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Suppose we walled off a 1000 km square. These walls would be made of unobtanium (so they don't break), and have no cracks or holes that go all the way through. They are also around 15 kilometres high, to stop birds flying over. No living thing can cross the walls.

This is a major element of my setting. This is definitely not a realistic option for enclosing a geographical area, but it has plot reasons.

However, something this big would have major meteorological consequences. Even a measly 2 kilometer high mountain range makes a huge rain shadow. This thing is quite definitely high enough to prevent some (if not all) clouds from crossing over it.

Basically, I have no idea what this would do to the weather of the area. Assume that the wall went up over the course of a year, and that the area enclosed... For simplicity's sake (as in, I don't have to provide a completely made-up map), let's centre this square on Italy, and keep the walls aligned to the compass. Rome to be precise. If the thing is to big to have an actual square, the corners should have right angles, and the distances form the walls that are opposite each other should be 1000 km measured from the middle to the middle. The wall would not be transparent For thermal stuff, let's say it has the characteristics of a hundred meter thick wall of limestone.

My question is, what would this do to the weather inside, and outside the walls? Could people live inside the enclosed area, or would it quickly become a barren wasteland? If it would become a wasteland, what could I do to keep the walls, but avoid this scenario?

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    $\begingroup$ The area of Europe is estimated at 10.18 million km², that's a lot more than 1000 km square (1,000,000 km²). Which one do you want us to think about? $\endgroup$ – mwarren Apr 24 '20 at 10:39
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    $\begingroup$ @mwarren The square. I had a look the other day, and it seemed like the square would cover a large chunk (50-80%) of Europe, which is why I wrote that. I'll get rid of that geographical reference. $\endgroup$ – Mark Gardner Apr 24 '20 at 10:42
  • $\begingroup$ Thanks for the edit. I'm not a weather expert so I won't make an answer of this, but I would be concerned about loss of wind. The jetstream apparently only goes up to 15 km and I don't know if sufficient pressure differences within the square would cause internal winds. $\endgroup$ – mwarren Apr 24 '20 at 10:57
  • $\begingroup$ (1) A square with a side of 1000 km has an area of 1,000,000 square kilometers. A square with an area of 1000 square kilometers has a side of about 32 km. (2) Rome is 25 km (16 miles) from the sea. (3) Italy is generally less than 200 km (125 miles) wide. (3) Unless a large part of the enclosed area is a sea, what you will get is an extremely dry desert. Water vapor is lighter than air, and will escape; rain clouds are much lower than 15 km, and won't come in. (4) A square with a side of 1000 km centered on Rome will include Nice (France), Sarajevo (Bosnia) and Zagreb (Croatia). $\endgroup$ – AlexP Apr 24 '20 at 12:04
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    $\begingroup$ One imagines these walls also sever seas and rivers? That water cannot flow beneath the wall? $\endgroup$ – user535733 Apr 24 '20 at 13:56
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From https://en.wikipedia.org/wiki/Troposphere

The troposphere is the lowest layer of Earth's atmosphere, and is also where nearly all weather conditions take place. It contains 75% of the atmosphere's mass and 25% of the total mass of water vapour and aerosols.[2] The average height of the troposphere is 18 km (11 mi; 59,000 ft) in the tropics, 17 km (11 mi; 56,000 ft) in the middle latitudes, and 6 km (3.7 mi; 20,000 ft) in the polar regions in winter. The total average height of the troposphere is 13 km.

So the upshot is that you basically won't have much weather flowing over the wall. Each walled area will be (mostly) independent. The sun each day will cause water of the seas to evaporate, but there will be only limited sea breeze to carry it inland. More fog, almost no rain. At night, the fog will precipitate out as dew, not rain.

The mountains of Italy will dry out and become barren as the vegetation dies for lack of water. The total amount of fresh water precipitated will be much, much less - rivers will dry and most land will become desert. A narrow strip of dew-dependent life will cling to the coasts.

Coastal Italy will become less moderate as the walls prevent massive heat movement by the oceans. Rome turns out to be pretty far north (like, say Chicago), and will have hotter (and humid) summers, and colder winters. The sea will moderate temperatures a bit (a few degrees, like Lake Michigan does with Chicago), but look for coastal sea-ice in winter.

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  • $\begingroup$ but the article says that in the middle latitudes (which is where I'm placing this) the troposphere is at 17 km height. As far as I can tell, that is higher than 15 km. Am I misunderstanding something? $\endgroup$ – Mark Gardner Apr 25 '20 at 7:31
  • $\begingroup$ If you think that measly couple km at the top of the wall, where the wind is fast, thin, and cold, will carry enough weather around to sustain life, think again. $\endgroup$ – user535733 Apr 25 '20 at 14:55

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