Can an atmosphere be thicker at higher altitudes?

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".

posted about 4 years ago

Matthew‭

27 reputation 8 46 3 1

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.

posted about 4 years ago

HDE 226868‭

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