Types of Deserts

Background Information:

Deserts are usually formed by a combination of conditions, but there are four basic kinds.

Inland Deserts:

Winds that sweep in from the ocean are full of moisture.  As these wet winds move over land they rise, cool, and release the moisture as rain.  But some places are simply too far from a coast to reap the benefits of those moisture laden winds.  By the time they've swept across hundreds and hundreds of miles and reach the middle of a large land mass, nearly all the moisture has already fallen.  An example of an inland desert is the Gobi Desert in Mongolia.

Cool Fact:  The Gobi Desert can have winter temperatures as low as -40 degrees F.

High-Pressure Deserts:

Air circulates over the Earth in set patterns which cause zones of permanent high and low atmospheric pressure.  In low pressure zones- like those near the equator-warm, humid, air rises, forms clouds and rains.  This is why the tropics are so wet.  In contrast, most deserts aren't located under these low pressure zones.  Instead they are under the nearly cloudless skies of high pressure zones north and south of the equator, often near the tropic of Cancer and tropic of Capricorn.  When the rain-spent tropical air circulates and descends from the equatorial regions to these high pressure zones, it warms.  This dry warmed air evaporates what little moisture is present in the desert's atmosphere before it gets a chance to fall as rain.  So no rain is brought in, nor is it allowed to fall.  Examples of high pressure deserts on the African continent are the Sahara, under the northern high-pressure band, and the Kalahari, under the southern band.

Rain-Shadow Deserts:

A mountain range acts as a rain barrier. As air masses move from ocean to land, they're forced upwards as they strike a mountain range.  As the masses rise, moisture condenses into clouds and  falls as rain.  By the time winds cross over the far side of the mountain peaks, the rain is depleted, and a rain-shadow desert results on the other side.  The Patagonian Desert in South America is an example of one, as is the Great Basin Desert in the United States.

Fog Deserts:

Icy ocean currents move from the poles toward the tropics along the western coasts of continents.  But cold wind over cold water carries very little moisture, so deserts often form along these coasts.  And while rain is almost nonexistent, these deserts are cold and usually blanketed by fog caused by cold ocean currents colliding with hot winds blowing off the land.   The Atacama Desert in Chile, and the Namib Desert in southern Africa - both fog deserts - are two of the driest places on Earth!

Cool Fact:  The Atacama gets an average annual rainfall of less than 1/25th of an inch!

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