What makes this the driest desert in the world?
The primary cause is a global climatic phenomenon that creates deserts at this latitude on the western coasts of all continents in the Southern Hemisphere. Large, stable high-pressure systems - the one here known as the Pacific Anticyclone -- hover just off the coast, creating easterly trade winds and pushing storm systems away. Meanwhile, the Humboldt Current carries cold water from Antarctica north along the Chilean coast, cooling the westerly sea breezes, reducing evaporation, and creating a temperature inversion (cold air immobilized below a ceiling of warm air), which prevents the formation of high rain-bearing clouds. What little moisture is borne onward by these ocean breezes condenses along the steep Pacific slopes of the Coast Range, creating highly endemic coastal ecosystems composed of cacti, succulents, and other xerophytic flora. The final factor contributing to the formation of the desert is the Andean cordillera, which in the north forms a broad, high-altitude volcanic plateau known as the Altiplano. But whereas in the south, the Andes serve to capture moisture blowing in off the Pacific, in the north the Altiplano seals Chile off from storms laden with moisture from the Amazon Basin, just to the northeast.