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I'm a newbie here and I'm slowly building a story based on Ancient Egyptian mythos that revolves around a primarily solar powered city in a vast desert. There will be magic and Gods involved, but I'd like parts of the world to be plausible.

A nearby river that this city was built by (loosely based on the Nile) completely dried up over the past hundred years. This is due to drought and extreme heat in the area, but there are larger bodies of water a great distance away. It's not feasible to transport water to the city, so my idea was to have the city build some sort of "Cloud Harvester" factory/technology that could suck clouds out of the upper atmosphere to supply water for the populace.

My questions are:

  • Can water be extracted from clouds?
  • How much water could you possibly get from a single cloud?
  • How would this impact the weather/water cycle of the world?
  • Would there be many clouds at all if the area is dried up enough to no longer sustain the river?

This is probably the first of many topics I'd like to brainstorm for this story's world, so any help would be greatly appreciated!

EDIT: I see the issue with having a "Cloud Harvester" in a dry climate, there won't be enough clouds. I'm curious enough about this idea to apply the question to areas outside of the area I described. Let's say this is happening in a wetter area for clouds to form, but the clouds aren't raining. Is it possible to suck up a cloud on a cloudy day and wring it out?

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    $\begingroup$ dry place dry for the reason, one of which is lack of clouds and prevailing air flows in- and around of that place $\endgroup$ – MolbOrg Sep 7 '16 at 19:58
  • $\begingroup$ My hunch says that canals are going to be way easier than cloud harvesting, and also yield a lot more water, even if the canals are very long. $\endgroup$ – Azuaron Sep 7 '16 at 19:58
  • $\begingroup$ Nice question skeletim, welcome to the site. For more info on writing good questions and answer check out the help center and once you hit 20 rep feel free to join us in Worldbuilding Chat $\endgroup$ – James Sep 7 '16 at 19:58
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    $\begingroup$ Not specifically how your society would do it but creating clouds for the purpose of rain is actually a real thing called "cloud seeding" but it typically involves releasing dry ice or silver iodine into the high atmosphere via airplane. Obviously you could attempt to use some form of this and the magic you might have in the story to create something of a mix. $\endgroup$ – Virusbomb Sep 7 '16 at 20:02
  • $\begingroup$ @Virusbomb cloud seeding is more about adding something that can be the nucleus of a raindrop to existing cloud. i.e. adding the right kind of fine particulates for rain to form. $\endgroup$ – Seeds Sep 7 '16 at 20:09
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The core idea you have is very similar to that of an air well, and these have been around since ancient times. You are simply talking about building yours higher up in the atmosphere.

The air well works by encouraging dew to form. By allowing air to flow around stones, the natural moisture in the air is cooled. As it cools, the water vapor in the air condenses and is collected.

Putting one of these in the clouds now becomes an architectural challenge. Today we have skyscrapers that meet this criteria, but your ancient Egyptian-like people might have more trouble. They did however build the pyramids, so it is plausible they might have expanded that concept to a massive scale. Since the air well works at low altitudes though, I doubt they would have bothered.

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  • $\begingroup$ Not necessarily. The building of the structure itself could be based on kites - or maybe kites itself. There have been demonstrations that show how the ancient Egyptians could have used the strong desert winds to lift obelisks. The only problem is that winds can be fickle. $\endgroup$ – Jesse Cohoon Sep 7 '16 at 23:34
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A nearby river that this city was built by (loosely based on the Nile) completely dried up over the past hundred years. This is due to drought and extreme heat in the area, but there are larger bodies of water a great distance away.

This means that it is not raining at all (extreme drought) in the area where the river has its sources and tributaries.

Weather patterns might have shifted so that instead of raining (and snowing) far in the South, feeding the river, a lot of humidity stops in the vicinity of your city. It will not condense into rain due to the temperature.

UPDATE: also you would need to make some reference to the water table - there's bound to be at least some wells in operation getting a bit of water from beneath the river.

At a height of about 100 meters (humid water is slightly lighter than dry air), you might be able to siphon 25 cubic centimeters of water from every cubic meter of air by cooling the air to below zero temperatures and re-heating with the waste heat when the water has condensed. To get 50 liters per day (comfortable drink, cook, wash, etc. for one person - you can do with much less in Sietch Tabr and using stillsuits), you would need to process 2000 cubic meters in 86400 seconds.

A reasonable vapour flow over the city could be about 2 meters per second, so one vent of one square meter aperture would get about 170.000 cubic meters per day -- or enough for 80 people.

(Incidentally, the condensing surface will grow linearly with the city radius while the water needs increases with the radius squared. So there will be a limit to the city size, and this limit might be varying in time. All well and good if it increases, but if it decreases...).

There might be some funny weather pattern when the dry, heated air is vented - you'd get an enormous, shimmering thermal over the city perimeter where the water condensers are.

But a much more noticeable effect of this situation wouldn't be the climate - it would be societal. You'd value water quite a lot, you'd need to recycle it, and if energy is cheap and water is expensive, you might get "water pirates" - people running moisture condensers in their basements, drying the air their neighbours breathe to fill their own tanks. On the other hand, drying one's house air would be OK - you'd be recovering the sweat, perspiration and drying losses.

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  • $\begingroup$ I really love your approach to this. With weather patterns bringing in more humid air, would a district of the city dedicated to massive moisture condensers be able to consistently extract the same amount of water, or would the condensers need to be spread out all over to reach more of the humid air? I'm imagining the whole city always emitting a low hum, as if the A/C was cranked up to 11. $\endgroup$ – skeletim Sep 8 '16 at 18:56
  • $\begingroup$ You would need to place condensers all around the city. And yes, you'd get a considerable humming from the air turbines. $\endgroup$ – LSerni Sep 8 '16 at 22:19
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You can harvest clouds, and it is done already! see here. And of course plants and animals harvest water from fogs (clouds) in a similar way. The idea is to create a surface were the water droplets can accrue. One sees this to a lesser amount on cobwebs and fences etc. in foggy weather.

Of course you may need to change your geology to enable the fogs/access to the clouds. The amount of water would be minimal, but may be able to sustain life, provide point irrigation etc. But would depend upon the scale.

Unless one had massive and continuous access to fogs, flowing water would be unlikely.

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