The atmosphere is divided into several layers,
based mainly on the vertical temperature gradients that vary
significantly with height. The lower 36,000 feet (11 km) of
the atmosphere is characterized by circulating air known as
the troposphere, where the temperature generally decreases
gradually, at about 14°F (6.4°C) per kilometer, with increasing
height above the surface. This is because the Sun heats the
surface, which in turn warms the lower part of the troposphere.
Most of the atmospheric and weather phenomena we
are familiar with occur in the troposphere.
The main components of the atmosphere are nitrogen,
oxygen, argon, carbon dioxide, helium, krypton, neon, and
xenon. Increasing levels of photochemical smog in the troposphere
is an increasing problem in many places. Smog is
made mainly of the gas ozone (O3), produced by secondary
chemical reactions from automobile and other pollution such
as hydrocarbons, although some is produced naturally. While
ozone in the stratosphere forms a protective shield against
ultraviolet radiation, low-level ozone is harmful. Ozone irritates
the respiratory system and also retards tree and crop
growth, and even causes rubber to break down. The formation
of low-level ozone is particularly bad near major urban
areas such as Los Angeles, California.
See also ATMOSPHERE.
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