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As title. What geographic features could cause a stable pocket of unmoving air?

I'm envisioning some kind of shrine, sacred place, or untouched grotto, which due to terrain features or certain aspects of the environment, has a naturally occurring, local weather system. In this system, there is to be a localised area in which air becomes still. This should occur predictably, if it's impossible for it to occur permanently. What kind of geographic features, elements of the environment, or other factors, might cause such a place to exist, where no wind blows and the air is as still as possible?

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    $\begingroup$ Does a glass dome constitute a geographic feature? $\endgroup$
    – Mathaddict
    Commented May 12, 2023 at 18:17
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    $\begingroup$ define unmoving, even sealed underground caves have circulation due to thermal differences. $\endgroup$
    – John
    Commented May 12, 2023 at 21:29
  • $\begingroup$ How big does this pocket need to be? Bedroom? Cathedral? Stadium? $\endgroup$ Commented May 12, 2023 at 22:17
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    $\begingroup$ I could swear we had a question the same as this a year or so ago... will have a look $\endgroup$ Commented May 13, 2023 at 4:56

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Caves with underwater entries

Some caves are isolated by a waterway, resulting in the air inside to be isolated, always at the same temperature and, if the water does not move, perfectly still. For the air to not escape through other cracks, the geology of the cave needs to be in such a fashion that it has little to no cracks.

If the waterway falls dry for some time of the year, then the air inside will move for that duration, but otherwise be perfectly still.

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    $\begingroup$ This is likely to result in an underground pocket of non-breathable gases unless it happens to be incredibly lush with plant life to filter the oxygen, which in turn implies the need for an entire self-contained ecosystem. $\endgroup$ Commented May 12, 2023 at 22:11
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A Deep And Narrow Pit at High Latitude

To shelter a location from the wind, it needs to be lower than the surrounding terrain on all sides. If it is lower, then wind will tend to travel above the floor of the valley. This effect is most significant when the valley air is colder than the surrounding air. To create this temperature difference, one needs to reduce the amount of sunlight which reaches the floor of the pit.

There are two effective ways of reducing the amount of sunlight which reaches the center of the pit. The first is to make the pit deeper while keeping the area constant. This makes it harder for sunlight to reach the bottom of the pit. With less sunlight, the pit will remain colder year round, making it more likely that outside winds will just blow over. The second is to place the pit at a high latitude. At high latitudes the maximum solar elevation angle can be lower than 30 degrees. If your pit's walls are almost cliffs, then at the bottom you can have a permanently shadowed area. This pocket will be much colder than the outside. Since cold air is denser than warm air, it will remain at the bottom of the pit and be stagnant.

For this pit to be stable over a long period of time, your story would have to occur in a hot high latitude desert. This pit is a basin with no outlet and minimal sunlight. Unless rainfall and snowfall is extremely low, it will fill with water and become a lake. For a hot high latitude desert to exist, your planet needs to be much warmer and much drier than Earth. In essence, your planet's oceans need to have evaporated off into space long ago.

The pit will likely be a refuge from the harsh conditions, having abundant slow growing vegetation adapted to life within the crater. For desert dwellers, it will be an alien environment completely unlike any other part of their world, well worth the descent down sheer walls.

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For a large scale example, see the article on Temperature inversion. A warm layer of air can trap a cold layer beneath it. If you are in a valley, this can trap your air. You can get a temperature inversion with natural weather conditions, but they can also be caused by a photochemical smog, which warms the upper atmosphere. The temperature invasion does not stop wind, but it can give a still lower layer.

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Bubble-Creating Lifeform whose bubbles harden after creation

Imagine a crab that builds a burrow, perhaps to lay its eggs in, and its secretions harden the walls. Separate secretions from the newborn (after devouring the parent of course) would let them out. This would be an anoxic environment.

Micro-biology could probably do a similar thing, but it would have to be a very specific soil in order to maintain its shape instead of the bubble popping. Possibly the microbes need to maintain specific atmospheres for specific life stages. This could be done on larger scale with cooperatives like Portugese-man-of-war , or mats of bacteria or algae that had slow reproduction and multiple life stages. Or perhaps they receive energy from radiation that can penetrate the bubbles but direct exposure is dangerous , or perhaps they enjoy neutron-activated specific minerals but not the originals.

In any case, "still pocket of air" is the same as "completely covered and sealed" because air always moves.

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