Some Physical Features of the Atmosphere:
i) The atmosphere is a mixture of gases. It is made up of 78.08% nitrogen; 21% oxygen by volume. There is only 0.03% of carbon dioxide, and small amounts of a few other gases. The presence of a thin layer of ozone is important for life on the earth. We also find in the atmosphere, a considerable amount of water vapour that evaporates from oceans, lakes and rivers etc. Now, let us talk in detail about some of the important gases constituting our atmosphere. Oxygen, a constituent of air is also called the breath of life. Without it, the chemical processes that sustain life are not possible. Human beings can live without food and water for some time, but not without oxygen for more than a few minutes. It is only oxygen in the air that makes combustion or burning possible. Burning of fuel produces heat, that is used for cooking food, for running automobiles, in industry and for various other purposes. Nitrogen is another important constituent of the atmosphere, and forms the great bulk of air we breathe. We cannot breathe pure oxygen for any length of time. If fire ever breaks out in an atmosphere of pure oxygen, it would be difficult to put it off. Thanks to the presence of nitrogen in air, such disastrous situations do not arise. Another very crucial constituent of the atmosphere is carbon dioxide. You have already studied that plants utilise this gas in the manufacture of food through the process of photosynthesis. The food thus produced serves as a source of energy for other organisms. The other gases, namely, argon, helium, krypton etc. are also present in the air. These are called inert gases, because they do not react with other substances. There is an ozone layer about 25 kilometers above the earth's surface. Ozone is a form of oxygen. Each molecule of ozone contains three atoms of oxygen (0,) while there are only two atoms in the ordinary oxygen (0,) molecule. The ozone layer plays a protective role, as it absorbs a great deal of ultraviolet rays of sunlight coming to the earth, and saves us from skin bums and other harmful effects.
ii) Besides gases, dust particles and smoke there are numerous forms of life in the atmosphere.
iii) The density of the air decreases as one goes up from the surface of the earth and at around 4000 meters it is difficult for man and many other animals to survive without supply of additional oxygen.