Desert Lands

As big as the earth is, over two thirds of its surface is covered in water from the oceans. The remaining one-third of the earth's surface is exposed as dry land for us to live on, but a third of that dry land is really dry. In fact, it's inhospitable desert. Much of the deserts in the world are clustered between 5 to 30 degrees north and south of the equator, in what are called subtropical zones. Scientists have theorized that these desert belts are due to two things:

1) Heat (read more about the Sun)

2) Lack of moisture

Duh? Anybody who's ever been outside on a hot summer day, all day, knows that. Just about every continent on earth that is inhabited by humans experiences seasonal weather changes, with a distinct winter and summer. Just because there's hot, dry weather during the summer, doesn't mean that where you live is going to turn into a desert. What makes the desert so hot and dry is the climactic conditions that are sustained almost continually, year round. Any part of the world that's hot and dry for long enough periods throughout the year won't be able to support much plant or animal life. Living things need water to survive.

Why is it so Dry All the Time?

First, the air in the earth's atmosphere is warmest around the equator (because that part of the earth is closer to the sun) so that warmer air rises and flows north and south of the equator. As the air "piles" up in the northern and southern latitudes, these zones of "piled-high" warm air become permanent high pressure zones. As the air at the "bottom of the pile" descends toward the earth it gets warmed up even more. Because this descending warm air has no clouds (i.e., condensing water vapor), that allows the burning sun to go right through the air and heat the land mass below even more. Hence, extreme heat.

Warm air can hold a lot more moisture (water vapor) than colder air. Unless this really warm air contacts some much cooler air (or cooler land mass), there's nothing to coax the moisture out of the air in the form of precipitation (rain, fog). Hence, lack of moisture.

What Goes Around, Comes Around

This hot air moves northward and southward of the equator, almost continuously in the form of reliable winds called the Trade Winds. As these warm winds circulate back around towards the equator they rise into the upper atmosphere again, cooling. The water vapor in the cooling air mass condenses and rains, and rains and rains all over the equator in the Tropical zones. All this rain makes the land mass around the equator the lushest, wettest, most densely forested in the world (plants love water!). It's ironic that the wettest and hottest places in the world occur within just a few thousand miles of each other.

Though the hottest place in the world, El Azizia, is a desert, not all deserts are hot. Antarctica, for example, is the driest continent on earth, getting less than 4 in/10cm of precipitation a year. What characterizes or defines a desert is the lack of precipitation - less than 10 in/25.4cm per year. In the Antarctic, there is very little precipitation in the form of rain or snow. Even though there's water, water everywhere it's locked up in the form of ice.

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