The Earth’s Atmosphere UCAR（地球的大气层UCAR）.pdf

THE SUN-EARTH SYSTEM III The Earth’s Atmosphere Composition and Distribution troposphere is heated by energy from the Earth, of the Atmosphere so its temperature decreases with altitude: from about 288 K at sea level to 216 K at its upper The composition of the atmosphere and the boundary. way its gases interact with electromagnetic Above the troposphere, from about 11 to 50 radiation determine the atmosphere’s effect on km altitude, is the stratosphere. This level con- energy from the Sun—and vice versa. Table 2 tains the ozone layer. Ozone, O3, is composed of lists the major components of our atmosphere three oxygen atoms. This molecule strongly and their relative concentrations. absorbs certain ultraviolet wavelengths, and the The atmosphere may conveniently, if rather absorption of this radiation from the Sun heats arbitrarily, be broken into four layers, depend- the air. So temperature increases with height in ing on whether the temperature is falling or the stratosphere, to about 271 K at its top. rising with increasing height above the Earth’s Moving upward, we come to a region in surface (Figure 7). The lowest layer, the tropo- which the temperature again falls with altitude, sphere, extends up to about 11 km. It contains because it has hardly any ozone to absorb radia- about 75% of the mass of the atmosphere and is tion. This region, the mesosphere, extends to 90 the region in which weather takes place. The km or so, where the temperature has fallen to about 183 K.