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In my world, a population of about a billion people lives in an underground cave that has growing grass, a forest of trees, an immensely deep lake to allow for rain to flow through the water cycle, and an artificial sun that provides light and warmth for everything under the ground. It evaporates the water to allow for rain as well.

My question: Would there be any wind at all that could reach this habitat, or would everything be deathly still? What would the tides on the lake be like, completely motionless?

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    $\begingroup$ If the population respires the same way as on Earth, you could also play around with the volume of the cave for total gas and note that CO2 is heavier than air, which could maybe create drafts. $\endgroup$
    – Raystafarian
    May 2, 2016 at 17:29
  • $\begingroup$ Have you tried the weather underground website? $\endgroup$
    – JDługosz
    May 3, 2016 at 1:28

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Wind is caused by changes in pressure. Changes in pressure are caused, among other things, by changes in temperature. The presence of a "Sun" - artificial or otherwise - will cause variances in temperature, which will undoubtedly cause some sort of changes in pressure.

Furthermore, this massive cave of yours, by its very size, is bound to have many access ways connecting it with other caves, and even the surface. Some sort of wind is basically guaranteed.

As far as tides go, they 100% affect underground bodies of water, much the same as surface ones. Google "tides in underground lakes?" for some very informative articles.

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  • $\begingroup$ I'm having trouble finding a reference, but the Mall of America (in Minneapolis, MN, USA) is a large enough structure that there are breezes inside of it - quite noticeable as you go from area to area (from personal experience). In fact, according to the Wikipedia page, even during winter the air conditioning may be on in places. $\endgroup$
    – Ghotir
    May 2, 2016 at 13:34
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Rather than actual rain, you would more likely get mists and condensation on surfaces.

Also, in open spaces airflow would be much less noticeable and may even be perceived as "still". In narrower areas connecting more open areas, you would have breezes (maybe even winds) moving from higher pressure to low.

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