英語聽力文摘 云朵是怎樣漂浮的

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How Clouds Float

形成云彩的暖濕上升氣流使得云彩能夠漂浮在空中。一片普通云彩的含水量約為200只成年公象的體重之和。但云彩中的水分被分解為細(xì)微的水滴和冰晶，因而不會(huì)傾盆而下。即使是最大的水滴，其半徑也只有0.1毫米。如此大小的水滴如果想要落到地面，則需要面對(duì)強(qiáng)大的空氣阻力，其下降的最大速度也只能達(dá)到30cm/s，對(duì)于漂浮的云彩來說，向下的重力與上升氣流向上的力達(dá)到了平衡。只有許多水滴結(jié)合在一起，體積足夠大的時(shí)候，才會(huì)形成降雨。而究竟多大的上升氣流才能讓云彩漂浮在空中呢?這取決于云彩的類型。平直廣闊的層狀云由每秒上升幾厘米的微弱氣流形成并支撐。形成大雨和暴風(fēng)雨的積雨云中含有的水滴體積較大，需要速度高達(dá)幾米每秒的上升氣流來托住它們。

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 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(結(jié)晶體). 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(墜入) 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(塵埃，微粒) swirl in a shaft(光線) of sunlight. Although the bits of dust are affected by gravity, even the gentlest air currents(氣流) 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(漂浮著的) cools, its water vapor (3)condenses(壓縮) and adds to the cloud’s droplets. At a certain point the droplets become heavy enough to (4)overwhelm(壓倒) the force of the rising air, and all that water falls to the ground.

注釋：

(1)plummet['pl?mit]

n.

1.墜子;測(cè)鉛

2.重壓

vi

垂直落下; 驟然失落

The damaged aircraft plummeted down to earth.

損壞的飛機(jī)垂直墜落于地上。

(2)mote[m?ut]

n.

塵埃;微粒[C]

thick as motes in the sunbeam

像陽光中的微塵那樣密集

(3)condense[k?n'dens]

vt.

1.壓縮;濃縮

condensed milk

煉乳

condensed soup

濃湯

Milk becomes condensed when water is taken out of it.

去除其中的水分,牛奶便成煉乳。

2.聚集(光線)

3.縮短,減縮(文章等)

He condensed his report from 2,000 words to 1,000.

他將報(bào)告從兩千字壓縮到一千字。

4.使冷凝,使凝結(jié)

vi.

1.(氣體)冷卻成液體(或固體)

Moisture in the atmosphere condensed into dew during the night.

大氣中的水汽在夜里凝成了露水。

2.濃縮;凝結(jié)

(4)overwhelm[,?uv?'hwelm]

vt.

1.戰(zhàn)勝;征服;壓倒

The defense was overwhelmed by superior numbers.

防守被優(yōu)勢(shì)的兵力摧垮了。

2.覆蓋;淹沒

The village was overwhelmed by ash from the volcano.

村子被火山灰覆蓋。

3.使受不了;使不知所措[H][(+by/with)]

I was overwhelmed by his generosity.

他的慷慨令我感激難言。