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Rain happens when the pressure of moist air drops enough to form droplets and the droplets get heavy enough to fall.

The trouble that I see is that the pressure will probably drop off slower than the "gravity" (which is opposite the situation on Earth).

There is no actual gravity in a spinning cylinder. The cylinder spins and the tangential momentum from that spin, pushes you into the cylinder.

If there was no atmosphere in the cylinder, there would be no "gravity" if you were not in contact with the surface. With an atmosphere, the atmosphere, through friction from the inner surface of the cylinder, gets dragged into the spin. The spinning atmosphere provides a lateral acceleration that pushes toward the surface of the cylinder. It only seems down because the surface is rotating in the same direction you are being pushed (the surface will move faster so you will fall "down" anti-spinward).

Since the spin (thus perceived gravity) decreases as you move toward the center, I'm concerned that any droplets that form near the center will not be pushed toward the surface.

So, how would we get "natural" rain in a spinning cylinder? I'm assuming a 1km radius but that is open to change.

The solution must allow clear air for the first 100-200 meters from the surface. So, super saturated moisture is likely out (we gotta breathe and live in there).

If possible, I would like to do away with the need to spray fake rain.

Addition:

While it would be amusing, I would also prefer to avoid bucket sized drops plunging from the center.

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    $\begingroup$ Don't forget that air is subject to laminar flow. Any water condensing in the center, even if it falls as a large drop purturbed from the center, will be subject to significant sheer wind forces on the way "down". This is very effectively cause a "normal" rain event. $\endgroup$ – Stephan Dec 1 '17 at 0:53
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    $\begingroup$ Fire protection system... a.k.a sprinkles😝 $\endgroup$ – user6760 Dec 1 '17 at 4:49
  • $\begingroup$ @user6760, I know. I want to avoid having to use that system. $\endgroup$ – ShadoCat Dec 1 '17 at 18:25
  • $\begingroup$ You have a big problem--O'Neill cylinders have some very nasty atmospheric effects. The air at the surface is moving at the rotational velocity of the cylinder. 1km diameter, this is 70m/s for 1g. Oops, 250m up it's only moving 35m/s. Can you say "vortices"?? $\endgroup$ – Loren Pechtel Dec 3 '17 at 5:09
  • $\begingroup$ @LorenPechtel, any chance of writing this up? This is the kind of thing I've been trying to discover. I just don't know enough meteorology or fluid dynamics to do it on my own. $\endgroup$ – ShadoCat Dec 4 '17 at 21:04
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Being large enough.

That's pretty much it. If the habitat is big enough for air to rise, cool, and form clouds, you can get rain. Even if the clouds are all crowded around the axis.

Since the spin (thus perceived gravity) decreases as you move toward the center, I'm concerned that any droplets that form near the center will not be pushed toward the surface.

That won't be an issue. Hovering at the center is an unstable equilibrium. Drops may hang out there for a while, but eventually any drop near the center will get nudged a little out towards the edge, which will result in a stronger push farther towards the edge, and so on. Even if the atmosphere starts out perfectly quiescent and co-rotating with the habitat, the movement of those first raindrops under coriolis effects will produce horizontal swirls, possibly developing into what Larry Niven called "eye storms" (essentially a hurricane flipped up on its side), thus introducing turbulence to the central regions which will ensure drops get transported out relatively efficiently.

Clouds can form pretty much arbitrarily close to the ground (after all, that's what fog is), so 1km radius is probably large enough, especially if the entire cylinder has a single day-night cycle so the air is allowed to cool (that way, it doesn't have to rise as much before clouds form). The bigger the cylinder is, however, the more "normal" the weather will seem. The official O'Neill Cylinders as designed by Gerard O'Neill would be 8 kilometers in diameter, which would be just big enough for cumulonimbus storm clouds to form... the effects of coriolis forces, and the crowding towards the middle, probably mean that you wouldn't actually get things that look just like Earthling cumulonimbus clouds, but that should still be plenty large enough for rain.

Use exotic materials to make the cylinder really big (say, 50km radius), so that you can get stratospheric pressures near the center despite the reduced pressure gradient, and you'd be able to get a clear cylindrical cloud deck layer that does not extend to the axis.

