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Air consists of 77% Nitrogen and 21% Oxygen, and other
substances like water vapor and other gases. This mixture surrounds
the earths atmosphere. You might be asking why doesn't this gas
mixture just float of into space? The answer is gravity. Gas is
made up of matter, It has mass thus weight is a factor that holds
it to Earth. Close to sea level the pressure is approx 14.7 pounds
per sq inch. This is occurs because air has weight. Air in the
atmosphere is stacked one on top another. If you stack one book on
top another on a table you note that the wight on that table
increases. A similar situation occurs in nature. When air is
stacked on top another the effect of that action is weight. That
weight shows up as pressure. Pressure is Force measured in terms of
its distribution over an area of opposing force. The pressure
decreases as altitude increases. This happens because when you get
higher and higher in the atmosphere there is less and less air
pushing on top of you.
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