Topographic effects

Relief and surface configuration have a marked effect on rainfall amounts in tropical regions, where hot, humid airmasses are frequent. At the southwestern foot of Mount Cameroon, Debundscha (9-m elevation) receives 11,160 mm yr-1 on average (1960 to 1980) from the southwesterly monsoon. In the Hawaiian Islands, the mean annual total exceeds 7600 mm on the mountains,

Figure 11.55 March rainfall along the southwestern coast of Africa (Gabon and Angola) associated with warm and cold sea-surface conditions.

Source: After Nicholson and Entekhabi; from Nicholson (1989), reprinted from Weather, by permission of the Royal Meteorological Society (redrawn).

Source: After Nicholson and Entekhabi; from Nicholson (1989), reprinted from Weather, by permission of the Royal Meteorological Society (redrawn).

Figure 11.56 The structure of the sea breeze in western Colombia.

Source: After Howell and Lopez (1967); from Fairbridge (1967).

with one of the world's largest mean annual totals of 11,990 mm at 1569-m elevation on Mount Waialeale (Kauai), but land on the lee side suffers correspondingly accentuated sheltering effects with less than 500 mm over wide areas. On Hawaii itself, the maximum falls on the eastern slopes at about 900 m, whereas the 4200-m summits of Mauna Loa and Mauna Kea, which rise above the trade wind inversion, receive only 250 to 500 mm. On the Hawaiian island of Oahu, the maximum precipitation occurs on the western slopes just leeward of the 850-m summit with respect to the easterly trade winds. Measurements in the Koolau Mountains, Oahu, show that the orographic factor is pronounced during summer, when precipitation is associated with the easterlies, but in winter, when precipitation is from cyclonic disturbances, it is more evenly distributed (Table 11.3).

The Khasi Hills in Assam are an exceptional instance of the combined effect of relief and surface configuration. Part of the monsoon current from the head of the Bay of Bengal (see Figure 11.23) is channelled by the topography towards the high ground, and the sharp ascent, which follows the convergence of the airstream in the funnel-shaped lowland to the south, results in some of the heaviest annual rainfall totals recorded anywhere. Mawsyuran (1400-m elevation), 16 km west of the more famous station of Cherrapunji, has a mean annual total (1941 to 1969) of 12,210 mm and can claim to be the wettest spot in the world. Cherrapunji (1340 m) averaged 11,020 mm during the same period; extremes recorded there include 5690 mm in July and 24,400 mm in 1974 (see Figure 4.11). However, throughout the monsoon area, topography plays a secondary role in determining rainfall distribution to the synoptic activity and large-scale dynamics.

Really high relief produces major changes in the main weather characteristics and is best treated as a special climatic type. In equatorial East Africa, the three volcanic peaks of Mount Kilimanjaro (5800 m), Mount Kenya (5200 m) and Ruwenzori (5200 m) nourish permanent glaciers above 4700 to 5100 m. Annual precipitation on the summit of Mount Kenya is about 1140 mm, similar to amounts on the plateau to the south, but on the southern slopes between 2100 and 3000 m, and on the eastern slopes between about 1400 and 2400 m, totals exceed 2500 mm. Kabete (at an elevation of 1800 m near Nairobi) exhibits many of the features of tropical highland climates, having a small annual temperature range (mean monthly temperatures are

Table 11.3 Precipitation in the Koolau Mountains, Oahu, Hawaii (mm).



Source of rainfall

Trade winds

28 May to 3 Sept 1957

Cyclonic disturbances 2 to 29Jan 1957

5 to 6 March I957

Summit 850 m 713

760 m west of summit 625 m 1210

7,600 m west of summit 350 m 329

499 S44 467

329 370 334

Source: After Mink (I960).

19°C for February and 16°C for July), a high diurnal temperature range (averaging 9.5°C in July and 13°C in February) and a large average cloud cover (mean 7 to 8/10ths).

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  • adiam girma
    What are the effects of topography on climate and weather?
    5 years ago

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