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Can a world exist where the atmosphere is actually thicker at some altitude than at sea level? If so, what mechanisms would be involved?

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    $\begingroup$ By "thicker" do you mean denser or more viscous? All of the answers so far seem to be assuming density. $\endgroup$ – Gene Feb 13 at 20:50
  • $\begingroup$ @Gene denser. Greater viscosity is optional. $\endgroup$ – Renan Feb 13 at 21:54
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    $\begingroup$ The magic phrase you may or may not have been searching for is Rayleigh-Taylor instability. It is energetically more favourable for the denser stuff to descend and displace the less dense stuff, so any disturbance will result in just that. $\endgroup$ – Starfish Prime Feb 13 at 21:58
  • $\begingroup$ They only method I can think of to allow this would be if the denser gas had a positive or negative charge, and the planet's surface had the same charge, while the less-dense gas had a neutral/opposing charge. I don't think this meets the necessary qualifications to answer a "reality check" question, though. $\endgroup$ – Liesmith Feb 14 at 0:28
  • $\begingroup$ @Liesmith I think the air would discharge the excess charges as lightning, keeping that from happening. $\endgroup$ – Renan Feb 14 at 4:14
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The density profile of a planet's atmosphere arises from two laws of physics: hydrostatic equilibrium and the ideal gas law. Put together, they require that the density $\rho(z)$ be a function of the form $$\rho(z)=\rho_0e^{-z/H}$$ where $H$ is the scale height, determined by the planet's surface gravity, composition, and temperature. The scale height is inversely proportional to temperature; on Earth, temperature increases and decreases in different layers of the atmosphere, but not at a quick enough rate to cause the density to decrease with height at any altitude.

A dense layer of the atmosphere could briefly form if there was extremely rapid cooling at a particular altitude. This could be maintained only by a complete lack of heat transfer via convection or radiation from nearby layers, which might require a thick band of optically thick clouds. However, presumably the atmosphere would eventually bounce back into equilibrium as the heat would be transferred back to the cooler layer.

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In general, I think the answer is "no"; buoyancy simply doesn't work that way, and you'd need to somehow circumvent buoyancy.

What might be plausible is to have a low spot in your terrain that is completely surrounded by higher terrain (maybe a caldera?) which, for some reason, is filled with a higher density substance. If this substance is unable to escape the basin, you might have higher pressure in the basin than at sea level, but this would be a very localized thing.

We do actually (sort of) have examples of this phenomena here on Earth. We usually call them "lakes"... although what they are full of isn't technically "atmosphere".

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To keep "thicker" layers up, the atmospheric system has to expend energy.

Consider dust clouds, or rain clouds. For a brief time, things that should fall to the ground can be kept in the air, but sooner or later in rains down.

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No, not without breaking the effect of gravity. Higher pressure at lower altitudes occurs because of gravity, and while this law can be temporarily disturbed by the asymetrical heating of the earth, weather systems are physics' way of constantly reverting the system back to higher pressure on bottom, lower pressure on top.

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    $\begingroup$ in fact we have a name where it is reversed ... the so called "inversion" weather .. where warm light air is layered on top of cold dense air - this can be stable for multiple days - but usually the warm air on top cools and thus ends the inversion - helped by winds $\endgroup$ – eagle275 Feb 14 at 12:44
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This kind of pressure inversion can happen at night over bodies of water, as the water releases the heat it stored during the day. The inverted density gradient in the atmosphere helps sound travel by reflecting it back down. I remember doing something about it at Uni 30 years ago, but I've forgotten the equations, something to do with refractive indices.

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