【单选题】

The atmosphere is a mixture of several gases.

There are about ten chemical elements which
remain permanently in gaseous form in the atmo-
Line sphere under all natural conditions. Of these per-
(5) manent gases, oxygen makes up about 21 percent
and nitrogen about 78 percent. Several other
gases, such as argon, carbon dioxide, hydrogen,
neon, krypton, and xenon, comprise the remaining
1 percent of the volume of dry air. The amount
(10) of water vapor, and its variations in amount and
distribution, are of extraordinary importance in
weather changes.Atmospheric gases hold in sus-
pension great quantities of dust, pollen, smoke,
and other impurities which are always present in
(15) considerable, but variable amounts.
The atmosphere has no definite upper limits
but gradually thins until it becomes imperceptible.
Until recently it was assumed that the air above
the first few miles gradually grew thinner and
(20) colder at a constant rate. It was also assumed that
upper air had little influence on weather changes.
Recent studies of the upper atmosphere, currently
being conducted by earth satellites and missile
probings, have shown these assumptions to be
(25) incorrect. The atmosphere has three well-defined
stratA、
The layer of the air next to the earth, which
extends upward for about 10 miles, is known as
the troposphere. On the whole, it makes up about
(30) 75 percent of all the weight of the atmosphere. It
is the warmest part of the atmosphere because
most of the solar radiation is absorbed by the
earth’s surface, which Warms the air immediately
surrounding it.A、steady decrease of temperature
(35) with increasing elevation is a most striking char-
acteristiC、The upper layers are colder because of
their greater distance from the earth’s surface
and rapid radiation of heat into space. The tem-
peratures within the troposphere decrease about
(40) 3.5 degrees per 1000-foot increase in altitude.
Within the troposphere, winds and air currents
distribute heat and moisture. Strong winds, called
jet streams, are located at the upper levels of
the troposphere. These jet streams are both
(45) complex and widespread in occurrence. They
normally show a waveshaped pattern and move
from west to east at velocities of 150 mph, but
velocities as high as 400 mph have been noteD、
The influences of changing locations and
(50) strengths of jet streams upon weather conditions
and patterns are no doubt considerable.Current
intensive research may eventually reveal their true
significance.
Above the troposphere to a height of about
(55) 50 miles is a zone called the stratosphere. The
stratosphere is separated from the troposphere
by a zone of uniform temperatures called the
tropopause. Within the lower portions of the
stratosphere is a layer of ozone gases which filters
(60) out most of the ultraviolet rays from the sun. The
ozone layer varies with air pressure. If this zone
were not there, the full blast of the sun’s ultra-
violet light would burn our skins, blind our eyes,
and eventually result in our destruction. Within
(65) the stratosphere, the temperature and atmospheric
composition are relatively uniform.
The layer upward of about 50 miles is the
most fascinating but the least known of these
three stratA、It is called the