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Detailed knowledge of each of the jet streams of the earth’s hemispheres- its location, altitude and strength-is critical
Detailed knowledge of each of the jet streams of the earth’s hemispheres- its location, altitude and strength-is critical
admin
2011-01-17
55
问题
Detailed knowledge of each of the jet streams of the earth’s hemispheres-
its location, altitude and strength-is critical to modern-day weather
forecasting, as well as to more specific applications such as the routing of
Line aircraft. By way of definition, a jet in fluid dynamics is simply a core of fluid
(5) moving at a higher velocity than the surrounding fluid, and although
complicated to describe mathematically, the jet streams in the atmosphere are a
straightforward, natural result of the meridional (equator-to-pole) temperature
gradient in the earth’ s atmosphere. Analogous flows exist on other planets with
substantial atmospheres having similar temperature gradients.
(10) The temperature gradient derives from the differential solar heating of the
spherical surface of a planet: the surface is generally warmest at the equator
and grows progressively cooler as one moves poleward. The centrifugal effects
of the earth’s rotation, often called the Coriolis force, deflect the north-south
transport of heat from the equator to the poles into the predominantly east-west
(15) motion of the jet stream. The relative strength, or velocity, of the jet stream is
proportional to the intensity of this thermal gradient. During the winter
months, when the equator-to-pole temperature disparity is at its greatest, the
jet stream reaches its maximum velocity, while during the summer months,
when the temperature gradient between the equator and the pole is considerably
(20) less, the jet stream reaches its minimum velocity.
The jet stream does not maintain a straight, zonal flow from west to east
but rather takes on a more serpentine look, often with dramatic dips to the
south or rises to the north. There are two major reasons for these nonzonal
motions: the temperature gradient between the equator and the poles and the
(25) presence of land masses on the earth’s surface. The meridional temperature
gradient between the equator and poles that gives rise to the jet stream also
produces secondary atmospheric circulations, or eddies which, referred to by
meteorologists as baroclinic waves, have a complex interaction with the jet
stream, one that is intriguingly two-sided. The eddies modify the distribution of
(30) temperature and kinetic energy within the atmosphere, a process that has a
pronounced effect on the location and movement of the jet stream, which itself
interacts with these waves, acting not only as a transport or steering
mechanism but transferring momentum and energy back to the waves.
The presence of land masses on the earth’s otherwise watery surface also
(35) modifies the distribution of temperature, because continents heat and cool at a
dramatically slower rate than do the oceans. The topography of the land also
influences the jet stream’s location-mountain ranges and plains on large
continents, for example, significantly imbalance the distribution of atmospheric
temperature, narrowing the jet stream. And since the jet stream is a thermally
(40) driven phenomenon, the more complicated the three-dimensional temperature
structure of the earth’s atmosphere, the more "wandering" will take place in
the course of the jet stream.
It can be inferred from the passage that during yearly equinoxes, jet streams tend to
选项
A、move at velocities slower than those at which they move the summer but faster than those at which they move in the winter
B、move from toward the east from the west in accordance with the deflection process caused by the Coriolis force
C、move at velocities faster than those at which they move the summer but slower than those at which they move in the winter
D、move at velocities equal to those of other planets that have similar polar temperature gradients
E、move at velocities equal to that of the centrifugal motion that causes the deflection of the jetstream’s direction
答案
C
解析
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本试题收录于:
GRE VERBAL题库GRE分类
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