英语听力文摘 English Digest 276、飞机的凝迹

　　Airplane Contrails
Have you ever seen a perfectly¹ cloudless sky?  Cloudless–that is–except for two or three long white lines smeared² across the perfect blue sky by airplanes like smudges on a pane³ of glass.  Why do airplanes sometimes leave contrails behind? Actually, contrails, short for condensation⁴ trails, are essentially⁵ clouds.  They are no different from the cirrus clouds（卷云） that naturally form in that region of the atmosphere.
High in the atmosphere, water can exist in one of two basic forms.  It can either be invisible water vapor⁶–a gas–or it can condense around microscopic⁷ dust particles into tiny droplets⁸ or ice crystals（冰晶）.  Every cloud you see is made of tiny droplets or crystals like this, and so is a contrail.

　　To make an airplane contrail, you need water and some kind of microscopic particle for the water to condense around.  When jet fuel（喷气燃料） burns, it releases both water vapor and microscopic particles（微观粒子） of exhaust.  A contrail occurs when the jet’s water vapor condenses around its own exhaust particles.
If you see a plane that’s leaving no contrail–or a very short one–this means the upper atmosphere is relatively⁹ dry.  Dry air reabsorbs the water as invisible vapor almost as soon as it condenses.  On the other hand, if you see a contrail that lasts a long time, this means the upper atmosphere is already (1)saturated with as much water vapor as it can hold.
Because of this, you sometimes can use contrails to predict the weather.  Quickly fading ones suggest a dry upper atmosphere and fair weather.  If you see a lot of long contrails in the sunset though, you might carry an umbrella the next day!
Notes:
(1)saturate¹⁰ with v.使饱和
If a medium other than water is to be used, saturate it with oxygen in a similar manner.
若使用的介质不是水也用类似方法以氧饱和之。

1 perfectly   

adv.完美地，无可非议地，彻底地

参考例句：

• The witnesses were each perfectly certain of what they said.证人们个个对自己所说的话十分肯定。
• Everything that we're doing is all perfectly above board.我们做的每件事情都是光明正大的。

2 smeared   

弄脏； 玷污； 涂抹； 擦上

参考例句：

• The children had smeared mud on the walls. 那几个孩子往墙上抹了泥巴。
• A few words were smeared. 有写字被涂模糊了。

3 pane   

n.窗格玻璃，长方块

参考例句：

• He broke this pane of glass.他打破了这块窗玻璃。
• Their breath bloomed the frosty pane.他们呼出的水气，在冰冷的窗玻璃上形成一层雾。

4 condensation   

n.压缩，浓缩；凝结的水珠

参考例句：

• A cloud is a condensation of water vapour in the atmosphere.云是由大气中的水蒸气凝结成的。
• He used his sleeve to wipe the condensation off the glass.他用袖子擦掉玻璃上凝结的水珠。

5 essentially   

adv.本质上，实质上，基本上

参考例句：

• Really great men are essentially modest.真正的伟人大都很谦虚。
• She is an essentially selfish person.她本质上是个自私自利的人。

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 microscopic   

adj.微小的，细微的，极小的，显微的

参考例句：

• It's impossible to read his microscopic handwriting.不可能看清他那极小的书写字迹。
• A plant's lungs are the microscopic pores in its leaves.植物的肺就是其叶片上微细的气孔。

8 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

9 relatively   

adv.比较...地，相对地

参考例句：

• The rabbit is a relatively recent introduction in Australia.兔子是相对较新引入澳大利亚的物种。
• The operation was relatively painless.手术相对来说不痛。

10 saturate   

vt.使湿透，浸透；使充满，使饱和

参考例句：

• We'll saturate California with the rise in its crime rate.我们将使加利福尼亚州的犯罪案件增长率达到饱和点。
• Saturate the meat in the mixture of oil and herbs.把肉浸泡在油和作料的卤汁里。