentration in the atmosphere.
The atmosphere sends back most of the sun’s heat rays into space during the day. It does not allow the heat of the earth to escape easily during the night. Therefore, days are not too hot and the nights are not too cold. So, plants and animals can survive on the earth. There is no atmosphere on the moon. There it is too hot during the day and too cold during the night. So plants and animals cannot survive on the surface of the moon.
The atmosphere is several hundred kilometres thick. It is not the same all the way up. It is thick near the ground and gets thinner as we go up, until it fades to nothing in outer space. Sufficient air for breathing is available only up to a height of about 15 km above the surface of the earth. The air gets thinner as we go higher and breathing become more and more difficult. This is why . mountaineers carry oxygen while climbing high mountains.
Layers of Atmosphere
The atmosphere acts as a shield for the earth. After a few hundred kilometres, the atmosphere ends and space begins. Space has no air.
This atmosphere is divided into following layers :
(1) Troposphere : The lower most layer of the atmosphere is troposphere. It extends upto 15 kilometres above the surface of the earth. All weather changes take place in this layer.
(2) Stratosphere : The stratosphere starts from where the troposphere ends and extends to about 35 kilometres. Compared to the troposphere, this layer of the atmosphere is less dense. The ozone layer, which absorbs the sun’s ultra-violet rays, is located in this layer. Jet planes usually fly in this layer.
(3) Mesosphere : Beyond the stratosphere lies the mesosphere. Temperature begins to fall in this layer, as we go higher. That is why it is also known as the ‘Cold layer’.
(4) Thermosphere : The thermosphere is also called the ‘Hot layer’ because it is, characterized by high temperatures. Space shuttles orbit in this layer.
Another layer called ionosphere is also a part of the thermosphere. The radio signals transmitted from the earth-are reflected back to the earth by this laver. So long distance radio communication is possible because of ionosphere.
(5) Exosphere : This is the outermost layer of the atmostphere. The air here is very-very thin. Beyond this exosphere is vast empty space without air.
The atmosphere protects us
(i) The atmosphere sustains life on the earth by providing oxygen for breathing. It also provides carbon dioxide to plants for making food.
(ii) The ozone layer in the atmosphere absorbs the harmful ultra-violet rave of the sun and prevents them from reaching us.
(iii) The atmosphere absorbs the excessive heat of the sun so that the earth does not get overheated during the day. It retains the heat at night so that the earth does not freeze.
(iv) The atmosphere helps in recycling water through the water cycle.
(v) Moving air can support parachutes, gliders and kites in the air, push sail. : hoats and turn the blades of the wind mill to draw water or generate electricity. Air is used to inflate tyre-tubes which support vehicles and make footballs.
Composition of Air
Air is a mixture of various gases. Air is not seen by us but it is felt only when it blows. We live at the bottom of the
Oxygen invisible ocean of the air. Air is mainly composed of oxygen, nitrogen and carbon Nitrogen di oxide. It contains other gases, watervapour and dust practices also.
Approximate 21% (20.94%) of the air is oxygen. All living beings breathe in oxygen. They use it to burn food inside their body to get energy. We can burn something like candle or fuel only in the presence of oxygen.
78.08% of the air is nitrogen. Animals and plants use it in their bodies as protein. They cannot take nitrogen directly from the atmosphere. Some nitrogen fixation bacteria and lightening changes it into consumeable form for plants. Animals can take this nitrogen from plants.
Air contains 0.04% carbon dioxide. Green plants make their food from carbon dioxide and water in the presence of sunlight. Animals breathe in oxygen and give out carbon dioxide during breathing. This balance of oxygen and carbon dioxide is maintained in the air.
Other gases like neon and argon are about 0.95% in the air. Neon and argon produce orange glow in electric tube-lights which are displayed in advertisements. These are called ‘neon signs‘.
In addition to these gases, air also contains variable amount of water vapour in it. Excess of water vapour in the air makes it humid. Amount of water vapour contained in the air is called humidity. Many of the weather conditions depend upon water vapour.
Dust, smoke, bacteria, virus and pollen particles are also found in the air in small and variable quantities.,
Properties of Air
(i) Air has weight
(ii) Air occupies spaces
(iii) Air exerts pressure