It rains only three years. |
期间只有三年下过雨 |
- That's incredible. - Exactly. |
-真是难以置信-的确如此 |
So less than one millimetre a year. |
难怪年平均降雨量会低于一毫米 |
On average, |
平均来说 |
it rains three one-hundredths of an inch a year. |
这里每年的降雨量仅为0.76毫米 |
It would take a century |
依照阿塔卡玛的降雨量 |
for Atacama's rainfall |
要灌满一个咖啡杯的雨水 |
to fill a coffee cup. |
需要花上一百年的时间 |
How does this compare with other deserts? |
那么与其他沙漠相比又如何呢 |
Here we have a cylinder, |
这是个量筒 |
and I'm going to show you the difference |
我用它来对比一下 |
between the amount of rainfall per annum here |
这里的年降雨量 |
and the amount of rainfall in other deserts. |
和其他沙漠的年降雨量 |
So if I fill this jar up, |
若我灌满量筒 |
right up to about there, |
大概灌到这个位置 |
that is roughly the rainfall that you get in the Sahara. |
这差不多相当于撒哈拉沙漠的年降雨量 |
Now if I pour most of that away, |
现在我把水倒掉一大半 |
we get to that level, |
差不多这个量 |
|
莫哈维沙漠:美国最大的沙漠 |
that represents what we have in the Mojave Desert, |
这便是莫哈维沙漠的年降雨量 |
five inches per annum. |
每年12.7厘米 |
If I pour all that away, |
若我把水全部倒光 |
except for that little drop in the bottom there, |
只剩下杯底的这几滴水 |
and that's the equivalent of what we have here |
这便是阿塔卡玛沙漠 |
in the heart of the Atacama Desert. |
中心地带的降雨量 |
That is such a small amount of rainfall |
而这微乎其微的降雨量 |
that it means it's the driest place on Earth. |
意味着这里是地球最旱之地 |
In his quest to find out why Atacama gets so little rainfall, |
为了探寻阿塔卡玛的降雨量如此之少的原因 |
Houston leaves the oasis behind |
休斯顿离开了这片绿洲 |
and heads into the desert. |
继续向沙漠前行 |
By the side of the Pan-American highway, |
在泛美高速公路 |
a road which runs the length of the continent, |
这条纵贯整个美洲大陆的公路的路边 |
he discovers the first clue. |
他发现了第一条线索 |
|
南回归线 南纬23度26分37秒 |
Well, here we are at the Tropic of Capricorn. |
我们现在位于南回归线上 |
This is one of the most important latitudes in the world |
这是世界上最重要的纬度之一 |
and it is absolutely critical in explaining |
它可以告诉我们 |
why the Atacama Desert is in this location here. |
为什么阿塔卡玛沙漠形成于此处 |
Most of the world's deserts |
世界上大多数的沙漠 |
straddle one of two special latitudes. |
都横跨南或北回归线 |
In the southern hemisphere, |
在南半球 |
|
北回归线赤道 南回归线 |
the Tropic of Capricorn runs through Atacama |
南回归线横穿阿塔卡玛沙漠 |
and Africa's Namib and Kalahari Deserts. |
非洲的纳米布沙漠和喀拉哈里沙漠 |
In the north, |
在北半球 |
完整版请点击 |
|
the Tropic of Cancer runs right |
北回归线正好穿越 |
through the vast Sahara. |
广阔无垠的撒哈拉沙漠 |
At these particular positions on the planet, |
在这些区域内 |
the air is extremely dry. |
空气异常干燥 |
This instrument is called a whirling hygrometer. |
这仪器叫做旋转式湿度计 |
What this does is to measure the relative humidity of the air. |
可以用来测量空气中的相对湿度 |
And the reading on here |
读数显示 |
gives us a relative humidity of 10%. |
这里的相对湿度是10% |
That's really low, really low. |
这实在是太低太低了 |
Um, there aren't many places in the world |
世界上没几个地方 |
where you'd get a relative humidity as low as that. |
能测到如此低的相对湿度 |
Back in the early 1700s, |
18世纪初 |
scientists discovered |
科学家们发现了 |
why tropical air is so dry. |
热带气团如此干燥的原因 |
European ships sailing to America |
驶往美洲的欧洲舰船 |
relied upon the trade winds |
借助信风 |
to power their crossings, |
才得以横越大洋 |
but English meteorologist George Hadley was mystified |
但英国气象学家乔治·哈德利很是费解 |
why they blew westward |
为何该吹北信风的时候 |
when they should blow directly north. |
却吹的是西信风 |
His studies would lead scientists |
他的研究引领着科学家们 |
to understand how air circulates around the Earth. |
领会地球上的空气是如何循环流通的 |
At the equator, |
在赤道地区 |
moisture-rich air gets heated by the sun and rises. |
饱含水分的空气受热后上升 |
As this hot, |
随着向赤道两侧 |
wet air flows away from the equator, |
不断流动的过程中 |
it quickly sheds its water as rain. |
这潮湿的热空气形成降雨来迅速释放水分 |
By the time it reaches the two tropic latitudes, |
等它到达南北回归线的时候 |
the air has lost nearly all of its moisture, |
空气几乎已释放光所有的水分 |
resulting in no rain on the land below. |
以至于无法再形成雨水降落到地面 |
The mystery, though, |
但不解之谜是 |
is why Atacama gets so much |
为什么阿塔卡玛沙漠的降雨量 |
less rain than anyplace else. |
远远小于其他任何地方 |
Scientists hope to crack the case |
科学家们希望通过找出阿塔卡玛沙漠 |
by figuring out how Atacama first formed. |
最初是如何形成的来破解这一谜团 |
On the hunt for clues, |
为了寻找线索 |
Houston travels deep into the true desert. |
休斯顿前往沙漠深处 |
This closely guarded location |
这个严谨保护着的地点 |
was discovered during routine mapping |
是地质学家们在上世纪70年代 |
by geologists back in the '70s, |
常规绘图时发现的 |
but the huge significance of their find |
但直到1998年 |
wasn't realised until 1998. |
他们才意识到这个发现的重要意义 |
This band of boulders is |
这片卵石带是 |
the single most important clue to Atacama's beginnings. |
有关阿塔卡玛沙漠起源的最重要线索 |
It's a delicate rock called gypsum. |
这是一种易碎的岩石名为石膏 |
A simple test shows how fragile it is. |
一个简单的实验就能证明它有多易碎 |
If I pour a little bit of water on top of that, |
如果我往它上面浇一点水 |
you will see that it very rapidly falls apart. |
就可以看到它迅速破碎 |
What's happening here, of course, |
这是为什么呢很显然 |
is that when I'm putting water on this, |
当我往它上面浇水时 |
you see it dissolve, |
它便溶解了 |
I mean, it's just going to fall apart. |
就这样分裂破碎了 |
The survival of gypsum as a solid rock |
这些仍呈固体状的石膏 |
tells scientists |
告诉科学家们 |
there hasn't been any heavy rain |
自从这些岩石形成之后 |
since the rock formed. |
这里就没下过大雨 |