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SUBTROPICAL JET STREAMS. These jets, like the polar-front jets, are best developed in winter and early spring. During summer, in the Northern Hemisphere, the subtropical jet weakens considerably, and it is only identifiable in sporadic velocity streaks around the globe. During winter, subtropical jets intensify and can be found between 20 and 50 latitude. Their maximum speed approaches 300 knots, although these higher wind speeds are associated with their merger with polar-front jets. The core is most frequently found between 35,000 and 40,000 feet. A subsidence motion accompanies subtropical jets and gives rise to predominantly fair weather in areas they pass over. These jets are also remarkably persistent from time to time, but they do fluctuate daily. Sometimes they drift northward and merge with a polar-front jet. Over Asia in summer, the subtropical jet is replaced by the tropical easterly jet stream.

TROPICAL EASTERLY JET STREAM. This jet occurs near the tropopause over Southeast Asia, India, and Africa during summer. The strongest winds are over southern India, but they are not as intense as the winds encountered in polar-front or subtropical jet streams. This jet is closely connected to the Indian and African sum-mer monsoons. The existence of this jet implies that there is a deep layer of warm air to the north of the jet and colder air to the south over the In-dian Ocean. This warm air is of course associ-ated with the maximum heating taking place over India in summer, while the colder air is over the ocean. The difference in heating and cooling and the ensuing pressure gradient is what drives this jet.

POLAR-NIGHT JET STREAM. This jet meanders through the upper stratosphere over the poles. It occurs only during the long winter night. Remember, night is 6 months long over the pole in which winter is occurring. The polar stratosphere undergoes appreciable cooling due to the lack of solar radiation. The horizontal temperature gradient is strongly established bet-ween the equator and the pole, and the pressure gradient creates this westerly jet. The temperature gradient breaks down intermittently during middle and late winter in the Northern Hemisphere; therefore, the jet is intermittent at these times. In the Southern Hemisphere the temperature gradient and jet disappear rather abruptly near the time of the spring equinox.

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