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Author Topic: Why is there no water in the desert?  (Read 13873 times)

Offline thedoc

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Why is there no water in the desert?
« on: 14/06/2012 22:30:01 »
Jon Kelling  asked the Naked Scientists:
Hi Chris,

My son would like to know:

Why is there little or no water in the desert?

He's 5, his name is Landon, and he would get a kick of hearing his
name on the air :)



P.S. love the show! Thank you!

What do you think?
« Last Edit: 14/06/2012 22:30:01 by _system »


Offline Lmnre

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Re: Why is there no water in the desert?
« Reply #1 on: 17/06/2012 15:13:19 »
If Landon keeps two potted plants indoors, and he regularly waters one, but not the other, he’ll see how not enough “rain” turns the ground into a “desert” where plants won’t grow. So, the short answer to Landon's question is that it doesn't rain much on deserts (because, as you'll find out below, the rains falls on the jungles instead).

Now for the longer answer. Air moves in patterns. One set of patterns is called "cells" — namely, the Hadley cells between the equator and latitudes 30°, the Mid-latitude cells between latitudes 30° and 60°, and the Polar cells between latitudes 60° and the poles. The lower right part of the diagram below shows how the air in those cells rotate like conveyor belts . 

The intense sunlight at the equator warms the air and causes it to rise up, which causes the water to  come out of the air as rain. That’s why there’s jungles at the equator — because of plenty of sunlight and rain, and plants grow well under those conditions. The air then moves away from the equator. By the time the air gets to about 30° latitude (north and south), it has very little water, and so, it doesn’t rain much at those latitudes. The sunlight there is also intense, but there’s very little rain. That’s why there’s deserts at the 30° latitudes — because of lots of sunlight but very little rain.

The air at about 60° latitude north and south does the same thing as the air at the equator — it rises up, drops its water as rain, and moves away (towards 30° latitude and the poles). When it gets to the 30° latitudes and the poles, it also doesn’t have much water either. The world map below shows that these air movements create deserts at the 30° latitudes.


Offline CliffordK

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Re: Why is there no water in the desert?
« Reply #2 on: 17/06/2012 20:29:59 »
The soil in some deserts can also be very porous so moisture is not effectively trapped on the surface, which leads to less availability of the moisture to plants, and less evaporation and formation of new rain.

I've been in rain storms in Nevada.  10 minutes after it stops raining, it is hard to tell that it had recently rained.

Offline evan_au

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Re: Why is there no water in the desert?
« Reply #3 on: 21/06/2012 12:06:32 »
Some of the most surprising deserts are near the North and South poles.
Because it is very cold there, there is not much moisture in the air, and so you get very little precipitation (which we call rain in warmer climates or snowfall in colder climates).

This technically makes them deserts, even though there may be large amounts of frozen water there, representing the slow accumulation of snow over many years.

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Re: Why is there no water in the desert?
« Reply #3 on: 21/06/2012 12:06:32 »


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