Depending on the definition of 'desert' used, deserts occupy between
one-third and one-fifth of the earth's land surface.
One definition is for an areas that receives less than 250 mm (10 inches)
of average annual rainfall. Some places in the Atacama have no recorded
rainfall between 1570 and 1971. When it does rain, the total annual
amount often comes as one sudden brief torrential storm. In the Atacama,
the rainfall is supplemented along the coast by regular daily and sometimes
heavy fogs, especially where rivers, carrying hot water, meet the cold waters
of the polar Humboldt Current. An example of this is found at the Parque
Nacional Fray Jorge, where the Rio Lamari empties into the Pacific Ocean
giving rise to a lush temperate rainforest.
Climatologists add a further criterion to this definition, based on the
rate of evaporation and the derived 'Moisture Index'.
Temperature is of less significance: the Antarctic, with winter mean
temperatures of - 30o C and only brief periods in summer where the
temperature may rise to 5o C is a true desert. On the other
extreme, temperatures in the Lybian Sahara and in Death Valley in the Mojave
Desert reach a sizzling 58o C, with soil surface temperatures
recorded as high as 80o C.
Hot Deserts, and this includes the Atacama, experience large temperature
differences between night and day, due to the lack of clouds that provide an
insulating mantle when the sun has gone down. The Atacama, with an
average annual daytime temperature between 16 and 20o C, is one of
the cooler 'hot deserts'.
In contrast to popular believe, sand is not the common surface layer in
deserts. Most often small rocks, pebbles and loose gravel make up this
layer, or occasionally just bare rock. Desert landscapes have
traditionally been divided into five types:
-
sand
-
stony
-
rock
-
plateau
-
mountain
