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Aliens build their city on Earth in such a way that it covers the entire planet but leaves the surface of the planet (including human cities) intact.

This is possible since they built their alien city on a huge platform which is supported by pillars rammed into the Earth's crust.

So with exception of the locations of these pillars the Earths surface remains as it was before the aliens arrival.

I'm interested in how such a city would impact the Earth's weather (I have intended for the alien city to be to be at a height of approximately 12 miles).

You can disregard the problem of sunlight as in how it could reach the ground since the alien city is in its way (the city will somehow let it through). Also disregard the problem of waste heat.

And a few notes: the city is built from extremely light but stable and firm material. I have intended that there are to be 12 pillars that hold up the city, which are located in the ocean and vast unpopulated areas such as deserts.

How would weather and other natural occurrences be affected in such a world? What type of weather patterns would the pillars produce?

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  • $\begingroup$ What's the size of the city? $\endgroup$ – Hyfnae Jul 4 '17 at 14:57
  • $\begingroup$ "it covers the entire planet". This is a good question. How much does the stratosphere on up contribute to weather? I look forward to reading the answers! $\endgroup$ – Willk Jul 4 '17 at 15:03
  • $\begingroup$ This will be a popular question, but I do not understand it: You state that you should have as little impact on the weather as possible. Fitting with that premise you also don't give a height. So why do you ask how it would impact the weather? Shouldn't the answer be "it doesn't"? I'm assuming you are not asking how high one could build such a city and then what the weather would be like below. I'm also guessing heat isn't a problem, the alien city doesn't have a temperature? Anything behaves as if it wasn't there? What doesn't? $\endgroup$ – Raditz_35 Jul 4 '17 at 15:18
  • $\begingroup$ @Raditz_35 I'm asking how high should I place my city to have as little effect on the weather as possible but since there is undoubtedly gonna be some sort of effect, I'm interested in what that effect would be. $\endgroup$ – JanT Jul 4 '17 at 19:00
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    $\begingroup$ @Raditz_35 Decided to change it, thank you. $\endgroup$ – JanT Jul 5 '17 at 11:14
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If the alien city you describe is located at

such a height that the weather of the Earth's surface would be as much similar to what it was before the construction of the pillar city as it can be

it is therefore built above the tropopause. The troposphere is the first layer of the atmosphere where most of weather phenomena take place, and above this we have the tropopause, where occasionally storms can extend

Vigorous thunderstorms, for example, particularly those of tropical origin, will overshoot into the lower stratosphere and undergo a brief (hour-order or less) low-frequency vertical oscillation

Since your city also is transparent to solar radiation it will have no practical effect on the weather. It can actually protect the heart from the consequences of huge volcanic eruption, as it may keep the ashes from diffusing above the troposphere and shielding solar light.

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The planet would boil to death.

If you want to know why, you can read the highly amusing series on http://www.irregularwebcomic.net/396.html which explains how a planet covering city would have no way to get rid of all the waste heat generated.

Now imagine living under this city. Since the alien city will somehow let the light through, which means it's basically invisible(??) you'll live in a giant glass house underneath a furnace.

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  • $\begingroup$ This only holds assuming the city isn't also invisible in the infrared spectrum (for example) or doesn't radiate heat itself. Since it is basically in space already and doesn't even reflect light given the premise, well, I'd be surprised if this was an issue $\endgroup$ – Raditz_35 Jul 4 '17 at 15:39
  • $\begingroup$ If you continue this path, you reach the point where this "city" is about as real as Russells teapot in orbit. $\endgroup$ – ths Jul 4 '17 at 17:49
  • $\begingroup$ @Raditz_35 Oh, it radiates heat. Its just that it can't do it fast enough and you end up doing Black Body calculations. $\endgroup$ – Draco18s Jul 6 '17 at 20:22
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Natural weather would be restricted, since solar rays, wind and rain drops would be blocked.

So light, wind and rain would have to be artificial unless, the upper city had holes that would let the elements pass through.

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    $\begingroup$ I dunno - if the city is high enough clouds could form underneath it, thus allowing rain. $\endgroup$ – Michael Jul 4 '17 at 15:08
  • $\begingroup$ @Michel Well, yes if we in fact ignore the sun blocking part, so if the sun could still pass through and the upper city high enough, the water could evaporate and condense. Also different air pressures could be created thus wind would still be a possibility. And with those conditions full weather could be possible $\endgroup$ – DGaspar Jul 4 '17 at 15:12
  • $\begingroup$ Yeah, and the OP already stated that sunlight gets through. $\endgroup$ – Michael Jul 4 '17 at 16:25
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The weather at the 12 pillars could be interesting, especially if the pillars are as massive (if we get to say they have mass, I guess) as I'm imagining.

