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So, in my world, I have a city built in a plain surrounded by a desert. The plain got to be there by magic and general hand waving and was sustained in a similar way. At some point, that stopped being possible (the one responsible died or something, it doesn’t matter), but the plain remained.

So, the question: how does the city get rain, or any kind of moisture for that matter? Keep in mind I am not an expert on these things, but moisture in the air can’t go through the desert because the humidity is too low (I’m simplifying a bit), nor can it go “over” that low-humidity zone because humidity decreases higher up. I know the moisture doesn’t technically go anywhere, but it works for my explanation of what I know.

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  • $\begingroup$ You haven't given enough details about the local geography (or climate) to really comment. For a start there are four different types of desert recognized by geographers and ecologists etc - hot and dry deserts, semi-arid deserts, coastal deserts, and cold deserts. The type of desert in question being determined by local geography and climatic conditions. You haven't defined any of that. Desert areas can have seasonal rains, river valleys fed by mountain glaciers or snow melt, fogs, springs & oasis etc feeding them water. You city potentially has lots of options, depending on how large it is. $\endgroup$
    – Mon
    Feb 28 at 10:49

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There is some humidity in the air over a desert, what lacks is a way to extract that humidity. In fact at night, when the temperature drops, it's possible for moisture to condense. That's how desert life often get their water.

If your plains have a way to elevate the airflows and cool them down, they have also a mean of extracting the humidity present in the air. These means are usually mountain ranges.

For a reference, Mount Kilimanjaro has bushland at its feet, while it gets a rain forest above that and even snow on the top.

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Ground Water

You don't need your plane to receive rain, you need it to receive water. If you plane is in a low-lying area compared to the desert then it may be closer to the water table which is being feed from the far side of the desert many hundreds of miles away.

The ancient wizard may or may not have been magical, but he was smart. He recognized the usefulness of the low-lands and terraformed them with a system of aquifers and irrigation fed by ground water. He may have used magic to to expose the ground water, but the actual aquifers are replenished by natural means.

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The rains on plains fall mainly in the plane

Of your planet's ring system, that is. It's not very visible from directly beneath, and the planet is no Saturn, but there actually is a nice bright ring around the planet as visible from anywhere else. Deorbiting ice chunks deliver periodic bursts of moisture to the air over your city. They tend to deorbit right over your city because your planet is a fairly standard oblate shape, but the ring did not form parallel to the equator. As the particles revolve over the equatorial bulge of atmosphere, they are slowed, and come falling out at your city's temperate location at the edge of the Hadley cell.

The plain was a good spot for deorbiting valuable mineral shipments from the ring - that was a fairly exotic asteroid that broke up, and it contains much more valuable bits than just water if you can find them and bring them down.

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