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The Earth's land surface is divided
into zones or biomes. These biomes are designated according
to climate and other physical characteristics. Each biome
has a distinctive combination of life forms that are able
to thrive in the particular conditions found there. Desert
is one biome. Others include: tundra, boreal forest, tropical
rainforest, savanna and mountain.
A desert is defined as an area that receives 25 cm of rain
or less a year. Natural deserts are part of the world's arid
zones. Arid means that the land is dry and barren. These lands,
together with the semi-arid lands that surround them, cover
one-third of the Earth's surface. Some deserts form in large
land masses in the interior of a country. This is because
after the wind has traveled a great distance inland from the
coast, it has lost all its moisture. Other deserts form on
the far side of mountain ranges because there is no rain left
in the clouds once they reach the interior of the mountain
range.
With little rain and extreme temperatures (very hot and very
cold), deserts can be the driest places in the world. Deserts
occur naturally all over the globe, but most are found near
the Tropic of Cancer and the Tropic of Capricorn. This is where the sun is very strong
and wind patterns produce many cloudless days. Without clouds,
there is nothing to shield the ground from the sun. During
the summer, the ground can bake from the heat. The desert
floor also becomes very cold at night when the heat is lost
back to the atmosphere. Temperatures in the desert range from
25ºC in the shade to the low 40ºC range in the sun. (There
are also cold deserts like the Antarctic, which are covered
with snow and ice all of the time.)
Deserts are often portrayed as vast
areas of blowing sand. This isn't always the case. There are
also deserts made of gravel, bare rock and boulders. Desert
sand starts out as rock, but over time, it is weathered down
to form fine particles of sand. Various sand dunes form as
a result of the blowing winds, creating different shapes such
as ridges, stars and crescents.
There are many living things that survive in the desert including
human beings, animals and plants. They have learned to adapt
to the harsh climate. For example, the hard outer shell of
a scorpion helps reduce water loss. And when the sun becomes
too hot, the Kalahari desert ground squirrel uses its tail
like an umbrella to protect it from the hot sun.
The camel, an animal that is often associated with the desert,
has many physical adaptations that allow it to survive and
act as a mode of transportation for humans living in the desert.
Camels have few sweat glands and are able to conserve water
because their body temperature can rise several degrees before
they start to sweat. They can also survive for long periods
without food by using the fat stored in their humps.
Desert plants, such as cacti, are able to store water more
easily because of their thick, waxy outer layer that protects
the plant from the intense sun. Some plants have long roots
to reach water located deep in the soil; others spread their
roots to gather water over a wide area, and some absorb dew
through their leaves.
People, too, have learned to live in the desert. Some are
nomads, which means they move from place to place when their
livestock need new pasture. Others live in permanent dwellings
such as mud huts, well adapted to local conditions.
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