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Reference
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 as you went up. 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.
