为什么会下雨？

又到梅雨季节，最近天天下雨，天气已经很久没有放晴了，那么为什么会下雨呢？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.
植物靠它生长，动物赖以生存，人体需要它来保持水分，维持机体正常运作。
Water is an essential building block to life and it comes to us from above.
水是生命的基本组成部分，它从天而降。
Rainwater fills streams,rivers,and lakes.
雨水注满了小溪、河流和湖泊。
It carries with it the potential for life to flourish.
承载着让生命蓬勃发展的潜力。
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.
它能把干旱的沙漠变成绿洲；在雨季把浅溪变成急流，给地球上最严酷的环境带来生命。
So how do clouds form to keep giving us the essential H2O we need?
那么，云是如何形成并持续提供我们所需的水的呢？
That's what we'll find out today, in this episode of The Infographics Show: Why Does It Rain?
这是我们在今天的“信息图表秀”要研究的：为什么会下雨？
Clouds are possibly the most interesting and beautiful of all weather phenomena.
云可能是我们见到的最有趣、最美丽的天气现象了。
They form in a wide variety of shapes and sizes, but are all made of the same thing...water.
它们大小不一，形态各异，但都由水而来。
Though we may not notice it, almost all the air around us is moist because it contains water in the form of vapor.
虽然我们可能注意不到，我们周边的空气几乎都是潮湿的，因为空气里有水分并以水蒸气的形式存在着。
You can't see the water vapor,because it's a gas,but it is still there.
由于水蒸气是气态的，所以我们看不到它，但它依然真实存在。
Water can be either solid as ice,liquid as water,or gas, which is a vapor.
水能以固态存在，比如冰；能以液态存在，比如水；或以气态存在，那就是水蒸气。
Vapor has no smell and it's invisible.
水蒸气无色无味。
But you can feel it.
但我们仍然能感觉到它。
Imagine a hot and sticky day in summer,or a cold foggy day in winter.
试想炎炎夏日里粘粘的空气，亦或是寒冬里的冷气 。
Those sensations are water vapor.
我们感觉到的这些都是水蒸气。
If you put steam or vapor under the microscope, you would see millions of tiny water droplets floating in the air.
如果在显微镜下观察水蒸气，我们能看到数以百万计的小水滴漂浮在空气里。
This is the same process that forms clouds - millions of tiny water droplets condensing out of the air to form liquid water.
这和云彩的形成过程相同-空气中数以百万计的水滴遇冷凝结成液态水。
But why does water condense out of air and become visible as a cloud?
那为什么水能从空气中凝结而成为可见的云呢？
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.
暖空气中的水蒸气比冷空气中的更多，所以当暖空气的温度降低时，就存不住那么多的水蒸气了。
The extra water vapor has to go somewhere.
多余的水分不得不去到别处。
It condenses out as water.
所以就凝结成了水。
To see this in action,go outside and stare up at a cloud.
要想看到这个变化，就要去外边盯住一朵云观察。
Be patient but keep watching the cloud,especially it's edges.
有点耐心，一直注视着它，尤其是它的边缘。
You will see the edges change,either growing larger or getting smaller.
你会看到它的边缘发生变化，要么变大，要么变小。
This is cloud formation in action.
下面是云的形成过程。
The cloud grows because more of those water droplets are condensing out of the air.
云变大是因为有更多空气中的水滴凝结在了一起。
And when it shrinks,you are seeing the droplets evaporating, or changing from visible liquid water into invisible water vapor.
变小是因为这些水滴正在蒸发，或是从可见的液态水变成了看不见的水蒸气。
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.
要产生雨水，云里凝结的水要足够重才能落到地面，否则这些小水滴就会悬浮在云层里，我们看到的雾就是这样。
To become heavier,the droplets need to grow into drops,and to do this, they have to acquire more water and become larger.
要变得更重，这些水滴就要聚集在一起, 水多了，水滴也变得更大。
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.
水滴间相互碰撞时，有些通过结合变得更大，有些则由空气直接凝结成水滴，持续变大，直到重得落向地面成为雨水。
Or if the weather is very cold,the tiny droplets freeze and fall down as snow,hail,or sleet.
