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I am creating a board game that requires an underground cavern world, with huge pipes that pump air for the civilization living there. The underground region is at about 35-40 degrees north latitude. I am wondering what sort of climate such a region would have. Is it really just no weather?

I am thinking that there will be convection from the air pumps, and if the ceiling is high enough, clouds of mist might form if it is humid enough. There will probably be bio-luminescent organisms, or rat-like creatures. The climate will be relatively cold for its latitude; because there is no sunlight, the only sources of heat will be the giant air pumps.

Does anyone have a similar logic for the climate, or is it different from what I thought?

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    $\begingroup$ You should research the New York City waste treatment facilities. They have underground settling tanks so large that they do have weather systems. Good source material for these sorts of questions. $\endgroup$
    – SRM
    Feb 6, 2018 at 4:19

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Underground temps vary little

The average annual temperature in Chicago is 51 F (11 C). If you are in a cave a mile below Chicago, it will be 51. It will be 51 in the day, and at night; in summer and winter. The average annual temperature on the surface will be roughly the temperature of your cave. If your cave is below Delhi, it will always be 25 C; if it is below Moscow it will always be 6 C. Pick surface climate and that will determine your cave temps.

Humidity depends on how much water is in your cave

The partial pressure of water in the air for a closed system will be dependent on the surface area of exposed water. If there are lots of underground lakes and rivers, humidity will be high; little of these and it will be low. Transpiration by plants would increase humidity, but without a sun, I don't know what kind of plants you might be thinking of.

You will never get mist

The chances of getting mist are pretty low in anywhere with a roof. When air cools, its ability to retain water drops. On the surface, air can move upwards and cool as pressure decreases; in a cave, air will cool by coming in contact with the walls. Once there, any moisture it loses will end up on the walls. You will get condensation, and possibly 'rain' as condensed water drips off of the ceiling, but not clouds of any sort. Since temperatures are steady, any area with rain will almost always be raining. Any area that isn't raining will almost never be raining.

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  • $\begingroup$ An unmodified cave in Chicago would stay 51. But if you are actively pumping outside air into the cave would cave temps not vary with the outside air you pump in? $\endgroup$
    – Willk
    Feb 6, 2018 at 16:37
  • $\begingroup$ @Will Not as much as you might think. The mass of air is low, but the mass of things in the cave (walls, buildings, rivers, whatever) is high. There isn't that much heat capacity in the air, compared to the things that are staying at about 51 F. $\endgroup$
    – kingledion
    Feb 6, 2018 at 18:22

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