What I see happening in the air and water at the pillar locations is similar to the effects on the current seen in a large flowing river spanned by a bridge held up on pillars. The current rushes along its direction as usual everywhere but at the pillars. There, the water immediately downstream from the pillar swirls around and back in a very beautiful way (someone with the right scientific terminology please feel free to describe this phenomena further in case OP would like that detail).

I can then further imagine additional fun yet inexplicable phenomena such as Bermuda triangle mysteries and oddly coincidental locations of visible anchoring structures like pyramids appearing all over the world. Although the pillars themselves are invisible due to reacting differently with photons than the matter we are familiar with, the observable effects such as strange swirling weather phenomena and anchors can provide many great plot launchers.

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I can't know for certain how much of an effect this will have, because you haven't specified the mass of the material, but by adding all this material in a shell around the earth you will be increasing its moment of inertia by enough to change its rotational period. Depending on the amount of change, this could affect global wind patterns caused by the Coriolis Effect. Regardless, if this is modern earth, it would easily be enough to be measurable by atomic clocks, so humans would notice.

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  • $\begingroup$ The OP said "extremely light" so how much would it it have to weight to make a difference? According to quora.com/What-is-the-total-mass-of-the-Earths-crust the earths crust is 0.099%+0.374% of the total mass of the Earth so presumably the city would weigh less than that... $\endgroup$ – Jerry Jeremiah Dec 20 '17 at 23:14
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You are putting the city right about where the ozone layer is (see this wiki).

Most weather would happen below the city. However, we have a heat problem. Underneath the city, we would either cook or freeze (I doubt the two sources of heat will balance evenly). We get heat from two sources, the Sun and our radioactive, molten core. The city will block the heat from the Sun and the Earth's heat will be reflected back from the underside of the city.

Now, assuming the alien's technology is good enough that they can exactly balance the two so the Earth's heat gain/loss rate remains the same, weather, as we know it will stop.

The differential heating from the Sun will no longer be a factor for driving weather. We will have convectional currents from hot air rising from the surface. So, we are likely to see a lot of vertically circular patterns. Most likely the air will rise around the pillars (since they will pick up heat from the crust and radiate it) and fall in the centers away from the pillars.

Also, they have to let enough of the right kinds of light through or plants die. Then herbivores die, carnivores die, we die, and the shrimp around the undersea vents become the inheritors of the planet.

Now the question becomes, why the heck they would bother with such a construction unless they want to fly the finger at Earth through some massive, expensive and spiteful construction.

The air pressure out there is such that it would need to be fully enclosed and pressurized unless they can operate in that low pressure. If they can survive that low of a pressure, they can build their city on the surface of Mars much more cheaply.

Less expensive still would be to just build space habitats for their population. Those would involve a lot less engineering.

From my pilot training, a pilot needs O2 at 12,000 feet if the aircraft is not pressurized to keep from falling asleep at the wheel. Pressurized airlines run at ~35,000 feet. 12 miles is 65,000 feet.

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The sunlight radiation is very important for the climate, so I understand the answer that is saying "no change, because the sun goes through". However, I don't believe this is quite right.

Your alien will need energy. No way out of this. They have computer/fridges/whatever, so they need to power it. If they don't get it from the sun, they need to get it from the planet. Let's consider a few options:

Sun

Ok, you said they won't. But the easiest way for them would be to take some solar energy. Even if they plant trees to feed themselves, trees are actually taking sun energy. And they are shading the earth below. If the sun goes 100% through their city, it means they are 100% transparent (the city and themselves). Highly unlikely.

If you can see them or their cities, they are blocking some solar radiation.

If they have vegetables, trees or solar panels, they are already using solar energy.

Wind

Ok, let's assume they didn't touch the sunlight. Everything is transparent in their cities. The sun goes 100% to the ground. So the first and most obvious energy source would be the wind. However, slowing the wind is not without effect. (Yes, if you take energy from it, you are slowing it). See for instance this article.

Actually, even if they don't take energy from it, it is likely that their cities in the sky are going to affect the wind.

Basically you can count on poorer energy redistribution. Warm places are going to get warmer. Cold places colder.

Water

They could condensate the water up there, directly from the cloud, and use it as it used in dams. So let it drop through their pillars and put a turbine at the bottom of it.

The effect would be to reduce the amount of rain on much of the planet. All the water would go down through pillars, no through rain. Everywhere would turn into a desert. Only exception would be the deserts, as you said the pillars are in there. So the area around pillar basis would get too much water, the rest would be dry.

Also consider the effet of releasing a lot of non-salty water in oceans. Likely to change ocean currents, like stopping the gulf stream.

Anything else

For instance stealing petrol or uranium from us. Will likely result in pollution and wars. Catastrophic for the climate. Just as we know it these days, just worse.

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