Map of the Atacama Desert

The Atacama Desert formed in this area due to several factors. The Andes Mountains created a rain shadow desert that prevents the warm, moist tropical air (brought by the tradewinds) from reaching the west coast. (Remember that the reverse is true on North America's west coast; see Figure 4.4 in Section 4.1). The Atacama Desert is on the leeward, not the windward, side of the Andes at these latitudes. At higher latitudes, this pattern reverses, which creates the prairie grassland of Patagonia in southern Argentina.

A cold ocean current along the west coast of South America lowers the air temperature, so little moisture accumulates. Once this air blows onshore, there is no potential for precipitation. At this latitude, a belt of high atmospheric pressure makes it difficult for air to rise and suppresses the formation of clouds. This creates a zone of low precipitation, which contributes to the already dry conditions of the Atacama Desert.