Highs (or ridges) at the surface imply subsidence at a few hundred metres a day, or less. This slow rate allows warming due to subsidence to be offset by cooling by radiation loss and by the evaporation of clouds. Also, the subsidence leads to an inversion on top of the boundary layer at about 1,000 m or so (i.e. a PBL inversion, Section 7.6), especially with cold highs in winter over continents. Where there is low-level moisture and air pollution it is trapped by the inversion, which leads to the sky becoming covered by persistent stratus cloud and thus to anticyclonic gloom. This occurs in Melbourne and Santiago in winter, for instance.
Generally, the subsidence characteristic of a subtropical high makes cloud and precipitation unlikely, yet it can occur in two ways. Firstly, the slight lowering of MSLP within a shallow trough moving across a subtropical high may be sufficient to trigger thunderstorms, especially in summer. Secondly, a turning of winds to become southerly or south-easterly, after the passage of a trough across a high, can induce light rain or drizzle along a coast facing southeast, even though the atmosphere is stable and subsiding. The rain is the result of coastal uplift and happens along south-east coasts of Brazil, South America and Australia, where it is an important source of rain in winter for places sheltered from westerly frontal rain by mountains. The case of Durban on the southeast coast of South Africa is shown on Figure 13.7 (Day 5).
Arid climates result from a prevalence of highs, as in the subtropics (Figure 10.3b). Northeast Brazil is arid, even at a latitude of only 8°S or so, because it protrudes far enough into the south Atlantic to be dominated by the high there. The same is true for the Galapagos Islands, dominated by the South Pacific high, even though they are at the equator. Droughts in midlatitudes are caused by a prevalence of highs, sometimes blocking highs, which deflect rain-bearing lows poleward.
Thus we end our consideration of the aspects of winds of the scale important in weather forecasting. These form the background to surface winds actually experienced at particular sites, discussed in Chapter 14.
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