新托福阅读：How Clouds Float

云彩漂浮在空中的小秘密

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。

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 standing 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 droplets(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)motes(尘埃，微粒) swirl in a shaft(光线) of sunlight. Although the bits of dust are affected 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 vapor (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['plʌmit]

n.

1.坠子;测铅

2.重压

vi

垂直落下; 骤然失落

The damaged aircraft plummeted down to earth.

损坏的飞机垂直坠落于地上。

(2)mote[məut]

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[kən'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[,əuvə'hwelm]

vt.

1.战胜;征服;压倒

The defense 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 generosity.

他的慷慨令我感激难言。