英语听力文摘 English Digest 216、云朵是怎样漂浮的

　　How Clouds Float
形成云彩的暖湿上升气流使得云彩能够漂浮在空中。一片普通云彩的含水量约为200只成年公象的体重之和。但云彩中的水分被分解为细微的水滴和冰晶，因而不会倾盆而下。即使是最大的水滴，其半径也只有0.1毫米。如此大小的水滴如果想要落到地面，则需要面对强大的空气阻力，其下降的最大速度也只能达到30cm/s，对于漂浮的云彩来说，向下的重力与上升气流向上的力达到了平衡。只有许多水滴结合在一起，体积足够大的时候，才会形成降雨。而究竟多大的上升气流才能让云彩漂浮在空中呢？这取决于云彩的类型。平直广阔的层状云由每秒上升几厘米的微弱气流形成并支撑。形成大雨和暴风雨的积雨云中含有的水滴体积较大，需要速度高达几米每秒的上升气流来托住它们。

　　Learn how clouds float on this Moment of Science.
One thing we at A Moment of Science love is having the opportunity to explain the science behind things you see every day in the world around you. For example, if you happen to be driving or standing¹ near a window, take a look up at the sky. See any clouds? If so, ask yourself this: how do clouds stay up in the air?
It’s a good question, and here’s why. Clouds are made of water droplets²（小水滴） and ice crystals（结晶体）. Together, these droplets and crystals weigh many tons. So if the typical cloud actually weighs as much as an elephant, why doesn’t it （1）plummet（坠入） to the ground?
Because unlike an elephant, a cloud’s weight is spread out over a very large area. Plus, the cloud’s droplets and crystals are very small–about one micron（微米）, or one-hundred-thousandth of an inch across. A cloud’s individual particles are so small, in fact, that warm air rising from the earth’s surface is able to keep them floating in the air. It’s similar to how dust （2）motes³（尘埃，微粒） swirl⁴ in a shaft（光线） of sunlight. Although the bits of dust are affected⁵ by gravity, even the gentlest air currents（气流） are enough to keep them dancing around in the air.
But clouds don’t stay up in the air forever, of course. When the warm air keeping clouds afloat（漂浮着的） cools, its water vapor⁶ （3）condenses（压缩） and adds to the cloud’s droplets. At a certain point the droplets become heavy enough to （4）overwhelm（压倒） the force of the rising air, and all that water falls to the ground.
注释：
（1）plummet['pl?mit]
n.
1.坠子;测铅
2.重压
vi
垂直落下; 骤然失落
The damaged aircraft plummeted⁷ down to earth.
损坏的飞机垂直坠落于地上。
（2）mote[m?ut]
n.
尘埃;微粒[C]
thick as motes in the sunbeam
像阳光中的微尘那样密集
（3）condense[k?n'dens]
vt.
1.压缩;浓缩
condensed milk
炼乳
condensed soup
浓汤
Milk becomes condensed when water is taken out of it.
去除其中的水分,牛奶便成炼乳。
2.聚集(光线)
3.缩短,减缩(文章等)
He condensed his report from 2,000 words to 1,000.
他将报告从两千字压缩到一千字。
4.使冷凝,使凝结
vi.
1.(气体)冷却成液体(或固体)
Moisture in the atmosphere condensed into dew during the night.
大气中的水汽在夜里凝成了露水。
2.浓缩;凝结
（4）overwhelm[,?uv?'hwelm]
vt.
1.战胜;征服;压倒
The defense⁸ was overwhelmed by superior numbers.
防守被优势的兵力摧垮了。
2.覆盖;淹没
The village was overwhelmed by ash from the volcano.
村子被火山灰覆盖。
3.使受不了;使不知所措[H][(+by/with)]
I was overwhelmed by his generosity⁹.
他的慷慨令我感激难言。

1 standing   

n.持续，地位；adj.永久的，不动的，直立的，不流动的

参考例句：

• After the earthquake only a few houses were left standing.地震过后只有几幢房屋还立着。
• They're standing out against any change in the law.他们坚决反对对法律做任何修改。

2 droplets   

n.小滴( droplet的名词复数 )

参考例句：

• Droplets of sweat were welling up on his forehead. 他额头上冒出了滴滴汗珠。 来自辞典例句
• In constrast, exhaled smoke contains relatively large water droplets and appears white. 相反，从人嘴里呼出的烟则包含相当大的水滴，所以呈白色。 来自辞典例句

3 motes   

n.尘埃( mote的名词复数 )；斑点

参考例句：

• In those warm beams the motes kept dancing up and down. 只见温暖的光芒里面，微细的灰尘在上下飞扬。 来自辞典例句
• So I decided to take lots of grammar motes in every class. 因此我决定每堂课多做些语法笔记。 来自互联网

4 swirl   

v.(使)打漩，(使)涡卷；n.漩涡，螺旋形

参考例句：

• The car raced roughly along in a swirl of pink dust.汽车在一股粉红色尘土的漩涡中颠簸着快速前进。
• You could lie up there,watching the flakes swirl past.你可以躺在那儿，看着雪花飘飘。

5 affected   

adj.不自然的，假装的

参考例句：

• She showed an affected interest in our subject.她假装对我们的课题感到兴趣。
• His manners are affected.他的态度不自然。

6 vapor   

n.蒸汽，雾气

参考例句：

• The cold wind condenses vapor into rain.冷风使水蒸气凝结成雨。
• This new machine sometimes transpires a lot of hot vapor.这部机器有时排出大量的热气。

7 plummeted   

v.垂直落下，骤然跌落( plummet的过去式和过去分词 )

参考例句：

• Share prices plummeted to an all-time low. 股票价格暴跌到历史最低点。
• A plane plummeted to earth. 一架飞机一头栽向地面。 来自《简明英汉词典》

8 defense   

n.防御，保卫；[pl.]防务工事；辩护，答辩

参考例句：

• The accused has the right to defense.被告人有权获得辩护。
• The war has impacted the area with military and defense workers.战争使那个地区挤满了军队和防御工程人员。

9 generosity   

n.大度，慷慨，慷慨的行为

参考例句：

• We should match their generosity with our own.我们应该像他们一样慷慨大方。
• We adore them for their generosity.我们钦佩他们的慷慨。