Contribution to a comparative meteorology of n10untain areas

56 HERMANN FLOHNincludes all scales from the laminar, millimetre-thick boundary layer up to the earth'satmosphere? (Proportional to the earth's radius the atmosphere is also very thin; 99per cent is concentrated in a layer 30 km thick and 40,000 km long.)Mountain areas offer us one ofthe best opportunities to study the interaction betweenthe different stages in this hierarchy of processes. Here we can measure the processes ofthe heat balance down to the lowest 1-10 em of the surface layer. Horizontal differencesin this layer produce the shallow (few metres thick) slope breezes which can cover manyhundreds of metres. These winds acting together make up the vaHey and mountainbreezes which have been studied for more than one hundred years. In turn, the sum ofthevalley and mountain breezes produces weak, but for weather very effective, thermallydriven, diurnal, large-scale circulations covering large mountain ranges (see p. 66).These winds were first quantitatively studied by Burger and Ekhart (1937) in the Alps,where they were found to have a.horizontallength ofapproximately 500-1000 km. Allthese processes occur, in forms varied by the earth's topography and radiative and thermalproperties of the snow cover, in all mountain areas."filii.2. Vertical distribution ofprecipitationIn contrast to many erroneous theories ofthe nineteenth century, properly installed andreliable stations in the Alps show an increase in the amount ofprecipitation with heightup to 3000-3500 m, above which few representative stations exist. This is actually asurprising result seemingly inconsistent with the fact that the absolute water-vapourcontent of the atmosphere at 3000 m is only about one-third of that at sea level. Similar,but not so reliable, evidence from other mountain areas in temperate and subtropicalzones yields the same results as in the Alps. In the Hindu Kush (Flohn, 1969a), thePamirs, and the northwestern Himalaya (Flohn, 1970a), ridge stations at 3200-4200 mshow a real increase in precipitation with height.In the tropics, however, especially in the equatorial region within 10 0 latitude of theequator, the pattern is different. In these areas, according to Weischet (1969), there is azone of maximum precipitation at 1000-1400 m perhaps with the exception of continentalmountains. This zone has been found in Java, Sumatra, Ceylon, west Africa, ..",central America, and the tropical Andes, and is especially striking on Kilimanjaro (fig.2B.l). Weischet differentiates a tropical convective type ofrain from an extratropical advectivetype. Under conditions ofpurely convective vertical mass exchange, the precipitationmaximum should lie at the level where the drops from the clouds begin to evaporate,i.e. the cloud base, which near the equator lies at 400-700 m at coasts and600-1000 m inland. Because in orographic rain, updraughts of 1-3 m S-I reduce the fallspeed of the drops - in cumulonimbus (Cb) cells updraughts of ;;;':10 m S-I can eventransport large drops upwards itis understandable that the zone ofmaximum rain lies afew hundred metres above the cloud base. In the region of the trade wind inversion, atKilimanjaro for example, areas above 2000 m are arid. Even in the area of the IntertropicalConvergence (ITC) with its cumulonimbus clusters, the amount ofprecipitationin areas above 3000 m decreases to about 100 em yr- I , which amounts to 10-30 per cent