或者在天气非常寒冷的时候，空气中的小水滴就会凝固，以雪、冰雹、雨夹雪的形式落下来。
So,there you go – that's what a cloud is,and how it creates rain.
所以这就是云和雨的形成过程。
But what causes the air to cool in the first place?
但首先，是什么原因导致空气变冷的呢？
We mentioned earlier,that cooling the air reduces its ability to hold water, and triggers the formation of water droplets.
我们之前说过，给空气降温会减少空气中的水蒸气含量，从而形成水滴。
To create clouds,and to have rain,the air has to be cooled.
要想产生云彩，进而下雨，空气遇冷是必要条件。
Most of the science journals we looked at state that there are three main ways in which this happens.
我们研究的大多数科学期刊都指出，雨的形成主要有三种方式。
These are known as relief rain,frontal rain,and convectional rain.
这些雨被称为地形雨、锋面雨和对流雨。
Let's take a look at how each of these work.
让我们看看这些类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.
地形雨的形成是当空气上升遇到物理障碍(如山脉或大山)时被迫冷却，然后就会产生这种现象。
As the warm air rises over the obstruction,it cools,and clouds form.
当暖空气上升到障碍物时，就会冷却，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.
地形雨在山区很常见，它可以导致更极端的局部降雨，因为雨云形成非常快。
Often one side of a mountain is warm and sunny,and the other side, only a few hundred feet away,is wet and rainy.
通常山的一边温暖而阳光灿烂，而山的另一边，就在几百英尺远的地方，潮湿而多雨。
Frontal rain develops when two bodies of air, an area of warm air and an area of cool air,meet.
锋面雨是由两股气流形成的，暖空气区和冷空气区相遇。
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.
较冷的气团比较热的空气重，因此较轻的空气——暖空气上升到较重、较冷的空气上方。
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.
随着暖空气上升到冷空气上方，它也开始冷却，出现凝结现象，云开始形成，导致降雨。
Where frontal rain develops,skies are typically grey and mostly covered by clouds.
在锋面雨形成的地方，天空通常是灰色的，大部分被云所覆盖。
And finally convectional rain.
最后是对流雨。
On a warm day,the sunshine heats up the ground,and the air above is also warmed,causing it to expand and rise.
在温暖的日子里，阳光使地面升温，上面的空气也变暖了，导致它膨胀上升。
As it continues to warm,it absorbs more water because,as we mentioned before, warm air can hold more water than cool air.
随着温度的升高，它吸收了更多的水，因为，正如我们之前提到的，热空气比冷空气能储存更多的水份。
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.
但随着暖空气升高，温度又开始下降，因为越往上越冷——大约每上升100米就会下降1摄氏度。
When the air reaches a height where the temperature forces the water vapor to start condensing,clouds begin to form.
当空气达到一定高度时，温度迫使水蒸气开始凝结，云就开始形成。
Clouds formed this way are called cumulus clouds,or cumulonimbus clouds, and they usually lead to very heavy rain,often with thunder and lightning.
这样形成的云被称为积雨云，它们通常会导致大雨，还经常伴有电闪雷鸣。
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.
在炎热的夏天结束时，会经历对流雨，空气会变得很重，我们可以感觉到暴风雨就要来了。
So,that's the process of clouds forming.
这就是云的形成过程。
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.
云通过反射阳光使地球表面降温，它们提供的水对植物、动物和我们人类的生存至关重要。
Have you experienced extreme or severe rain,when the water we need turns from friend to foe?
你经历过大雨或暴雨吗？彼时我们赖以生存的水从朋友变成了敌人。
Describe the worst storm you've ever been in,in the comments below.
在下面的评论中描述一下你经历过的最大的暴风雨。
Also, be sure to check out our other video called What Is Life Like In The Smallest Country In The World?
还有，一定要看看我们的另一个视频——《世界上最小的国家的生活是什么样的？》
Thanks for watching,and,as always,don't forget to like,share,and subscribe.
感谢您的收看，和往常一样，请不要忘记点赞、分享和订阅。
See you next time!
下次再见！

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