為什么會(huì)下雨？

又到梅雨季節(jié)，最近天天下雨，天氣已經(jīng)很久沒(méi)有放晴了，那么為什么會(huì)下雨呢？雨水是如何形成的呢？

【原文】

Plants need it to grow,animals need it to survive,and the human body needs it to stay hydrated and keep functioning.
植物靠它生長(zhǎng)，動(dòng)物賴(lài)以生存，人體需要它來(lái)保持水分，維持機(jī)體正常運(yùn)作。
Water is an essential building block to life and it comes to us from above.
水是生命的基本組成部分，它從天而降。
Rainwater fills streams,rivers,and lakes.
雨水注滿(mǎn)了小溪、河流和湖泊。
It carries with it the potential for life to flourish.
承載著讓生命蓬勃發(fā)展的潛力。
It can turn an arid desert in to an oasis; convert a stream to white water rapids in the monsoon season,and breathe life into the harshest places on earth.
它能把干旱的沙漠變成綠洲；在雨季把淺溪變成急流，給地球上最嚴(yán)酷的環(huán)境帶來(lái)生命。
So how do clouds form to keep giving us the essential H2O we need?
那么，云是如何形成并持續(xù)提供我們所需的水的呢？
That's what we'll find out today, in this episode of The Infographics Show: Why Does It Rain?
這是我們?cè)诮裉斓摹靶畔D表秀”要研究的：為什么會(huì)下雨？
Clouds are possibly the most interesting and beautiful of all weather phenomena.
云可能是我們見(jiàn)到的最有趣、最美麗的天氣現(xiàn)象了。
They form in a wide variety of shapes and sizes, but are all made of the same thing...water.
它們大小不一，形態(tài)各異，但都由水而來(lái)。
Though we may not notice it, almost all the air around us is moist because it contains water in the form of vapor.
雖然我們可能注意不到，我們周邊的空氣幾乎都是潮濕的，因?yàn)榭諝饫镉兴植⒁运魵獾男问酱嬖谥?br>You can't see the water vapor,because it's a gas,but it is still there.
由于水蒸氣是氣態(tài)的，所以我們看不到它，但它依然真實(shí)存在。
Water can be either solid as ice,liquid as water,or gas, which is a vapor.
水能以固態(tài)存在，比如冰；能以液態(tài)存在，比如水；或以氣態(tài)存在，那就是水蒸氣。
Vapor has no smell and it's invisible.
水蒸氣無(wú)色無(wú)味。
But you can feel it.
但我們?nèi)匀荒芨杏X(jué)到它。
Imagine a hot and sticky day in summer,or a cold foggy day in winter.
試想炎炎夏日里粘粘的空氣，亦或是寒冬里的冷氣 。
Those sensations are water vapor.
我們感覺(jué)到的這些都是水蒸氣。
If you put steam or vapor under the microscope, you would see millions of tiny water droplets floating in the air.
如果在顯微鏡下觀察水蒸氣，我們能看到數(shù)以百萬(wàn)計(jì)的小水滴漂浮在空氣里。
This is the same process that forms clouds - millions of tiny water droplets condensing out of the air to form liquid water.
這和云彩的形成過(guò)程相同-空氣中數(shù)以百萬(wàn)計(jì)的水滴遇冷凝結(jié)成液態(tài)水。
But why does water condense out of air and become visible as a cloud?
那為什么水能從空氣中凝結(jié)而成為可見(jiàn)的云呢？
Warm air holds water vapor better than cool air, so when warm air starts to cool,it can no longer hold as much of the vapor.
暖空氣中的水蒸氣比冷空氣中的更多，所以當(dāng)暖空氣的溫度降低時(shí)，就存不住那么多的水蒸氣了。
The extra water vapor has to go somewhere.
多余的水分不得不去到別處。
It condenses out as water.
所以就凝結(jié)成了水。
To see this in action,go outside and stare up at a cloud.
要想看到這個(gè)變化，就要去外邊盯住一朵云觀察。
Be patient but keep watching the cloud,especially it's edges.
