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Introduction
Half of the Earth's atmosphere lies within 5 km of the surface of the
earth, three-fourths is located within ten km and 90% is within 16 km (10 miles).
Among the many functions of this atmosphere are two that are critical to life
as we know it: the atmosphere shields the surface of the earth from lethal
ultraviolet radiation and allows life to exist on the surface of the
planet. It also serves to process energy that comes from the sun and energy that is reradiated from the surface of the earth.
Absorption of this energy establishes an average surface temperature
that allows water to exist in all three phases -- liquid, solid, and
vapor. The favorable environment established through these two
functions allows for the flourishing of life on the planet over the past million years.