olelog

Two months ago I wrote about the wettest place on earth. My trip to Greece reminded me that there are also dry places. There is a bit of disagreement, however, where the driest place is. One outstanding candidate is a low spot in the Lut Desert of eastern Iran.

I will however, as so many have done before me, keep it to the Atacama Desert in South America. The Atacama Desert is a virtually rainless plateau in South America, extending nearly 1000 km between the Andes mountains and the Pacific Ocean. Parts of Chile's Atacama Desert haven't seen a drop of rain since record-keeping began. Here a place called Arica gets just 0.76 millimetres of rain per year. At that rate, it would take a century to fill a coffee cup. The precipitation (moisture equivalent to rain) in Atacama averages less than 1 centimetre per year from fog. Measurable rainfall (more than a millimetre of rain) occurs every five to 20 years and heavy rains fall only two to four times a century. No vegetation grows here. It is what is termed ‘absolute desert’.

The desert is to a great extent created by the cold Humboldt current (also known as Peru Current). A great mass of ice cold water surges out of the Antarctic Ocean and flows north along the South American continental shelf. The shallowing land forces the cold deep waters up to the sea surface where the waters encounter warm winds that blow land-ward. The warm air cools as it moves across the cold current and the air becomes too cold to hold much moisture. No rain clouds, therefore, can reach the coast and the land dries into a hostile area for life. In the winter, fog rises from the upwelling cold currents, blankets the desert, and gives moisture to the land. The mountain ranges also play a role. The Atacama is blocked from moisture on both sides by the Andes mountains to the east and by coastal mountains to the west. The trade winds blow westward on the east side of the Andes, but the desert lies in the rain shadow of the Andes.

I would like to compare the situation with the Kalahari or Namib Desert and the cold Benguela current in Southern Africa. The Namib Desert is considered to be the oldest desert in the world, having endured arid or semi-arid conditions for at least 55 million years. Its aridity is caused by the descent of dry air cooled by the cold Benguela current along the coast. It has less than 10 mm of rain annually and is almost completely barren. The cold waters of the north-flowing Benguela current move from the western coast of South Africa and Namibia towards north and Northwest up to the line where it joins the southern equatorial current which is a warm current. Its waters are cold because there are very deep waters that were brought upward due to the rotation of Earth from west to east. This upward movement of deep waters are sometimes increased by southern Trade winds which blow west from the Kalahari Desert towards the ocean. The cold current creates the desert conditions of the shore of Namibia, and the persistent fogs of the Skeleton Coast.