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  • $\begingroup$ The question is looking for how large or any other conditions. Even if the drops get nudged out of the center, they will be falling very slowly at first. Will they fall fast enough to avoid evaporation? $\endgroup$ – ShadoCat Nov 30 '17 at 19:41
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    $\begingroup$ You don't have to be that large to have inside rain $\endgroup$ – Gary Walker Nov 30 '17 at 20:08
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    $\begingroup$ Even bucket sized drops should break up on the way down like water plunging over a waterfall turns into spray. It's the water colliding with the air on the way down that would make them unstable. This doesn't mean that you might not get a "burst" of heavy rain in one location though. $\endgroup$ – Josh Belmont Nov 30 '17 at 20:42
  • $\begingroup$ the answer with @GaryWalker is a complete answer to the question, as my opinion goes. $\endgroup$ – MolbOrg Nov 30 '17 at 21:25
  • $\begingroup$ @ShadoCat Yes, if conditions are right to form them in the first place, they would fall fast enough to avoid evaporation. As for how large... as I said, 1km radius ought to be enough, if all you care about it making rain. But the larger you make it, the more "normal" and Earthlike it will seem. $\endgroup$ – Logan R. Kearsley Nov 30 '17 at 22:19
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I've been considering this for a while (see my other cylinder hab-related questions) and this is what I've come up with so far:

On Earth the conditions are:

  1. Evapouration – atmospheric water vapour
  2. A thermal/pressure gradient – to carry and bring vapour together
  3. Condensation – atmospheric dust forms the nucleus of droplets
  4. Gravity – to draw the droplets to the surface

The first three three conditions would be present in an O'Neill or McKendree cylinder, but the nature of "gravity" in such an environment requires a substitute. That substitute is momentum, or wind, itself.

Aboard the habitat:

  1. Evapouration – Wind, heat and artificial light with the correct properties will evapourate water and keep it in vapour form just as on Earth.
  2. Given a sufficient thermal gradient hadley cells should form and carry moisture with them, from the warmer surface and higher air pressure to the cooler, lower-pressure central shaft: Diagram of hadley cells in a cylindrical atmosphere

  3. Condensation – dust is unavoidable. It may be necessary to periodically "seed" clouds by spraying particulates into the air.

  4. Winds will carry droplets through the atmosphere toward the central shaft, where they will collide with others and be carried through, back toward the surface. Unlike rain on Earth, this will "fall" in every conceivable angle, possibly looping and spiraling until finally connecting with a surface.

Assumptions

This assumes a closed/windowless model with artificial light and heat provided by a mechanism running through the central shaft.

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  • $\begingroup$ Something that factored into my thinking is: if the light and heat runs through he central shaft, shouldn't the center be warmer than the outer layers? That might kill this kind of heat engine. $\endgroup$ – ShadoCat Nov 30 '17 at 22:34
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    $\begingroup$ Would Hadley cells work in that convex environment? On Earth they are aided by a convex Earth which helps to force rising air streams appart. But in a Cylinder that logic is turned on its head. $\endgroup$ – Slarty Dec 1 '17 at 0:20
  • $\begingroup$ @ShadoCat Heat transfers to the air by contact with the ground, producing thermal columns. If the air at the centre is thinner there's less of it to absorb and retain heat. $\endgroup$ – rek Dec 1 '17 at 2:12
  • $\begingroup$ @Slarty The diagram is overly simplified, but the principles of heat rises, cold descends should hold. Cells would probably spiral as they ascend and be less defined at the core. $\endgroup$ – rek Dec 1 '17 at 2:12
  • $\begingroup$ Yes true - I could see there being one giant spiral cell $\endgroup$ – Slarty Dec 1 '17 at 14:24
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Couldn't you force rain just by having long spikes on the central shaft that are cooled below the ambient air temp, so water condenses on them and are "flung" off the tips? By adjusting the temp of the spikes you could adjust where/when the rain fell in areas along the cylinder. This ought to work even in the very low gravity center. Water collects on the cooled spikes and centrifugal force moves them to the tips where they should land in a relatively predictable area. Moisture can be actively collected from even the driest of earth climates via cooling condensation (windmill driven condensers working in the desert so this should work even if the humidity in the cylinder is below what could cause natural clouds to form. Targeted rainfall would allow for green areas/open water without showering everything (although periodic showers in other areas can wash away dust).

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  • $\begingroup$ This is an interesting and different take on the problem. $\endgroup$ – ShadoCat Dec 1 '17 at 18:23

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