有點(diǎn)耐心，一直注視著它，尤其是它的邊緣。
You will see the edges change,either growing larger or getting smaller.
你會(huì)看到它的邊緣發(fā)生變化，要么變大，要么變小。
This is cloud formation in action.
下面是云的形成過(guò)程。
The cloud grows because more of those water droplets are condensing out of the air.
云變大是因?yàn)橛懈嗫諝庵械乃文Y(jié)在了一起。
And when it shrinks,you are seeing the droplets evaporating, or changing from visible liquid water into invisible water vapor.
變小是因?yàn)檫@些水滴正在蒸發(fā)，或是從可見(jiàn)的液態(tài)水變成了看不見(jiàn)的水蒸氣。
But not every cloud has rain pouring out of it.
但并不是每片云彩都有雨。
So,how does the formation of a cloud become actual rain?
那么是如何因云成雨的呢？
For rain to be produced, the water condensing in the clouds has to become so heavy that it will fall to Earth,or else the tiny droplets will just stay suspended in the cloud, in the same way we see fog just hang there.
要產(chǎn)生雨水，云里凝結(jié)的水要足夠重才能落到地面，否則這些小水滴就會(huì)懸浮在云層里，我們看到的霧就是這樣。
To become heavier,the droplets need to grow into drops,and to do this, they have to acquire more water and become larger.
要變得更重，這些水滴就要聚集在一起, 水多了，水滴也變得更大。
As the droplets collide with each other,some will combine and become larger, and others will grow as more water condenses out the air directly into the droplet,until they become condensed and heavy enough to fall to the earth as rain.
水滴間相互碰撞時(shí)，有些通過(guò)結(jié)合變得更大，有些則由空氣直接凝結(jié)成水滴，持續(xù)變大，直到重得落向地面成為雨水。
Or if the weather is very cold,the tiny droplets freeze and fall down as snow,hail,or sleet.
或者在天氣非常寒冷的時(shí)候，空氣中的小水滴就會(huì)凝固，以雪、冰雹、雨夾雪的形式落下來(lái)。
So,there you go – that's what a cloud is,and how it creates rain.
所以這就是云和雨的形成過(guò)程。
But what causes the air to cool in the first place?
但首先，是什么原因?qū)е驴諝庾兝涞哪兀?br>We mentioned earlier,that cooling the air reduces its ability to hold water, and triggers the formation of water droplets.
我們之前說(shuō)過(guò)，給空氣降溫會(huì)減少空氣中的水蒸氣含量，從而形成水滴。
To create clouds,and to have rain,the air has to be cooled.
要想產(chǎn)生云彩，進(jìn)而下雨，空氣遇冷是必要條件。
Most of the science journals we looked at state that there are three main ways in which this happens.
我們研究的大多數(shù)科學(xué)期刊都指出，雨的形成主要有三種方式。
These are known as relief rain,frontal rain,and convectional rain.
這些雨被稱(chēng)為地形雨、鋒面雨和對(duì)流雨。
Let's take a look at how each of these work.
讓我們看看這些類(lèi)型的雨是怎么形成的。
Relief rain,sometimes referred to as orographic rain,is caused when air is forced to cool as it rises over a physical obstruction,such as a mountain range,or big hills.
地形雨的形成是當(dāng)空氣上升遇到物理障礙(如山脈或大山)時(shí)被迫冷卻，然后就會(huì)產(chǎn)生這種現(xiàn)象。
As the warm air rises over the obstruction,it cools,and clouds form.
當(dāng)暖空氣上升到障礙物時(shí)，就會(huì)冷卻，形成云。
Relief rain is common in mountainous areas where it can lead to more extreme local rainfall patterns,because the rain clouds form very quickly.
地形雨在山區(qū)很常見(jiàn)，它可以導(dǎo)致更極端的局部降雨，因?yàn)橛暝菩纬煞浅？臁?br>Often one side of a mountain is warm and sunny,and the other side, only a few hundred feet away,is wet and rainy.
