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I was thinking about people who once told me that I must feel the rain earlier than them (I'm tall). So I thought of a world where the rain falls so slowly that the humans of different size living on it may not feel the rain at the same time.

Let's set a few boundaries, because I want this world to have a similar kind of humankind (at least in size and shape, no problem if they are actually looking like lizardmen). I need an answer fitting these elements:

  • Most of the so called humans must be from 20cm to 2.5m tall (newborn to tall grown up)
  • The size of the planet isn't really a matter, but I need the gravity on it to be the same as on earth: 9.807 m/s²
  • We must have the same kind of biomes as on our earth, all with a "slow rain"
  • No particular expectation on how the water acts when on the ground in a big body of water (sea)
  • No expectation about frozen water nor snow

I've put these boundaries up, because the only way I already thought of was to lower the gravity on Earth, but I'm not sure it would have actually worked.

On the Earth we may have a difference between someone tall and someone that is small (difference of height about one meter), but it is about 1/10 seconds (speed of rain drop). And I would like it to become many seconds or even minutes.

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  • $\begingroup$ @Raditz_35 It would be better if it's all rain, but maybe too hard to answer in this case, so frequently would be great already! $\endgroup$ – Calaom Feb 12 '18 at 10:28
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    $\begingroup$ You can add some other forces to equation. If your rain drop have electric charge with same sign as planet it will slow down them. $\endgroup$ – talex Feb 12 '18 at 12:18
  • $\begingroup$ @talex, nice tought here, so for now we have theterminal velocity possibility and the electric charge that seems great at solving the problem. But I wonder if theses solutions can slow the drops enough that it will be perceptible without killing humanlike lifeform $\endgroup$ – Calaom Feb 12 '18 at 12:53
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    $\begingroup$ it won't kill anyone. Just make word pretty weird. Everyone will have strange haircut. Because hairs will looks like you just wear synthetic pullover. All point up. But no lightning bolt hitting everything or something like that. $\endgroup$ – talex Feb 12 '18 at 13:00
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    $\begingroup$ I don't understand the premise to this question. Each raindrop can only hit one person, and the chance of being hit by a raindrop is proportional to the ground area one covers. A square meter on the ground is more likely to feel rain first before a 10-meter tall giant with a cross section of 0.5 square meters. In your rain storms, do all rain drops fall at the exact same time? $\endgroup$ – Sam Feb 12 '18 at 15:50
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Increase creature's perception of time

Instead of trying to twist the planet into doing something impossible, how about modifying the creature? Creatures have an innate perception of time passing. If your creatures have an enhanced resolution or perception of time, then for them rain falling could feel much slower than it does for a human. What if their eyes could see at 600 fps and their brain process it much like how slow motion looks to us? Our normal rain would appear as slow rain to them. The gotcha is that everything would appear to be moving slow not only rain.

One second will always be one second, but take a computer for example. To it what it can do in one second is totally different than what a human can do with the same one second. Your creature physically might not be able to move any faster than us, but from our perspective their reaction time and reflexes would seem far superior.

If you want more information on how human's perceive time there is a Wikipedia article on it.

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  • $\begingroup$ Nice answer, do you have any idea how they could communicate at this speed? I'd prefer the taller to be able to say "I feel rain" before the smaller feels it $\endgroup$ – Calaom Feb 15 '18 at 15:09
  • $\begingroup$ @Calaom it is not unreasonable for a species which thinks and process reality faster, would also have a faster language than us. This could be done with shorter or more complex sounds. As a side effect communicating between them and humans would be a frustrating experience for them. $\endgroup$ – Anketam Feb 15 '18 at 16:56
  • $\begingroup$ I just simply didn't tought of that, but actually the speed of sounds is enough ^^' You are getting the right answer besause it fits very well (even though that a lot of good answer arose) $\endgroup$ – Calaom Feb 15 '18 at 17:08
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What you need is much, much lower terminal velocity for raindrops.

