jet stream(redirected from Sub-tropical jet stream)
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jet stream,narrow, swift currents or tubes of air found at heights ranging from 7 to 8 mi (11.3–12.9 km) above the surface of the earth. They are caused by great temperature differences between adjacent air massesair mass,
large body of air within the earth's atmosphere in which temperature and humidity, although varying at different heights, remain similar throughout the body at any one height.
..... Click the link for more information. . There are four major jet streams. Although discontinuous at some points, they circle the globe at middle and polar latitudes, both in each hemisphere. The mean position of the stream in the Northern Hemisphere is between lat. 20 and 50 degrees N; the polar stream is between lat 30 and 70 degrees N. Wind speeds average 35 mi (56.3 km) per hr in summer and 75 mi (120.7 km) per hr in winter, although speeds as high as 200 mi (321.9 km) per hr have been recorded. Instead of moving along a straight line, the jet stream flows in a wavelike fashion; the waves propagate eastward (in the Northern Hemisphere) at speeds considerably slower than the wind speed itself. Since the progress of an airplane is aided or impeded depending on whether tail winds or head winds are encountered, in the Northern Hemisphere the jet stream is sought by eastbound aircraft, in order to gain speed and save fuel, and avoided by westbound aircraft.
an air current in the upper troposphere and lower stratosphere with an almost horizontal axis. A jet stream is characterized by high velocities, relatively small transverse dimensions, and large vertical and lateral wind shears. It resembles a gigantic jet within the relatively weak winds of the surrounding atmosphere. Jet streams are thousands of kilometers long, hundreds of kilometers wide, and several kilometers deep. The maximum wind speeds are observed at the axis of a jet stream and may vary from 108 km/hr to 250–350 km/hr. Jet streams may have a significant effect on the ground speed of modern airplanes; the strong turbulence in the vicinity of jet streams also affects flight.
Tropospheric jet streams outside the tropical latitudes arise from frontal zones (polar fronts) between air masses of the troposphere. The strong lateral temperature gradients in these zones give rise to strong pressure gradients and strong winds in the upper troposphere and lower stratosphere. The axes of jet streams are most frequently near the tropopause at altitudes between 7 and 12 km; the altitudes are higher during the summer than during the winter. These jet streams are displaced and develop in connection with cyclonic activity at fronts. At the highest latitudes, jet streams are less intense and are found at lower altitudes owing to arctic and antarctic fronts. At subtropical latitudes (25°–40°), more stable, subtropical jet streams are observed with axes at altitudes of 12 to 14 km. These jet streams result from the convergence of antitrades and air currents of the middle latitudes and are related to subtropical fronts, which are found only at the highest layers of the troposphere.
The major direction of air transport in all tropospheric jet streams is from west to east. Jet streams should therefore be considered an intensification of the general air transport from west to east in the upper troposphere and the lower stratosphere. Near the equator, at altitudes between 15 and 20 km, equatorial jet streams often arise in connection with the intertropical convergence zone. The predominant wind direction in these jet streams is easterly, corresponding to the direction of the general air transport at these latitudes. Stratospheric jet streams are also observed with axes at altitudes between 25 and 30 km; they are westerly at high latitudes during the winter and easterly at low latitudes during the summer.
Jet streams are significant components of the atmospheric circulation. Because of this and because jet streams are of practical importance to air transport, they were intensively studied on an empirical and theoretical basis in the 1950’s and 1960’s.
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S. P. KHROMOV