From where comes the raindrop

First off all, I've no knowledge of how all this works. I've only got some common sense of "how things work"... But I've been wondering this for quite a long time now, so maybe you guys can help me.

The question is rather simple: Where does the raindrop come from? And with that I mean: The raindrop that just hit the ground, where did that come from? From the clouds that are right above me, in front of me, or back of me?

So let's say that we've got these 3 possibilities, which one is the most likely one?

The first one, where the rain is traveling with the same speed horizontal as the cloud it came from: (so it came from the cloud directly above me)

The second one, where the rain is traveling faster than the clouds (due to wind?). So that the rain came from the cloud that yet has to pass me.

The third one, (And i do think that this is the right one) is that the rain is falling slower (horizontally seen) than the cloud (because the cloud is lighter). So that the rain that hits the ground came from the cloud that already past me.

An another possibility is that the rain is going left and right, (keeping it 2D for simplicity) because of the different wind regions (in the high clouds, the wind goes from left to right, but lower it goes from right to left).

Is there even an "right" answer for this? Or are there too many variables that could change everything?

And could you please explain it as simple as possible? I've got zero knowledge in this area...

Thank you in advantage!

-Also, i don't know if this is the right stackexchange site to ask...

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I would be highly surprised if there is a universal answer. It would depend on the atmospheric conditions, which can be anything. – Bernhard Jul 29 '14 at 13:27
@Bernhard, I already thought so, but maybe there's an general overall rule that fit's in most situations... – Mathlight Jul 29 '14 at 13:30
The cloud and the raindrop are carried by the mass of air, except that the raindrops fall because they are bigger and heavier than cloud droplets. They just do what the air tells them to do. – Mike Dunlavey Jul 29 '14 at 13:37
The title put me in a zen mood. – BMS Jul 29 '14 at 13:50
+1 for actually thinking and pretty pictures. – jpmc26 Jul 29 '14 at 17:13

Suppose a rather simple case where the wind is completely uniform, so it has the same strength and direction at all altitudes and all times (highly unlikely)! Then the horizontal motion of the raindrop will be ambivalent to the wind, moving with it from the moment it leaves the cloud until the moment it falls on your head. The consequence is that the cloud will be directly overhead at the moment of impact, though it was far upwind when the drop began to fall.

Now most likely, the wind will become less strong, and also backed (more to the left) closer to the ground. The effect is that if you stand with your back to the wind, the cloud from which the rain landing on your head came will be slightly ahead of you and slightly to your right.

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Thank you for your brief explanation. This is very understandble ( for me ) and sounds right. And the conclusion is that if everything is perfect, the clouds will move a little upfront of the raindrop, right? – Mathlight Jul 29 '14 at 13:41
A little Fermi estimation puts the cloud ahead of the raindrop by a tens of metres, and off to the side by a few metres, so perhaps 5 degrees to your right, looking downwind. – Holographer Jul 29 '14 at 14:05
That is good enough for me ^_^ Thank you very much! – Mathlight Jul 29 '14 at 14:08
The effect of the ground (or any surface) on wind is called the boundary layer. And yes, it does slow the wind down. The difference of wind speed at different altitudes is known as the the wind gradient. If you want to know more about how real wind behaves (instead of idealized wind) google these two terms. – slebetman Jul 29 '14 at 19:01
@slebetman, thank you for that info. Will google them in the future! – Mathlight Jul 29 '14 at 19:25

The droplets are falling pretty much exactly vertically down from the viewpoint of the moving air mass at the given place because friction would almost immediately eliminate any relative motion of the droplets and the air (except for its vertical portion which is driven by gravity).

In other words, the horizontal speed in the $x,y$ directions is equal to the local velocity (speed with direction) of the wind.

If there is no wind, the droplets are falling from the point directly above you.

Obviously, there is no correlation between the direction from which the droplets are coming and the direction where you turned your head.

The vertical speed of the droplets is of order 10 m/s or less. So if the wind speed is also 10 m/s, the angle where the droplets are coming from is 45 degrees. If the wind is faster, the trajectories of the droplets are more horizontal than 45 degrees; if the wind is slower, the trajectory is more vertical.

The wind speed may also depend on the altitude.

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Thank you. Ofcourse does the wind not listen to the direction of my head. But what i meant to say is that when i stand with my back in the wind, will it past me or not ( the cloud ) before the droplets hit me or the ground... – Mathlight Jul 29 '14 at 13:43
The cloud from which the droplet came is probably in the direction from which the wind blows. – Luboš Motl Jul 29 '14 at 13:46
wind speed "may" depend on altitude? It definitely does. It's common knowledge that the wind veers and increases when you go up and it backs and decreases when you go down – Jim Jul 29 '14 at 14:28
I agree with you, Jim. However, one must also be realistic about the altitude from which the droplets are actually falling. The altitude may be as small as 1200 meters in which the case the variation of the wind may be limited. – Luboš Motl Jul 29 '14 at 14:39

This is in addition to Luboš Motl's answer.

Due to the friction, the rain drops move with the wind while falling.

So, on windy days, with the wind in the back, the cloud will always be behind you.

When we say, the falling speed of $10 \frac{\mathrm m}{\mathrm s}$ is fixed, the angle $\vartheta$ corresponds directly to the wind speed over $$\vartheta = \arctan{\left( \frac{\text{wind speed}}{10 \frac{\mathrm m}{\mathrm s}} \right)}$$

Lets say the wind speed is $10 \frac{\mathrm m}{\mathrm s}$ as well, then the angle $\vartheta$ is $45^\circ$.

If the wind speed is $0 \frac{\mathrm m}{\mathrm s}$, so that there is no wind at all, the angle $\vartheta$ is $0$ $\leftrightarrow$ the cloud is directly above you.

So, when your with your back to the wind, the cloud is always behind you.

So, because the clouds move with the same speed than the drops/wind speed while the drop is falling, even on windy days, the cloud will always be above you.

But this is just an approximation. E.g. in reality, the wind speed varies with height.

Sorry for the wrong answer at first!

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But that means that Holographer had it wrong? Or do i miss something? – Mathlight Jul 29 '14 at 14:19
No, I was wrong. I forgot that while the drop is falling, the cloud does not stop moving. So when we say the drop moves with wind speed, the cloud of course does it as well and will therefore be above you when the drop reaches you. – smoneck Jul 29 '14 at 14:34

You can actually see rain falling from distant clouds. The best situation to see it is a hot summer day that gives birth to a relatively isolated cloudburst a few miles away, where you can watch from the top of a hill. Before the rain, the bottom of the cloud is more or less well-defined and cloudy-looking, with same sort of "tufty" texture that you're accustomed to seeing around the tops of clouds. The rain, by contrast, looks like a curtain of grey fog that takes a half-minute or a minute to grow from the bottom of the cloud to the ground. Compared to the size of the cloud, the rain goes straight down.

Here are some excellent photos of rain beneath a thundercloud via Phil Plait:

You can clearly see the rain — a poorly-defined grey cloudiness — under the thunderhead in the second photo. This looks like a pretty big storm, so the rain has some lateral motion to it, but for the most part it is more or less directly under the cloud.

If you have some time to watch a this time-lapse video, the cloudbursts start about a minute and a half in.

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protected by Qmechanic♦Aug 1 '14 at 21:08

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