Using mathematical terms, terminal velocity—without considering buoyancy effects — is given by$$ V_\text{t}= \sqrt{\frac{2mg}{\rho A C_\text{d} }} $$

where

  • $V_\text{t}$ represents terminal velocity,
  • $m$ is the mass of the falling object,
  • $g$ is the Earth's gravity,
  • $C_\text{d}$ is the drag coefficient,
  • $\rho$ is the density of the fluid through which the object is falling, and
  • $A$ is the projected area of the object.

You want as low $V_\text{t}$ as possible. Considering you want it to still be a rain as we know it, $m$ and $A$ are there to stay. You also locked $g$.

What's left is $C_\text{d}$ and $\rho$ — For a slower rain, you need to make your atmosphere denser or increase drag. You can't really change raindrop shape too much, so to increase drag you would have to increase what happens at the water - atmosphere border. 4 times denser or 4 times higher drag will make rain fall 2 times slower. 9 times more dense = 3 times slower, and so on. Note that it would caused severe problems with breathing far earlier than slow-down would reach the level you want. If you want minutes, you are starting to be in the values found in liquids, not gases.


You could make raindrops smaller. $m$ will decrease faster than $A$ due to square cube law. But it would not look like rain anymore. It would be a wall cloud that goes all the way down to the surface. Good thing is - this actually happens in real life sometimes. But I don't believe this is what you really need.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – James Feb 14 '18 at 15:44
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I've come up with two ideas, not very much steeped in reality, but close enough.

  • Similarily to Molot's answer, a different sort of atmosphere: You could say that there is a chemical composing enough of the atmosphere, different from anything in our own, which slows the falling of raindrops. Name it whatever you like, Calaomium, Densium, whatever, or pick a dense gas on the periodic table.
  • The ground of the planet could be a high enough temperature that water evaporates before reaching shorter people. It could be a matter of the humidity being raised enough over the course of a storm that people shorter than the tallest finally feel droplets.

Additionally, let's consider...

On the Earth we may have a difference between someone tall and someone that is small (difference of height about one meter), but it is about 1/10 seconds (speed of rain drop). And I would like it to become many seconds or even minutes.

In relation to my first example, there are some interesting implications. Because you want people to feel rain between many seconds to minutes apart, meteorology could be radically different here. According to National Geographic, clouds can be between 2,000 meters and 15,000 meters in the sky. I really want to avoid using actual math or science here, but if you're looking at the, maybe, 0.5 to 0.25 meter difference being many seconds to minutes, clouds could have rained days or weeks prior to even the first droplet hitting the ground. This interaction makes me really fond of my first example.

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    $\begingroup$ "I really want to avoid using actual math or science here" Why? $\endgroup$ – Brythan Feb 12 '18 at 15:30
  • $\begingroup$ Because I don't think fiction has to have any basis in reality. It could limit the kind of story Calaom wants to tell. $\endgroup$ – Dispenser Feb 12 '18 at 15:33
  • $\begingroup$ Thanks for the answer, it gives new actual matter to think of. I don't really think about telling a story right now but I came up with this idea inside my head and kept thinking about ways to make it possible. Of course if it is science based, it would be better to think we can, one day, have a human step on a planet that actually have this particularity. And would easily open our minds about the infinity of the universe and what we can find inside. $\endgroup$ – Calaom Feb 12 '18 at 15:51
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This answer also is about terminal velocity, but just in the oposit way:

Accelerate the medium in which the drop is falling

Do you know these machines? enter image description here

Well, I was thinking of something similar. The link you provide states that the terminal velocity of a rain drop is about 9m/s, so if you have a wind going upward at about 8.99 m/s, the drop would take 100 seconds to go down 1 meter.

Feasability

Speed of wind

The speed of the wind is not too strong for people to stay on the ground. 9 m/s is about a force 5 beaufort, which is descibed as :

Small trees in leaf begin to sway; crested wavelets form on inland waters. Source: wikipedia.

Wind going upward

Well, this is probably the difficult part. You need to admit for the phenomena to be localized, i.e. not the whole world will be like that, only a region.

Let's admit there are two regions separated by a very high mountain range. Below the mountains are a network of caves, but instead of having one big entry/exit hole, they have trillions of very very small ones. Your people are living just on top of them. When the atmospheric pressure is different between the two regions, wind is forced through the caves. It will appear to come out of the ground (or be sucked in on the other side).

