Mountains force winds to rise, creating orographic cloud and rain (Figure 10.4 and Figure 10.5) in a complicated way. First, any moist and stable airstream is forced by the wind to rise over a mountain chain, and will lose some of its moisture upon ascent to the
Table 10.1 Effect of latitude and season on rainfall
Latitudes Dec-Feb Jun-Aug
Rainfall (mm) during three months, averaged over ten degrees
crest (Figure 7.2). This mechanism is important at high latitudes. Second, orographic uplift may destabilise any nearly-unstable airstream (Section 7.4), causing deep convection and additional rainfall. Third, the heating of the sides of an isolated mountain draws winds from the surrounding plains (Chapter 14), creating a convergence which causes uplift. This happens in summer when there is no prevailing wind. Any or all of the three processes may occur.
Even quite modest elevation can enhance rainfall greatly. The most rainfall on St Helena (16°S) is near the central peak of only 600 m, where it is 1,300 mm/a, compared with 250 mm/ a on the north-west shore. More striking is the close connection between elevation and rainfall across New Zealand (Figure 10.7).
Rainfall tends to decrease at places above a kilometre or two up a mountain, after the air has lost water lower down and temperatures of the ascending air have fallen to the extent that little water can be held as vapour. The maximum rainfall on Kenya's Mt Kilimanjaro (5,895 m high) is at 2,800 m or thereabouts, and rainfalls in central Java are at a maximum between 1-3 km elevation. Near Mt Wilhelm in Papua New Guinea, rainfalls are around 3,200 mm/a below 500 m, but only 2,300 mm/a above 2 km.
In general, orographic rainfall is promoted by strong winds of moist air impacting a mountain chain at right angles, without inversion layers to impede uplift. For instance, 3,500 mm/ a is measured where steady winds from the warm Indian ocean strike Tamatave at 18°S on the east coast of Madagascar (Chapter 16). Similar amounts are recorded along the coastal hills of northern Queensland, especially in areas where the coast is oriented north-south, across the moist easterly winds.
Cities may increase summertime convective rainfall by around 25 per cent, for instance, at distances of 0-60 km downwind (Section 9-3). An extreme example is Mexico City which experienced rapid growth between the early 1950s and the late 1970s, and the ratio of city to nearby rural rainfalls increased from 1.13 to 1.75, i.e. by 55 per cent. The rise could be caused by additional convection due to urban heating (Section 3.7) or by extra aerosols due to air pollution. A consequence is a slight augmentation of rainfall on weekdays, compared with weekends.
Sometimes it is claimed that a forested surface increases the rainfall. Computer simulation of the atmosphere over Europe (Chapter 15) indicates that a forested surface might induce 30 per cent more frontal rain in some circumstances. But the effect may be small since rainfall depends on atmospheric conditions well above the surface (Section 9.1, Note 10.J).
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