I'm working on a science fiction project and hoping someone who knows something about climate can answer this question for me: if Earth had 25% less gravity, how would that affect precipitation? I read that air at lower density holds more water vapor. Would this mean a lower gravity version of our planet would be more misty? Would it rain more?

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    $\begingroup$ This question would be better to ask on the Worldbuilding StackExchange site. $\endgroup$ May 25, 2022 at 11:22
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    $\begingroup$ I’m voting to close this question because it belongs in worldbuilding.stackexchange.com $\endgroup$
    – arkaia
    May 25, 2022 at 13:41
  • $\begingroup$ Please don't. I came here because world building wasn't giving me the answer :( $\endgroup$
    – Elhammo
    May 25, 2022 at 14:30
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    $\begingroup$ I’m voting to close this question because this was cross-posted to Worldbuilding SE, and they've given you answers there. $\endgroup$
    – Spencer
    May 25, 2022 at 16:13
  • $\begingroup$ @Spencer - I'm trying to get as much good info a possible, and I assume there are more actual earth science experts here. Why does it matter if I ask this question here or there or both places? $\endgroup$
    – Elhammo
    May 25, 2022 at 16:36

1 Answer 1


I read that air at lower density holds more water vapor.

I suspect you may have misinterpreted what you read. Given two air masses at the same pressure and temperature but with different humidity levels, the air mass with the higher humidity level will be have a lower density than the air mass with the lower humidity. This is because $\ce{H_2O}$ has a molar mass of about 18 grams/mole while nitrogen gas ($\ce{N_2}$) and oxygen gas ($\ce{O_2}$) have molar masses of about 28 grams/mole and 32 grams/mole, respectively. This difference in molar mass is what makes high humidity air less dense than low humidity air, for air masses of the same pressure and temperature.

  • $\begingroup$ Thanks for the clarification! I think I'm still a bit confused though. What if the air pressure is decreased? Would there be more water vapor due to water evaporating more easily under lower air pressure? Also, how could lower density air be at the same pressure as higher density air? At the same gravity and a lower molecular weight, shouldn't the pressure be lower if the density is lower? $\endgroup$
    – Elhammo
    May 25, 2022 at 16:33

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