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Look at the earth - isn't it beautiful? Blue oceans, white polar ice-caps,
and verdant green land. Some brown as well, where there aren't as many plants.
But why are
those parts of our earth barren, and others green? I mean, deserts cover much of Africa and Australia,
but not Europe!
Europeans, it turns out, discovered the answer, but in the most unlikely of places
- the oceans! When sailing from Europe, trade winds pushed ships enthusiastically southwest
to the equator, where suddenly, the winds would die. These were the doldrums. And for
sailors, they were a pain.
They were also annoying to scientists like Galileo, Kepler, and Halley,who all had
theories about why the wind blew to the southwest: Did the wind somehow follow the sun from morning
til night? Or did it have trouble keeping up with the ground spinning beneath it?
In 1735, a London lawyer and amateur meteorologist named George Hadley
came up with an even brighter idea that ultimately helps explain not just ocean winds, but also
why our planet has rainforests in a belt near the equator and deserts just north and south
Hadley figured that, since the sun warms the Earth
most at the equator, air to the north and south
must be cooler--and therefore more dense. Just as cold air rushes in through an open
door in winter, cool air north and south of the equator must flow toward the warm air
in the middle, bringing sailors with it.
There, in the doldrums, the air didn't actually stop moving, it just
headed upwards, heat rising to make way for the denser air flowing in from both sides.
And here's where the earth's greens and browns come in:
As warm, humid air at the equator rises, it cools, and--since cool air can't
hold as much moisture as warm air--it rains. A lot. Enough to make rain-forests.
At an altitude of about 17 kilometers, the rising (and drying) air hits the stratosphere,
which acts kind of like a ceiling, causing the warm air to spread out and separate -- some
goes north, some south. As the air departs from the equator, it rains away more moisture,
becoming denser and slightly
cooler, until finally dry, it sinks, creating the arid bands where many of the world's
famous deserts lie.
This giant atmospheric conveyor belt, officially called a Hadley cell, brings
us both tropical rainforests and deserts. Of course, neither rain nor sand nor scientific
understanding stopped European adventurers and traders from traipsing across the globe,
bringing disease with them and treasures back home - but that's a story for another day.


Why Does Earth Have Deserts?

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VoiceTube 2013 年 4 月 24 日 に公開
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