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为何地球上有沙漠?

Why Does Earth Have Deserts?

看看这个地球,它是不是很美丽?蓝色海洋,白色的极地冰川以及
Look at the earth – isn’t it beautiful? Blue oceans, white polar ice-caps, and verdant
绿色的土地。也有一些棕色地区没有很多植被。但是为什么我们地球上那些部分
green land. Some brown as well, where there aren’t as many plants. But why are those parts
很贫瘠,其他地方为绿地?我的意思是,沙漠覆盖了大片的非洲和澳洲,
of our earth barren, and others green? I mean, deserts cover much of Africa and Australia,
但是没有覆盖欧洲!
but not Europe!
事实证明,欧洲人发现了答案,却在最不可能的地方―海洋!
Europeans, it turns out, discovered the answer, but in the most unlikely of places – the oceans!
当从欧洲开始航行,凭借风力从东南驶向赤道,
When sailing from Europe, trade winds pushed ships enthusiastically southwest to the equator,
到那儿时风力会忽然消失。那里是赤道无风带。对于水手而言,
where suddenly, the winds would die. These were the doldrums. And for sailors, they were
是很痛苦的事情。
a pain.
他们也常常困扰像伽利略,开普乐,哈雷这样的科学家,他们都有
They were also annoying to scientists like Galileo, Kepler, and Halley, who all had theories
关于为什么风往西南吹的理论。难道风向在某种程度上是跟随太阳从早
about why the wind blew to the southwest: Did the wind somehow follow the sun from morning
到晚的?或者可能是它没能跟上下面地球的自转?
til night? Or maybe it had trouble keeping up with the ground spinning beneath it?
在1735年,一个伦敦的律师和一个叫乔治.哈得利的业余气象爱好者提出了一个更为清楚的
In 1735, a London lawyer and amateur meteorologist named George Hadley came up with an even brighter
观点,最终不仅解释了海洋风也说明了为什么我们的星球上会呈现雨林
idea that ultimately helped explain not just ocean winds, but also why our planet has rainforests
带在赤道附近以及沙漠为什么仅出现在北部和南部。
in a belt near the equator and deserts just north and south of that.
哈得利指出,因为太阳照射地球最多的地方是赤道,北部及南部
Hadley figured that, since the sun warms the Earth most at the equator, air to the north
的空气就会相对凉爽,也更稠密一些。就像冬季冷空气涌入
and south must be cooler–and therefore more dense. Just as cold air rushes in through
打开的门内一样,赤道南北凉爽的空气定会向
an open door in winter, cool air north and south of the equator must flow toward the
中间温暖的气体涌动,同时带动船员一起。
warm air in the middle, bringing sailors with it.
因此,在赤道无风带,实际上空气不会停止移动,只是被抬高了而已,
There, in the doldrums, the air didn’t actually stop moving, it just headed upwards, heat
受热上升,为南北两边密集的空气流动让出道路。这儿就成了地球上
rising to make way for the denser air flowing in from both sides. And here’s where the earth’s
绿色和棕色聚集地。
greens and browns come in:
当温暖潮湿的气流上升,就会冷却,因为冷空气不能容纳
As warm, humid air at the equator rises, it cools, and–since cool air can’t hold as much
和暖空气一样多的水分,这时就会下很多雨足以形成雨林。
moisture as warm air–it rains. A lot. Enough to make rain-forests.
当海拔达到17千米后,上升的暖空气遇到平流层,
After reaching an altitude of about 17 kilometers, the rising (and drying) air hits the stratosphere,
就像遇到天花板,使暖空气分散,一些涌向
which acts kind of like a ceiling, causing the warm air to spread out and separate — some
北方,一些涌向南方。当空气离开赤道,就会伴随更多的降雨,
goes north, some south. As the air leaves the equator, it rains away more moisture,
空气由密集,冷却直到最后的干燥,下沉,造成不毛之地,
becoming denser and slightly cooler, until finally dry, it sinks, creating the arid bands
就是世界上许多有名的沙漠。
where many of the world’s famous deserts lie.
这个巨大的大气传送带,官方称为哈得利室,它给我们即带来了热带
This giant atmospheric conveyor belt, officially called a Hadley cell, brings us both tropical
雨林也带来了沙漠。所以地球上出现的绿色,棕色地带是
rainforests and deserts. So the locations of the greens and browns of the earth come
很多的热空气的杰作。
courtesy of a lot of hot air.

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https://www.youtube.com/watch?v=T6Us1sPXBfA

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