There are some rocks that are porous, so it's not impossible to build a city on top of something that let the air go through.

I am very good at drawing, so I decided to impress you and did an illustration of my idea:

enter image description here

I think this solution is interesting, because it uses the same setting as the earth, no gravity change, no alien breathing heavy gases...

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  • $\begingroup$ Wouldn't there be chances that the rain makes little floating flakes just above the ground until they become too heavy and fall again? And there's also risks that the porous rocks are filled with the flow of water $\endgroup$ – Calaom Feb 15 '18 at 15:13
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    $\begingroup$ @Calaom I'am not sure what will happen to the water once it (nearly) reaches the ground. But if it enters the porous rock, after the rain it could evaporate because of geothermal heath, escape again from the bottom generating a tropical climate for your people and preparing big clouds for the next raining event. $\endgroup$ – Legisey Feb 16 '18 at 7:17
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    $\begingroup$ You're right, it could work this way and it would result into some kind of environment that we don't actually have on earth but that is still possible! Great work! $\endgroup$ – Calaom Feb 16 '18 at 7:38
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Of course, ✳here✳ on Earth, heavy rain and light rain is merely a quantitative difference in the amount of water falling. Heavy rain is more water, light rain is less water, but people of varying heights caught in either shower feel it roughly at the same time.

 

But there are some out of the way places in the polyverse where people in certain worlds experience heavy rain and light rain quite differently than we do. For example, on Yeola, apart from there being quantitative distinctions of less and more, more interesting still are the the qualitative distinctions between light, moderate, and heavy.

 

  • ✳There✳, when folks speak of light rain, they mean that the drops of water are not very dense. They're soft, with perhaps a more open architecture, maybe airier would be an apt description. In any event, such rain drops are more prone to hanging about in the air. Like mist, but more substantial. When these kinds of rain drops fall from the clouds, they float down more gently, glide about, enjoying the ride in the breeze and may even rise up again before easily settling down into puddles. At best, light rain is a gentle caress those folks standing at about eight foot or so tall feel upon their faces and shoulders as much as fifteen seconds before the lesser folks, who stand around four foot tall do.

 

  • On the other hand, by heavy rain, they mean that the quality of the drops is different, more dense, harder if you will. It's as if more water and denser water were packed into a rain drop. When these little bastards jump from the clouds, they mean business! No floating about in the upper airs for these rain drops! No, heavy rain seeks to meet or exceed terminal velocity and from the start, they make a speedy bee-line straight for the earth far below, and they don't care who gets in the way! Unlike their lighter cousins, who often don't even really cause one to become wet, heavy rain drops love to aim themselves right at you. Whether vertically, from directly above in big splashsome plops or horizontally, and especially directly into your face, they seem almost to be guided by a malicious will. Heavy rain, in fact, is so mean and nasty that it literally pelts everyone, regardless of height. Often painfully and nearly simultaneously. Best you can do, whether you're eight foot tall or four only, when a heavy rain begins to fall is crouch under a sturdy tree and hunker down until the storm gives up!

 

  • Light and Heavy rains don't always travel together. They don't get along well, for even the light rain doesn't much like the company of heavy rain. Heavy rain drops don't pause to say begging your pardon, but might I pass by? No ma'am! They just barrel through the airs above without care or concern who they bump into. And when a shower of heavy rain passes through the territory of a shower of light rain, high up in the sky, it often happens that the heavy rain drops will capture and incorporate light rain drops in their zeal to pelt the very earth and all living creatures walking or growing upon it. Now, you might think that this is the end of the light rain drops so devoured, but not so! In fact, what ends up happening is that, in spite of themselves, the heavy rain drops' nature is itself transmuted, softened slightly.

    In these cases, a new hybrid form, moderate rain drops, are formed. More variable in temperament and more labile in nature & behaviour than either parent, a moderate rain drop may fall speedily, but not recklessly and generally without malice; or yet it may fall gently, but without dalliance. Moderate rain on Yeola, in fact, is much like ordinary rain ✳here✳ on Earth. The tall eight footer might feel a moderate rain only half to perhaps three-half seconds in advance of his four foot tall companion.

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