How dry does it get?
Q: What is the driest desert on Earth?
A:  That's debatable. The Atacama of northern Chile is the driest, declares the United States Geological Survey. The
Encyclopaedia Britannica casts its vote , instead, for a low spot in the Lut Desert of eastern Iran where the summer heat
and low humidity are "unsurpassed".
Figure 1. [USGS] The Atacama Desert.
Either place is almost unbelievably dry. The precipitation (moisture equivalent to rain) in Atacama averages less than a
half inch (1 centimeter) per year from fog. Measurable rainfall (more than a millimeter of rain) occurs every five to 20
years and heavy rains fall only two to four times a century. No vegetation grows here.
Mountains to the west and east sandwich the length of the 700-mile cool desert. Summer temperatures average 65 degrees Fahrenheit. Weatherworn mountains,
grey with dust layered by winds sweeping the desert, crouch along the coast. The desert begins here and stretches towards the Andes like an enormous salt pan,
devoid of life-not even an insect. You could almost be on Mars.
A cold sea current, the Humboldt, creates the desert. The great mass of frigid water, surges out of the Antarctica 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 landward.
The warm air chills as it scrapes 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 an area as hostile to life as any place on our planet. In the winter, fog rises from the upwelling cold currents, blankets the desert, and gives
moisture to the land.
 Our other candidate for the driest desert on Earth, the Lut, is not a coastal desert but rather lies in the
northern of two deserts belts which circle our globe. The Sun heats these belts of land more than any other
part of Earth and creates deserts.
Figure 2. [J.T. Daniels, Goddard NASA] A dune field in the Lut Desert
The Lut is so forbidding that not even bacteria can live. Research groups bring sterilized milk into the Lut
and then store it uncovered in temperatures that can exceed 160 degrees Fahrenheit in the shade. The milk
remains sterile.
The Lut Desert consists of several large basins separated by worn mountains and ridges, covering an area
of about 200 by 100 miles. The west desert contains wind-swept corridors separating high ridges. The east
is a sea of sand. Winds pile the sand into dunes up to 500 feet high, as tall as Washington's monument.
Alfons Gabriel, one of the first explorers in 1938 describes the sea of sand as a "confused mass of impassable tangled dunes."
(Answered by April Holladay, science correspondent, May 9, 2001)
Further Surfing:
USGS: Deserts-Geology and Resources
Carnegie Mellon U, Atacama Desert trek of Mars rover
NASA: Geomorphology from space
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