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Again referring to the world rainfall map, it will be seen that all west coasts in latitudes 15° to 30° are extremely dry, generally with less than 10 in (25 cm) of rainfall annually. The Atacama Desert of Chile and the Namib Desert of coastal southwest Africa are perhaps the most celebrated of these deserts, but they exist also in Lower California, the Moroccan coast of Africa, and the west coast of Australia. The arid belt extends continuously eastward to inland continental tropical deserts.
2. Main article
Does it not seem strange that extreme dryness exists immediately along the shores of the oceans, close to possible sources of moist maritime air masses? The interior tropical deserts, already discussed, are logically explained by land-centered high-pressure cells into which moist air masses cannot easily drift, but the dry west coast strips are located between the oceanic and continental high-pressure cells where we might expect to find some development of fronts and convergence of air masses. The key to this problem seems to lie in the fact that the oceanic subtropical high-pressure cells are inherently dry on their east sides. The circulation in these cells is thought to be such that the air on the east sides is subsiding as it moves outward, hence, is adiabatically heated and its humidity reduced. The result is dry, stable air masses that bring an arid zone not only to the coast but extending far seaward as well. Strangely enough, there are dry deserts over the oceans in these tropical latitudes.
3. References
In what respects do the dry west coasts differ in climate from the interior continental deserts, which they adjoin? The principal difference is in temperature. The coastal deserts are relatively cool, with annual average temperatures around 65°F (18°C), whereas the interior continental deserts average some 10°F (5°C) higher. The presence of cool upwelling and equatorward-flowing currents, such as the Humboldt and Benguela currents, explains the lowered temperatures. The annual range is only 11°F (6°C). In contrast, the temperature cycle of Aswan, Egypt (24° N), shows both a higher average and a greater range.
Although the cool west coast desert climate is treated here as a separate climate from the tropical desert climate, not all climate classifications make the distinction. In the original Koppen classification, the cooler desert west coasts arc designated by the symbol BWn, in which n means frequent fog (from the German, nebel, meaning fog). Persistent coastal fog banks form in the cool lower air layer overlying the cool ocean water. In the latest versions of the Koppen climate system, the symbols BWh and BWk are applied to the West-Coast deserts. The BWk climate is limited to the South American and southwest African coasts in latitudes 20° to 32° S, where the cool ocean currents are most influential.
Vegetation and soils of the cool west coast deserts are essentially similar to those of the interior deserts. An unusually high incidence of fog on these coasts leads to growth of some specialized plants, which can exist, on condensed moisture close to the shore.
