|MadSci Network: Physics|
There are a couple of reasons that work together to explain this situation. The first one has to do with how raindrops form.
Now, it may seem silly to make such an obvious statement as, "Rain always falls from clouds." But once it has been said, we can then ask, "What is going on inside those clouds?" And that question leads us to plenty of relevant information!
Do you remember the last time your local weather was described as being "hot and humid"? Do you remember what it FELT like? Do you also remember what the air LOOKED like? The reason I am asking these questions is because it is important to know that ordinary water is commonly found in one of three forms: solid, liquid, or gas. The liquid form is what we call "water", while the solid form we call "ice", and the gas form we call "water vapor". Water vapor is colorless, odorless, and invisible. But you sure can FEEL it on a hot and humid day!
Next, do you remember the last time you encountered some fog? Think about what IT looked like. If you looked real close, you would have seen zillions of tiny tiny droplets of water floating in the air. And, if you remember the last time you saw some steam, you might think that it looked just like fog (except it was hotter). Steam is NOT pure water vapor!
Well, it happens that the description, "zillions of tiny tiny droplets of water floating in the air", applies to every fog, to every mist, to every burst of steam -- and to every cloud in the sky. If you sit back and simply watch the clouds for a while, you will see that some clouds grow, some shrink, and some do nothing but move across the sky. What is happening to the growing or shrinking clouds? When a cloud grows, water vapor in the air gradually "condenses" to form lots more tiny tiny droplets. When a cloud shrinks, many droplets are completely "evaporating" to become invisible water vapor.
As the winds move air across the sky, carrying clouds along, fragments of wind known as "eddys" can do strange things to the shapes of clouds. A common game is to study a cloud's shape and guess what it looks like. Also, winds from different directions can collide with each other, which often has great effects upon clouds. It depends on how cold or warm the colliding winds are, and how much water each wind carries (only the differences matter, not whether the water is in the form of vapor or tiny drops).
For rain to happen, the weather conditions inside the clouds, and surrounding them, has to cause the droplets to grow. Some droplets will grow by colliding and combining with each other -- just what you Asked about. Some droplets will grow by acquiring more and more water vapor from the air. Some will do both. But the important thing to remember now is that MOST of the droplets in the clouds grow at the same time!
BUT, the droplets do not all grow at the same rate; some grow faster than others. As soon as ANY get big enough, they start to fall out of the cloud. Water is heavier than air, after all. Depending on the wind conditions, those first raindrops might be blown back up into the cloud, where they can grow some more. Eventually, however, they get too big to stay up in the air, so they fall all the way to the ground. And the slower-growing droplets will fall after them, and the slower-growing-yet droplets will fall after THEM, and so on. This is the essence of the raining process.
As I said at the start, there are a couple of reasons why the raindrops don't combine to make a single stream. The first reason is simply that raindrops start falling just as soon as they are too heavy to stay in the air. The second reason is that they are all falling together DOWNWARDS, not sideways. If the wind blows them sideways, they all move TOGETHER sideways. That isn't good enough to cause them to meet each other, while falling, so they could combine to form a single stream!
Finally, there is one more reason that only partly applies. THIS reason applies to existing streams of water that move through the air, and causes them to break up into droplets. In other words, if rain DID happen to fall out of the sky in a stream, this reason would turn it into raindrops!
Two phenomena work together to create that reason. They are "surface tension" and "air resistance". I will not explain surface tension here; I have already done that in another Answer to another Question (about water staying in a straw when you pull it out of a glass, with your finger over the top end). And air resistance you already know about, even if you don't know that you know. Did you ever stick your hand out a car window, while the car was moving? What you felt was air resistance!
When a stream of water moves through the air, it encounters resistance that deforms the shape of the stream. This HELPS the property of water known as surface tension to create droplets! And so the stream breaks apart. The farther through the air the water moves, and the more air resistance it encounters, the more it can break up into droplets. If an initial stream of water fell thousands of feet from the clouds to the ground, it would become completely broken up into a pretty ordinary (but heavy) rainfall.
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