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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.

Look at the earth - isn't it beautiful? Blue oceans, white polar ice-caps,

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為什麼地球上會有沙漠? Why Does Earth Have Deserts?

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