Detailed knowledge of each of the jet streams of the earth's hemispheres- its location, al

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 "wand

A．interpret data

B．explain research methodology

C．evaluate a conclusion

D．suggest a new technique

E．describe a phenomenon