Amounts Of Cloud

About 52 per cent of the globe is covered by cloud at any time, with a slightly higher fraction in the southern hemisphere where there is more ocean (Figure 8.14). On the whole, there is least cloud over North Africa, northern Australia and Antarctica, and most north of 60°N, over high-latitude oceans (especially at 60°S) and south-east Asia. There are persistent decks of stratocumulus over the cool oceans (Chapter 11) off the coasts of Namibia, northern Chile and Peru, especially in summer. On the other hand, high clouds are particularly common on the equator north and north-west of Australia. The South Pole is much less cloudy than the North, because the South Pole is higher, colder and further from the sea, so there is less atmospheric moisture. In general, proximity to the sea and mountains increases cloudiness, though there is a decrease in some places right at the coast on account of reduced convection from a cool ocean surface.

Cloudiness varies with season. There is more (convective) cloud in summer than winter in most parts of the Americas and in southern Africa, but more (frontal) cloud in winter in much of Europe, for instance. The cloudiest regions in the southern hemisphere in July are near the equator, and there is a band of cloud around Antarctica at about 60°S, with least cloud at 15°S and the South Pole. In January, there is least cloud at 30°S.

The chain of mountains along the west coast of the south island of New Zealand gives rise to the so-called 'long white cloud' that notably

Figure 8.12 Comparison of (a) the GMS cloud photograph at noon Greenwich Meridian Time (GMT) on 8 November 1995, and (b) the synoptic chart from ground measurements at the same time (from the Australian Bureau of Meteorology). The cold front across south-west Australia on the chart corresponds to the band of cloud in the photograph. Isolated convective cloud can be seen at low latitudes.

Figure 8.13 The ranges of cloud-top temperature and albedo of various kinds of cloud in the New Zealand region.

reduces the amount of bright sunshine there (Figure 8.15). However, central Australia is usually free of cloud. For instance, there are usually five months each year with over 10 hours of bright sunshine each day in Alice Springs and the annual average is 9.6 hours of bright sunshine daily, i.e. 80 per cent of the daylength of 12 hours. Even in west Tasmania (with less than 5 hours daily on average) the figure is more than the 4.2 for London, for example. Figure 8.16 shows that there is a notable seasonal variation of sunshine in Canberra and Adelaide, due to both the variation of the length of day (Table 2.1) and a difference between amounts of frontal cloud in winter. The different curve for Darwin results from the monsoonal climate (Chapter 12).

It is now generally agreed that there has been a global increase of cloud since about 1950, possibly because of more chimney emissions of sulphates to nucleate cloud droplets (Section 8.2). There was 2.3 per cent more cloud in the northern hemisphere in 1981 than in 1952, and a 1.2 per cent increase in the southern hemisphere, where there is less industrial pollution. The increase was mainly of altocumulus and altostratus.

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