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I have a medieval fantasy world made up of an earth-like planet over which "islands float in the sky", a few kilometers high. These islands are fixed above the planet and fixed between each other. The distance between them can vary from a few meters to several dozen. Their size varies from that of a small village to several times that of a large city. They float because of some magic that, among other effects, produces a wind of about 100 km/h from the ground to the sky (the wind is not responsible for the "floating", it's just a side effect of the magic).
The people do not live on the planet but on the islands.

I saw a few questions about "what would make the islands float", but that's not the point.

I wonder: what would happen in the landscape if such a world existed and what would need to happen to make life there possible?

Here is what I already made up:

How to (naturally if possible, magically if not) fix the problem inherent to that world to make life possible?

  • I am thinking especially about the water: if it's floating islands, rivers would end up at an end of an island and water would fall off and not be available for plants or animals. Would the construction of cisterns and artificial lakes on mountains be sufficient?
  • The magic that makes the islands "float" produces a lot of wind coming from below. How to prevent the formation of tornadoes all over the world? I thought of building pieces of wall at some places on the borders of islands to "break the wind" but I'm not sure it would be efficient.
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closed as too broad by Mołot, L.Dutch, JDługosz May 16 '17 at 12:56

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ Is there a solid planet below or is it just a cluster of floating islands somehow all sharing a single atmosphere? $\endgroup$ – Hyfnae May 16 '17 at 10:12
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    $\begingroup$ Welcome to WorldBuilding Octachore! If you have a moment please take the tour and visit the help center to learn more about the site. You should only ask one question at a time, otherwise this might be too broad. You can wait a few days for feedback and then incorporate that feedback into a follow-up question. In general questions about "the effect on society" are almost always too broad, as there are too many things to consider. I'd recommend to focus on your number 1 for now. Have fun! $\endgroup$ – Secespitus May 16 '17 at 10:17
  • $\begingroup$ I agree with @Secespitus in that this question is probably too broad. I answered an earlier edit of it where you were just seemingly asking specifically how wind/water would work. Now that you've thrown customs, behaviour, and the possibility of life itself into the mix, it's too vague. I highly suggest focusing on the original issues in this question. You can always post additional questions! Welcome to Worldbuilding.SE. $\endgroup$ – type_outcast May 16 '17 at 10:20
  • $\begingroup$ @type_outcast I am not sure which revision you are referring to. a4android and I have only edited a few typos. $\endgroup$ – Secespitus May 16 '17 at 10:23
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    $\begingroup$ I see you already accepted the existing answer. That's fine, it's your decision if that answer was helpful. But I have yet another tip for the future for you: you should wait a day or two before accepting an answer. People using WorldBuilding live all over the globe in different timezones. After 2 hours with not even 50 views only a very small part of the community has seen your post and some people might be discouraged from answering if they see you already accepted something. You might be amazed to see how creative people can be and there could also be discussions with the existing answer. $\endgroup$ – Secespitus May 16 '17 at 12:03
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Wind

The 100 km/h updrafts from the ground would be very strong effect—nearly hurricane strength already! I'm not a weather expert, but depending on how large of an area the updrafts would exist in (and how densely packed your islands are), this could indeed create a lot of other dangerous eddy currents that could result in some very strange weather indeed.

These updrafts would also cause a big, direct problem for anything that needs to fly via conventional lift mechanics (including non-magical airplanes and birds). The thought of some poor flock of geese getting violently splattered on the underside of one of these islands is not a pretty thought. I suppose you could have the flighted animals evolve a little better awareness of subtle pressure changes or island shadows and navigate around them.

Honestly the best way to deal with the overall weather/wind issue would probably be to just reduce the wind strength significantly. If that's not possible, then the next best thing I could tell you is to perhaps have the effect start at a higher altitude (at least a few hundred meters off the surface), and at a gradual gradient, or maybe with some kind of magical barrier to prevent the wind from creating "spin off" currents. (My bad weather puns, you can have for free.)

Water

The top end altitude for rain clouds is about 2 km. You're going to struggle to get any water at all up there at your altitude of "a few km".

If you can lower your islands, great. Otherwise, they'll either need to be big enough to support their own water cycle (how big, I'm not exactly sure, but big: they'd need to retain at least a large lake/small sea worth of water and have a very large and mountainous land area to support wind patterns and temperature/pressure differentials), or you'll need to come up with some magical high-altitude rain clouds.

If you can get rain, and have enough soil depth and large-scale dips in the surface (lakes) such that it doesn't all run off immediately, you'll at least be able to keep water around long enough for it to be useful for plant and animal life.

Erosion is going to play a big role; water is going to tend to carve out rivers with time, and there's a good chance that one or two large rivers are going to dump all of the water off of the island, and it's all going to land in one spot. Fortunately all that water is quickly going to hit terminal velocity (about 200 km/h) on the way down so it won't instantly start carving huge canyons, but it's still going to hit very hard if a lot of water is concentrated in a relatively small area.

Alternative Water Sources

Irrigation is an option, although you will need either a very powerful multi-staged mechanical or magical pumping system to move water up by several km. Could create some interesting trade opportunities if, say, the lowlands have a monopoly on water, but the island people have a monopoly on other resources like sunlight crops, wind, or whatnot.

Surface Sunlight

Big islands create big shaded areas. Plants on the surface might suffer a bit if there are a lot of these islands, especially if they don't move around independent of the surface. Something to keep in mind, not an insurmountable issue.

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