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If two planets were tidally locked and shared an atmosphere, as described in this question, what would the weather patterns of the planets be like? What weather patterns would form, and could these weather patterns travel from one planet to the other?

For the sake of simplicity, assume both planets are Earth-like in composition and mass, with high amounts of water on the surface, liquidy interiors, metal cores, humid oxygen/nitrogen-filled atmospheres etc. One can assume that the planetry pair are about the same distance from there star (which is of the same type as the Sun) as Earth is from the Sun.

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  • $\begingroup$ humid oxygen? What should it be? $\endgroup$
    – L.Dutch ♦
    Commented Mar 20, 2019 at 3:30
  • $\begingroup$ I think tox123 meant an athmosphere made up by oxygen and nitrogen, which contains noticable amounts of water vapour. $\endgroup$
    – DarthDonut
    Commented Mar 20, 2019 at 7:43
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    $\begingroup$ @L.Dutch I think that entire section between the preceding comma and the period is supposed to be a description of the atmosphere all together. "humid oxygen and nitrogen filled atmospheres" So it would be an atmosphere with lots of oxygen and nitrogen and it would be humid. I don't think humid is an adjective for oxygen in that sentence. $\endgroup$
    – Loduwijk
    Commented Mar 20, 2019 at 20:31
  • $\begingroup$ @L.Dutch Is that edit better? tox123: If I misunderstood, feel free to roll back the change. $\endgroup$
    – Loduwijk
    Commented Mar 20, 2019 at 20:33

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Planets sharing an atmosphere would produce a highly unstable situation. At the closest point between the two planets gravitation would be negligible and would result in huge amounts of water, foam, rock and debris being drawn up into the atmosphere.

It is likely that this region would form a rotating storm between the two planets. Given the high energy content of storm systems and the very low net gravitational forces in the area it would result in a storm of biblical proportions and would absorb any storms approaching it from either planet. No Storm would be able to cross this boundary.

Weather would be dominated by the ring storm between the two planets. The weather systems would be powered by the energy from the sun coupled with the difference in ground speed between the point of closest approach and the far side of each planet (similarly on Earth there is a difference in ground speed between the poles and the equator that drives our climate).

This would cause all of the weather systems on each planet to spiral from the far side towards the point of closest approach where they would be consumed by the world storm system circulating in that area. Similarly on Earth storm systems spiral away for the fast moving equator to the stationary poles.

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  • $\begingroup$ I'm sorry, this is just wrong. Yes there would be a zero g point between the planets but that would not extend down to the two surfaces. $\endgroup$
    – Tim B
    Commented Mar 20, 2019 at 11:40
  • $\begingroup$ No it is correct. I said negligible not zero. With atmospheres touching the gravitational forces of each planet would almost cancel out so it would take very little energy to move huge amounts of material up in the air and a storm system contains huge amounts of energy. $\endgroup$
    – Slarty
    Commented Mar 20, 2019 at 11:48
  • $\begingroup$ It would be cancelling out at the point where the atmospheres touch. When you descend from that point towards the surface gravity would increase until you had real gravity by the point you reached the surface. The gradient would be different as you rose but surface gravity would be present. $\endgroup$
    – Tim B
    Commented Mar 20, 2019 at 12:01
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    $\begingroup$ The net gravitational acceleration difference between a point on the surface of the Earth and a point 50km up is around 0.15m/s/s. This would be the net gravitational force on the surface of both planets at the point of closest approach – about one tenth of the gravitational acceleration on the surface of the Moon. Imagine a storm under those conditions. $\endgroup$
    – Slarty
    Commented Mar 20, 2019 at 12:15
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    $\begingroup$ In the classic rocheworld configuration there is an 80km separation and 6 hour rotation period - so yeah you are right, there is a centripetal effect but it's only 0.00034g. Interesting. I've just tried doing some research on the effective g force across the rocheworld surface and not found any good sources. $\endgroup$
    – Tim B
    Commented Mar 20, 2019 at 17:36

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