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  3. 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

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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.
According to the passage, which of the following statements about temperature gradients is true?
选项 A、These temperature gradients tend in most cases to be impossible to describe mathematically.
B、The lower the contrast between the extremes of the gradient, the lower the resulting velocity of the jet stream.
C、Jet streams are responsible for most of the north-south transfer of heat between the equator and the poles.
D、The temperature gradient tends, on a non-negligible level, to augment the effects of the earth’s rotation.
E、The gradient typically refers to differences in jet stream velocity at various longitudinal positions around the globe.
答案B
解析
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