The atmosphere is a mixture of numerous gases. This gaseous cover of the earth is held around it by gravitational attraction and rotates with it. Like the lithosphere and hydrosphere, the atmosphere too is an integral part of the planet earth and is inseparable from it.
However, the density of the atmosphere decreases rapidly with altitude. About 97 percent of the air is concentrated in the lower 29 km.
In fact, the earth’s surface, which sets the stage for all natural as well as human activities, may be deemed to represent the bottom of this shallow ocean of the air. That is why it is all the time being influenced by the changes occurring in the air ocean.
Actually this gaseous cover penetrates to a certain depth in land and water. Since there is unequal distribution of land and water on the earth surface, the atmosphere is present everywhere and every time.
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The atmosphere differs from the lithosphere and the hydrosphere in many respects. Air is a colourless, odourless and tasteless substance. Besides, it is mobile, elastic and compressible. The most interesting fact about air is that we do not feel its presence unless there is horizontal motion in it.
Air in horizontal motion is known as “wind’. Although air is not as dense as land or water, it has weight, and the pressure it exerts on the surface is called the atmospheric pressure. Air pressure is the most important climatic element. The atmospheric pressure at sea level is 1034 grams per square centimeter (14.7 pounds per square inch).