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In the troposphere, which has the bulk of the atmospheric mass, it's colder at higher altitudes due to adiabatic lapse. Gas giants have several different chemicals that condense into cloud decks at various altitudes. On Jupiter, for example, water clouds occur about 100 km below ammonia clouds.

Suppose a gas giant got so cold that Hydrogen and finally Helium began to condense in the upper atmosphere. This does not happen in our solar system because there is too much sunlight and internal heating, but it would likely be the fate of an orphan planet left to cool for a long enough time. Unlike the case with water or methane, the entire upper atmosphere tries to condense into cloud droplets. How would such an atmosphere behave?

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    $\begingroup$ The interiors of many gas giants are already most likely condensed into a metallic phase of hydrogen and above that there is a supercritical layer in which there is no difference between gas and liquid phase. The transition between the two phases around the critical point is quite amazing, which can be demonstrated in the lab with carbon dioxide: youtube.com/watch?v=yBRdBrnIlTQ, youtube.com/watch?v=GEr3NxsPTOA. I find this to be one of "coolest" experiments in all of physics. One can only imagine what it looks like at the scale of weather systems the size of earth! $\endgroup$ – CuriousOne Jun 1 '16 at 22:50
  • $\begingroup$ @CuriousOne I assume as it cooled there would be a time window where you would have a normal gas layer between the growing liquidish/solid interior and the condensing upper atmosphere. The causes of those too liquids are very different. $\endgroup$ – Kevin Kostlan Jun 1 '16 at 22:54
  • $\begingroup$ Hydrogen and Helium atoms are relatively easy to ionize and if in equipartition with other gases, would have a higher thermal speed. If ionized, they can be extracted from the atmosphere by electromagnetic forces that greatly exceed those of gravity. Therefore, they are generally more difficult for planets to "hold on to" than more massive atoms. $\endgroup$ – honeste_vivere Jun 3 '16 at 12:06

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