In this answer, I describe a low-density water world with a surface gravity of 1g and a surface atmospheric pressure of 20 ATM. The air temperature at the 1 ATM altitude (naively, assuming a temperature lapse rate of 0) of 25.25 km is 15°C.

However, if we assume a temperature lapse rate of 5°C/km, which is typical on Earth for wet air, which is highly probable on a water world, this would mean that (again, naively) the surface temperature would be about 141°, and at 20 ATM, the boiling point of water would be about 210°C. This would be the equivalent of having a surface temperature of about 67°C at 1 ATM, far closer to the boiling point of water than 15°C

So, I'm wondering if the temperature lapse rate might not reverse at some point in the lower atmosphere below the 1 ATM altitude, with a cloud layer increasing the planet's albedo and reducing the solar energy that reaches the surface.

Would this be a reasonable assumption? If so, at what altitude might such a lower cloud layer form?

Secondly, is it reasonable to assume that the weather on this planet would be more dramatic than on Earth, with higher winds and rainfall, even 25+ km above the surface?

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    $\begingroup$ You are putting a lot of values out there but with a gravity of 1G, how are you expecting to get a 20atm air pressure? Are you expecting a different composition of the atmosphere? $\endgroup$
    – Michael H.
    Nov 23, 2021 at 21:49
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    $\begingroup$ @MichaelH. Just a lot more air... pretty much the same composition, with a little Helium. $\endgroup$
    – Monty Wild
    Nov 24, 2021 at 0:01
  • $\begingroup$ You would start having your atmosphere would not work out like you think. $\endgroup$
    – Michael H.
    Nov 24, 2021 at 15:05
  • $\begingroup$ @MichaelH. Gravity and atmospheric pressure are almost entirely unrelated. Venus is smaller than Earth and has 90 times more air. Titan is much smaller than Earth, and still has about 4 times the surface pressure. "Just add more air" is completely reasonable, especially for a water world which obviously had more volatiles to start with. $\endgroup$ Feb 23 at 0:07
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    $\begingroup$ @MichaelH. Uh... no, it's not. Air is highly compressible, which is really, really easy to personally verify by squishing a syringe or a pneumatic cylinder. You can compress air quite significantly with just finger strength, and there are practical consumer products which depend on the high compressibility of air. Just add more air, and the pressure (and, yes, also the density) will go up, without having to change gravity at all. Total atmospheric mass is a far larger contributor to surface pressure than gravity is. $\endgroup$ Feb 25 at 17:12

1 Answer 1


This should be reasonable, as geoengineering already provides us with the answer, a reflective cloud layer that is very reflective should be able to reflect solar radiation, decreasing temperature on the surface, the humidity of the planet means that cloud formation is completely possible. The density of the atmosphere means that the cloud layers would be pretty low as the water vapour is pushed down by the atmospheric weight. Judjing by cloud layers on earth and a 20 atm atmospheric pressure, I'd say the clouds would be at a height of around 1km on the high end.


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