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I am writing a fan-fiction (thus the world that my story is set in is not mine though some modifications can be done in accordance to my story's timeline and setting in relation to the canon universe) where at some point the characters travel to a moon (which has a breathable atmosphere) of the main planet.

I plan that this moon has a giant wall, reaching the Karman line of the atmosphere (it does not really matter how high this line is, after all clouds are usually way below that arbitrary line), surrounding the entire circumference from pole to pole, separating the visible side, barren and moon-like, from the "far side", Earth-like and thriving with life and a civilization.

I try imagining how this world looks like from space, seeing a sudden and abrupt separation between an Earth-like landscape with oceans and trees and a barren white/grey cratered land. However I have difficulty seeing how clouds would look like.

Since the moon is tidally locked then the clouds would look like diagonal stripes opposite of each other on both hemispheres, like this image: https://upload.wikimedia.org/wikipedia/commons/thumb/b/b9/TerraformedVenus.jpg/250px-TerraformedVenus.jpg

However, with a wall surrounding the planet from pole to pole (in other words surrounding an entire meridian), I find it hard to know how clouds can work or move in such a situation.

So how will the clouds look like from space, in terms of overall shape and their movement through the atmosphere, when there is a giant "meridian wall" that practically reaches the Karman line of the moon's atmosphere?

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  • $\begingroup$ How can the side which never gets any sunlight be "earth like and thriving with life and a civilization"? $\endgroup$ – RonJohn Aug 14 '18 at 13:38
  • $\begingroup$ "far side" would be a better expression, so I've edited. $\endgroup$ – James K Aug 14 '18 at 19:47
  • $\begingroup$ Ron John, the far side is not always bathed in darkness. Much like how our Moon's far side receives light when it's in its New Moon phase. And when I originally write the question I did forget that term for a while. $\endgroup$ – Victorbrine Cassini Aug 15 '18 at 3:55
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My guess would be that the clouds would look normal in the "middle" of the walled off areas whilst you'd have a, probably broken, ring of clouds at the walls

On earth there are orographic effects where winds push the clouds against a mountain range. The clouds rise, therefore cool down and dissolve because they begin to precipitate rain.

And your wall is, in principle, just a big mountain range with no way around or above. So winds would push the clouds against the walls where they'd rain off. These regions would often suffer heavy rain. The rest of your world should look like a normal earth weather, at least cloud-wise.

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Your walls are massive to a point not many can actually comprehend, As such they will have a disproportionate effect on weather. Your walls are likely dozens to hundreds of kilometers tall which is quite a large amount of surface area to catch sunlight. Your walls are going to warm up and shove heat into the atmosphere around them causing updrafts which will cause the air to rise near the wall, where it will encounter more hot wall and continue to rise. I think that this would cause any clouds that are close to the wall to look like horizontal funnel clouds/tornadoes/hurricanes depending on pronounced this effect is. Of course the darker your wall is the more heat it will absorb from the sun.

Another thing to note is since the moon is tidally locked it only rotates once every orbital period (in the case of our moon I believe this is about every 28 days). This would lead to horizontal clouds due to the Coriolis effect to be less pronounced.

Overall I think clouds would either move towards or away from the wall (I'm not sure which) so I think you would have clouds that appear as semi circles when close to the wall and some what normal once you got far enough away.

You should also take note that the effect of the wall will not occur during night (or based on how quickly it releases heat it will become less pronounced at night and may actually be cold and cause precipitation at night, and a complete reverse of the effect described above)

On the barren side it seems like it would be reasonable to assume there are no clouds

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On the barren side of the wall, there would be no clouds.

Clouds are water that has evaporated and risen upwards and then condensed into a vapor. For this to happen, you need an atmosphere and water. From your description, it sounds like the wall is designed to stop the atmosphere on one half from getting to the other, so I am guessing that the barren side has no atmosphere. Even if it does, the barren side probably would not have water, so no clouds.

For there to be water (if you have ice), you need heat. Heat is normally captured by the atmosphere, and that keeps the planet at a steady temperature. Our moon on Earth has no atmosphere, so its temperature varies from +100°C during the day to -100°C at night. For clouds, you need a sustained temperature. The temperature has to be warm enough for liquid water to evaporate, and cool enough higher above the surface for it to condense.

On the other side, everything would probably look normal?

Your moon is tidally locked with a planet. So is Earth's moon. I'm not a meteorologist, but I don't think there would be much difference.

On earth, our weather is separated by the equator. Because of the planet's rotation, hurricanes don't cross the equator, like a lot of weather phenomenon. Air currents start at the poles and move toward the equator.

So I think that the clouds on that side should look similar to clouds on earth.

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The large scale air movements on such a world are still going to be driven by heat input, in this case the input will be from the planet the moon is orbiting, if I've read the description properly. Air and the cloud systems it carries will rise at the equivalent of the sub-solar point and then spread out towards the meridian wall losing heat as it goes and sink down at the wall before blowing back across the surface towards the main heat input zone. The overall effect will be a bit like a human iris from space but the pupil will be white cloud. Horizontally it'll be like Hadley Cells arranged radially around the planet.

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