Atacama - Aridity

Aridity

The Atacama Desert is commonly known as the driest place in the world, especially surroundings of the abandoned Yungay town (in Antofagasta Region, Chile). The average rainfall in the Chilean region of Antofagasta is just 1 millimetre (0.04 in) per year. Some weather stations in the Atacama have never received rain. Periods of up to four years have been registered with no rainfall in the central sector, delimited by the cities of Antofagasta, Calama and Copiapó, in Chile. http://ngm.nationalgeographic.com/ngm/0308/feature3/ Evidence suggests that the Atacama may not have had any significant rainfall from 1570 to 1971. It is so arid that mountains that reach as high as 6,885 metres (22,589 ft) are completely free of glaciers and, in the southern part from 25°S to 27°S, may have been glacier-free throughout the Quaternary, though permafrost extends down to an altitude of 4,400 metres (14,400 ft) and is continuous above 5,600 metres (18,400 ft). Studies by a group of British scientists have suggested that some river beds have been dry for 120,000 years. However, some locations in the Atacama receive a marine fog known locally as the camanchaca, providing sufficient moisture for hypolithic algae, lichens and even some cacti – the genus Copiapoa is notable among these. Geographically, the aridity can be explained by the following reasons:

• The desert is located on the leeward side of the Chilean Coast Range, so little moisture from the Pacific Ocean can reach the desert.
• The Andes are so high that they block convective clouds, which may bring precipitation, formed above the Amazon Basin from entering the desert from the east.
• An inversion layer is created by the cold Humboldt current and the South Pacific High.
• The rain that would change the climate of the land mostly falls at sea instead. Largely this is caused by the cold waters of the Humboldt current just off shore. The temperature change causes most of the clouds and the rain to occur over the ocean instead of over the land. The Humboldt Current transports cold water from Antarctica towards the north the length of the Chilean and Peruvian coasts; this water that makes the western sea breezes cold, reducing evaporation and creating a thermic inversion –cold air immobilized under a cover of tepid air--, preventing the formation of large clouds which produce rain.