【英语时差8,16】浮云

 Fact: liquid water is heavier than air. Fact: clouds, which consist of water droplets² and ice crystals, seem to hover³ in the air. One final fact: the water droplets in a typical cloud together weigh many tons. Which raises a question: how do clouds stay up in the sky?  Clouds form when warm, moist air rises to a greater altitude, causing it to expand and cool. Eventually, water vapor⁴ in the cooled air condenses into the droplets that form clouds. The condensed droplets are on average roughly eight-one-hundred- thousandth of an inch across. In other words, a single droplet¹ is very, very small.

Despite their tiny size, the droplets still have mass and are therefore affected⁵ by gravity. They are so small, however, that rising air keeps the water droplets aloft. The effect is similar to how dust appears to float in a shaft⁶ of sunlight: although gravity works on the bits of dust, even the gentlest currents are enough to keep them swirling⁷ around in the air.

Clouds, then, are not so much able to float as they are held up by surrounding air. In fact, the tiny cloud droplets do sink, but so slowly that it would take an hour for a single droplet to sink only one yard. And although the cumulative⁸ weight of a cloud's droplets is many times that of the surrounding air, the droplets are spread out over a large enough area to render the gravitational pull on any single drop almost negligible. When the droplets become larger, gravity overcomes the dragging forces of the rising air and the droplets fall to the earth as rain.   

1 droplet   

n.小滴，飞沫

参考例句：

• The rate of droplet growth under different conditions can be evaluated.可以计算在不同条件下的云滴增长率。
• The test results showed that increasing droplet size was associated with better stability.试验结果表明，增加液滴尺寸将使稳定性提高。

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 hover   

vi.翱翔，盘旋；徘徊；彷徨，犹豫

参考例句：

• You don't hover round the table.你不要围着桌子走来走去。
• A plane is hover on our house.有一架飞机在我们的房子上盘旋。

4 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。

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 shaft   

n.(工具的)柄，杆状物

参考例句：

• He was wounded by a shaft.他被箭击中受伤。
• This is the shaft of a steam engine.这是一个蒸汽机主轴。

7 swirling   

v.旋转，打旋( swirl的现在分词 )

参考例句：

• Snowflakes were swirling in the air. 天空飘洒着雪花。 来自《现代汉英综合大词典》
• She smiled, swirling the wine in her glass. 她微笑着，旋动着杯子里的葡萄酒。 来自辞典例句

8 cumulative   

adj.累积的，渐增的

参考例句：

• This drug has a cumulative effect.这种药有渐增的效力。
• The benefits from eating fish are cumulative.吃鱼的好处要长期才能显现。