The atmosphere of Earth is made up of several distinct layers.
The lowest layer, the one in which humans live and breathe,
is the troposphere (Figure 9.7a). It contains 90 percent of
Earth’s atmospheric mass and is the source of all weather on
the planet. At Earth’s surface, usually referred to as sea level,
the troposphere has an average temperature of 15°C (288
K) and an average pressure of 1.013 bars. Within the troposphere,
atmospheric pressure, density, and temperature all
decrease with increasing altitude. For example, at an altitude
of 5.5 km (18,000 feet, which is a few thousand feet below the
summit of Denali in Alaska), the atmospheric pressure and
density are only 50 percent
of their sea-level values and
the average temperature
has dropped to –20°C.
Still higher, at an altitude
of 12 km, where commercial jets cruise, the temperature is
a frigid –60°C and the density and pressure are less than
one-fifth what they are at sea level. Mountain climbers and
astronomers are very much aware of this behavior of Earth’s
troposphere. At the Mauna Kea Observatories in Hawaii,
even the most dedicated astronomers, surrounded by thin
air and subfreezing temperatures, have been known to gaze
longingly at the sunny beaches some 4 km below, where it is
a pleasant 30°C warmer.