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Air Composition
Our atmosphere is composed almost entirely of oxygen and nitrogen in
their diatomic forms (two atoms bound together by chemical forces).
Diatomic nitrogen makes up about 78% of the total molecules in the
atmosphere. Diatomic oxygen represents nearly 21%. The inert noble gas,
argon, accounts for about 0.9%, and the remaining 0.1% is composed of
many trace gases, the most significant being carbon dioxide and water
vapor. Carbon dioxide makes up only 325 parts per million of the
atmosphere by volume. Water vapor is present in highly variable
quantities ranging from 0 to 4% by volume.
Air Density
If the atmosphere were like water and incompressible, pressure would
decrease uniformly while ascending. In reality, the atmosphere is
compressible and density (mass per unit volume) is proportional to
pressure. This relationship, called Boyle’s Law, implies that density
decreases with height in the atmosphere: As height increases, less mass
remains above a given point; therefore less pressure is exerted. At sea
level, the density of air is about 1 kg per cubic meter (8 oz per cubic foot).
Both pressure and density decrease by about a factor of 10 for every 16
km (10 miles) increase in altitude.
Density does not depend solely on pressure. For a given pressure, density
is inversely proportional to temperature. This relationship, known as
Charles’ Law, implies that the depth of an air column bounded by two
constant-pressure surfaces will increase as the temperatures in the column
decreases.
Density varies mostly with pressure over large vertical distances; at
constant height, pressure variation with temperature becomes important. In
the low atmosphere, air is heavy, with a stable mass of roughly one
kilogram per cubic meter (1 oz/cubic foot). A room of 500 cubic meters
(650 cubic yards) thus contains 0.05 metric ton of air. At an altitude of 3
km (2 miles), however, density is 30% less than at sea level.
This difference in air density can cause variations in flow readings from
one location to another when elevations are quite different and no
corrections are made.
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