The air we breathe is basically a mixture of gases. About 78% consists of nitrogen, 21% oxygen and remaining 1% is a mixture of carbon dioxide and other gases. Air is never dry, it also includes water vapor. The amount of water vapor varies from place to place and from time to time. The ocean of air surrounding earth has no well defined boundary. Scientists generally consider the thickness of air to be 300 miles. The atmosphere above the earth’s surface is divided into four distinct layers.
The lowest layer troposphere averages about 10 miles in depth. It contains about 75% of all of the air in the atmosphere, and almost all of the water vapor (which forms clouds and rain). It is where most of the storms are born.
The next layer stratosphere lies between 10-50 miles above the earth. Commercial airlines fly in the lower stratosphere as it is calm compared to the turbulent troposphere. It contains relatively higher concentration of ozone (O3), a form of oxygen. It is constantly being produced there and then broken down again into oxygen by the absorption of ultraviolet radiation from the sun.
The third layer of atmosphere is called mesosphere is quite cold extending from 30 to 50 miles out in space. Visible trails of meteor are formed here.
Beyond lies the extremely thin thermosphere where air is electrically charged.
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