The Atacama is the highest and driest of the Earth's deserts. It occupies a strip of land 1000 km long and 160 km wide on average, extending from about 30º to 18º S between the Pacific Ocean and the Andes mountains, at an average altitude of 2000 m. To the west lies the Coastal Chain, which rises abruptly from the Pacific ocean along most of the Atacama length. The raised depression between the Andes and the Coastal Chain is filled with lava, sediments and salares.



The Atacama is so dry that rainfall has not been recorded for centuries in some places. The aridity of the Atacama has two main causes. One is the high Andes chain of mountains, averaging 4000 m with peaks exceeding 6000 m, which blocks the moist laden air formed above the Amazon basin and causes precipitation on the eastern slopes. Moist air from Amazonia therefore does not reach west of the Andes. The other is the Humboldt Current, which flows northward along the South American coast bringing cold water from higher latitudes. The cold water causes a thermal inversion in the atmosphere, with a cold layer below a warmer higher layer. The cold layer reduces evaporation from the ocean, and the warmer layer above it prevents the scarce moisture gathered from the ocean to rise and develop into rain-producing clouds. Consequently, apart from a few oasis fed by melting ice from the high peaks of the Andes, travellers in the Atacama can drive hundreds of kms without seeing a single dry twig.



To the east of the Atacama, on the feet of the Andes and extending into the altiplano, lies the Central Andean puna grassland, a region of montane grasslands and shrublands. The puna lies above the treeline at about 3 500 m, and below the permanent snow line at 4 500 to 5 000 m. The high daily thermal range and low rain fall has caused the evolution of a unique assemblage of species.