通常山的一邊溫暖而陽(yáng)光燦爛，而山的另一邊，就在幾百英尺遠(yuǎn)的地方，潮濕而多雨。
Frontal rain develops when two bodies of air, an area of warm air and an area of cool air,meet.
鋒面雨是由兩股氣流形成的，暖空氣區(qū)和冷空氣區(qū)相遇。
The colder air mass is heavier than the warmer air,and so the lighter, warmer air rises over the top of the heavier,cooler air.
較冷的氣團(tuán)比較熱的空氣重，因此較輕的空氣——暖空氣上升到較重、較冷的空氣上方。
As the warmer air rises over the cold air it also starts to cool, condensation occurs,and the cloud forming process begins,leading to rain.
隨著暖空氣上升到冷空氣上方，它也開(kāi)始冷卻，出現(xiàn)凝結(jié)現(xiàn)象，云開(kāi)始形成，導(dǎo)致降雨。
Where frontal rain develops,skies are typically grey and mostly covered by clouds.
在鋒面雨形成的地方，天空通常是灰色的，大部分被云所覆蓋。
And finally convectional rain.
最后是對(duì)流雨。
On a warm day,the sunshine heats up the ground,and the air above is also warmed,causing it to expand and rise.
在溫暖的日子里，陽(yáng)光使地面升溫，上面的空氣也變暖了，導(dǎo)致它膨脹上升。
As it continues to warm,it absorbs more water because,as we mentioned before, warm air can hold more water than cool air.
隨著溫度的升高，它吸收了更多的水，因?yàn)椋缥覀冎疤岬降?，熱空氣比冷空氣能?chǔ)存更多的水份。
But as it rises,the temperature starts to drop again,because the atmosphere gets cooler as you go higher - by roughly one degree centigrade for every 100m of altitude.
但隨著暖空氣升高，溫度又開(kāi)始下降，因?yàn)樵酵显嚼洹蠹s每上升100米就會(huì)下降1攝氏度。
When the air reaches a height where the temperature forces the water vapor to start condensing,clouds begin to form.
當(dāng)空氣達(dá)到一定高度時(shí)，溫度迫使水蒸氣開(kāi)始凝結(jié)，云就開(kāi)始形成。
Clouds formed this way are called cumulus clouds,or cumulonimbus clouds, and they usually lead to very heavy rain,often with thunder and lightning.
這樣形成的云被稱(chēng)為積雨云，它們通常會(huì)導(dǎo)致大雨，還經(jīng)常伴有電閃雷鳴。
Convectional rain is experienced at the end of a hot summer day, like when the air feels heavy and we can tell a storm is brewing.
在炎熱的夏天結(jié)束時(shí)，會(huì)經(jīng)歷對(duì)流雨，空氣會(huì)變得很重，我們可以感覺(jué)到暴風(fēng)雨就要來(lái)了。
So,that's the process of clouds forming.
這就是云的形成過(guò)程。
Clouds cool the Earth's surface by reflecting sunlight, and they supply water that is essential for the survival of plants,animals,and us humans.
云通過(guò)反射陽(yáng)光使地球表面降溫，它們提供的水對(duì)植物、動(dòng)物和我們?nèi)祟?lèi)的生存至關(guān)重要。
Have you experienced extreme or severe rain,when the water we need turns from friend to foe?
你經(jīng)歷過(guò)大雨或暴雨嗎？彼時(shí)我們賴(lài)以生存的水從朋友變成了敵人。
Describe the worst storm you've ever been in,in the comments below.
在下面的評(píng)論中描述一下你經(jīng)歷過(guò)的最大的暴風(fēng)雨。
Also, be sure to check out our other video called What Is Life Like In The Smallest Country In The World?
還有，一定要看看我們的另一個(gè)視頻——《世界上最小的國(guó)家的生活是什么樣的？》
Thanks for watching,and,as always,don't forget to like,share,and subscribe.
感謝您的收看，和往常一樣，請(qǐng)不要忘記點(diǎn)贊、分享和訂閱。
See you next time!
下次再見(jiàn)！

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