The driving force of any wind is the local pressure gradient, expressed as ?p/?n, where ?p is the difference between the pressures at points separated horizontally by a distance ?n. It is like the slope of a hillside (i.e. the gradient of elevation) that governs the speed of water flowing downhill. Similarly, the speed of surface winds depends on the gradient of mean sea-level pressures (MSLP) (Section 1.5)
Mean sea-level pressures around the world can be averaged in time and corrected for elevation to obtain a map like Figure 12.7, after joining places with the same pressure by lines called isobars. An isobar is like a contour line on a map which connects places of equal height. Places of maximum pressure are generally marked H, for 'high pressure', and likewise L for 'low-pressure'. It is important to emphasise that places marked H in Figure 12.1 and Figure 12.7 do not have high pressures at every moment. High-pressure systems are mobile (Chapter 13), but linger in the places marked H so that the annual average pressure there becomes high. Likewise for the low-pressure regions, marked L in Figure 12.7.
Particularly high pressures occur over Asia in winter (Figure 12.1) because of the low temperatures there (Figure 3.4). The cold air contracts, leaving room above for adjacent air to converge, adding to the weight of the column which causes the pressure (Note 1.G). For the same reason, relatively low sea-surface temperatures lead to high pressures over the subtropical oceans, especially in summer. Conversely, the MSLP is then generally low over the continents, on account of high surface temperatures there, leading to atmospheric expansion and so a spilling away of upper air, i.e. a reduction of the amount of air in the column above the continent. However, a different process operates at about 55° around Antarctica, where a ring of lows constitute the circumpolar low. The remarkably low pressures there (much lower than in the northern hemisphere) result from the shallowness of the troposphere at high latitudes, and the consequent warmth of the tropopause (Figure 1.9).
High pressures dominate at around 30° latitudes. The highs are centred over the oceans in summer (Figure 12.1), and adjacent continents in winter (Figure 12.7), whichever is the cooler. The belt of these subtropical highs expands equatorward in winter (Section 12.3). For instance, the South Pacific high-pressure zone is centred at 23°S in July but 32°S in January (Figure 12.1). The shift drives the Trades into the other hemisphere and contributes to the monsoon there (Figure 12.6).
Overall, the variation of zonal-mean MSLP with latitude is shown in Figure 1.8, which also indicates the Equatorial Trough. There are steep gradients between 30-60°S (where zonal winds happen to be strong—Figure 12.5) but the curve is flat at 30°S and the equator, where winds are light. The relationship between the north-south gradient of pressure, and the strength and direction of the zonal winds, is due to the Coriolis effect, which has now to be discussed.
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