Strong westerly winds extend over most of the upper troposphere just below the tropopause (Figure 12.10). They are esentially thermal winds (Note 12.F), due to the meridional gradient of near-surface temperatures at midlatitudes. The westerlies are strongest near the polar front (Figure 12.12), where the temperature gradient is steepest. They are not found higher than the tropopause because the latitudinal gradient of temperature is actally reversed at such elevations, i.e. it is colder at lower latitudes (Figure 1.9 and Figure 12.10), especially in the summer hemisphere. In short, the belt of strongest winds, called the jet stream, is above the polar front and just below the tropopause.
The speed and direction of the jet stream are evident in the path of a balloon launched from New Zealand and tracked for 102 days as it was carried along at 200 hPa (Figure 12.13). It travelled about 1,000 km/day. Another balloon flight in the upper westerlies is described in Note 12.I. Airplane flights from Sydney to Santiago (Chile) at a level of nearly 11 km take about an hour less than the return flight in summer, and two hours in winter, on account of the upper westerlies.
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