Page 315 - Fundamentals of Air Pollution 3E

V. General Circulation 271

terns—cyclones and anticyclones, which are quite mobile, causing frequent
changes in weather at any given location. In contrast to the systems near
the earth's surface, the motions aloft (above about 3 km) have few closed
centers and are mostly waves moving from west to east. The core where
speeds are highest in this zonal flow is the jet stream at about 11-14 krn
aboveground (Fig. 17-27). Where the jet stream undergoes acceleration,
divergence occurs at the altitude of the jet stream. This, in turn, promotes
convergence near the surface and encourages cyclogenesis (formation of
cyclonic motion). Deceleration of the jet stream conversely causes conver-
gence aloft and subsidence near the surface, intensifying high-pressure
systems. The strength of the zonal flow is determined by the zonal index,
which is the difference in average pressure of two latitude circles such as

Fig. 17-27. Average position and strength of the jet stream in January between 11 and
1
14 km above the earth's surface (speeds are in m s" ). Source: After Battan (1).