Spatial Differences

We will now describe patterns of rainfall in terms of the annual or monthly average values, as we did for temperature (Section 3.2). Of course, average rainfall may not be typical— the mean of nine drought years and one flood year does not resemble the rainfall in any year. So alternatives to the arithmetic mean are sometimes more useful (Note 10.H). For instance, there are the 'modal' and 'median' rates of rainfall. The mode is derived after data have been grouped into sequential ranges; it is the mid-value of the range with the most values. But it is possible to obtain the mode only when many values are available. The median annual rainfall is exceeded in half the years, and is less affected than the average by outlying values.

Large-scale maps of average rainfalls (Figure 10.3) show great differences between seasons and spatially, affected mainly by the latitude (Figure 6.13). Notably, rainfall at sea (1,140 mm/a) is more than that over the land (730 mm/a). This means that the southern hemisphere with its greater area of ocean (Note 1.A) is slightly wetter than the northern half of the world, with 1,080 mm/a instead of 960 mm/ a. It also means that a major factor governing the rainfall at places on land is proximity to the sea. Other factors are topography, elevation,

Figure 10.3 (a) Mean annual precipitation (mm/a) around the world.

(b) Mean monthly precipitation (mm) in January and July in the southern hemisphere.

Figure 10.3 (a) Mean annual precipitation (mm/a) around the world.

(b) Mean monthly precipitation (mm) in January and July in the southern hemisphere.

latitude and the coastal sea-surface temperature, which will now be considered.

Distance from the Sea

Remoteness from the sea tends to lead to low rainfalls, especially in the 'rainshadow' of mountain ranges, where the air's descent and consequent warming (Note 7.E) lead to the evaporation of any cloud, as in Patagonia east of the Andes. Rainfalls on the westerly windward side of New Zealand's Alps in the South Island are up to 10,000 mm/a, but only 500 mm/a on the leeward side (Figure 10.4). (This corresponds to the pattern of cloudiness in Figure 8.15.) Similarly, a transect at about 30°S from Australia's east coast, across the Dividing Range, into the interior desert, shows an annual rainfall of 1,658 mm at Coffs Harbour

Figure 10.4 Annual mean rainfall in New Zealand.

on the coast, 796 mm/a at 130 km inland, 473 mm/a at 416 km and 210 mm/a at about 1,000 km.

An exception to the rule about most rain at the coast is the situation around 20°S in southwest Africa, for instance. Precipitation is less than 100 mm/a at the coast, but over 400 mm/a at about 150 km inland. This is due partly to the low sea-surface temperature along the coast (Chapter 11), making onshore air masses too stable for the uplift needed to create rain, and partly to the winds being often easterly, i.e. offshore, making the coastal fringe downwind of high land. The same applies at similar latitudes on the west coast of South America; the world's driest place is Arica (at 18°S, on the coast of Chile), where only 1 mm of rain was measured over forty-two years. Several places near the coast of northern Chile have recorded no rain for one or two decades. These dry coastal regions extend thousands of kilometres offshore of south-west Africa, northern Chile and also western Australia (Figure 10.3).

Other exceptions are found at tropical islands, many of which receive more rainfall inland than at the coast on account of uplift in the centre of fairly flat islands (such as Bathurst and Melville islands just north of Darwin in Australia), due to the convergence of sea breezes (Chapter 14). On larger, mountainous islands, such as Papua New Guinea, the increase of rainfall inland (Figure 10.5) is due to orographic uplift behind the coastal plain. Moreover, when the wind blows from one prevailing direction, the downwind coast may be relatively dry. For instance, Suva on the southeast coast of Viti Levu (Fiji, 18°S) receives 3,024 mm/a, while Nadi

Figure 10.5 Effect of elevation near Port Moresby (9°S) on the annual rainfall.

on the north-west coast receives less than half that amount, most of it in summer when the prevailing south-easterly winds weaken or vanish.

The driest parts of Australia are well inland. The most arid region is around the ephemeral Lake Eyre, at 28°S in South Australia, where the median annual rainfall is less than 100 mm. It has been suggested that the aridity could be remedied by flooding Lake Eyre, in the hope of thereby increasing local rainfall. The idea is nonsense (Note 10.I).

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