<blockquote>
<p>I read that air at lower density holds more water vapor.</p>
</blockquote>
<p>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 <span class="math-container">$\ce{H_2O}$</span> has a molar mass of about 18 grams/mole while nitrogen gas (<span class="math-container">$\ce{N_2}$</span>) and oxygen gas (<span class="math-container">$\ce{O_2}$</span>) 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.</p>

CC BY-SA 4.0
2022-05-25T16:33:07.110Z
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?
Elhammo
CC BY-SA 4.0
2022-05-25T12:33:49.677Z
2022-05-25T12:33:49.677Z
David Hammen
<p>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?</p>

CC BY-SA 4.0
2022-05-25T11:22:24.513Z
This question would be better to ask on the [Worldbuilding](https://worldbuilding.stackexchange.com) StackExchange site.
David Hammen
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2022-05-25T13:41:58.480Z
I’m voting to close this question because it belongs in https://worldbuilding.stackexchange.com/
arkaia
CC BY-SA 4.0
2022-05-25T14:30:16.763Z
Please don't. I came here because world building wasn't giving me the answer :(
Elhammo
CC BY-SA 4.0
2022-05-25T16:13:53.533Z
I’m voting to close this question because this was [cross-posted to Worldbuilding SE](https://worldbuilding.stackexchange.com/questions/230487/how-might-a-lower-gravity-planet-sustain-a-warm-earth-like-atmosphere), and they've given you answers there.
Spencer
CC BY-SA 4.0
2022-05-25T16:36:53.487Z
@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?
Elhammo
CC BY-SA 4.0
2022-05-26T00:09:32.980Z
As others alluded to, Earth Science SE tends to be focused on reality rather than hypotheticals, that's why it's not a good fit here (it's a bit like asking "what if humans had a different immune system" in a medical group, or asking the bus driver how he'd drive if he had a different route.... it can be fun for some to daydream, but most like to focus on more practical realities, so there's a special site for those who like to focus on such abstract theorizing).
JeopardyTempest
CC BY-SA 4.0
2022-05-26T00:10:47.720Z
And StackExchange generally encourages a single post at one site because this means one thread with the full information rather than bits and pieces in different spots, and prevents duplication of efforts (you can post a question link in chat at other sites if it would be topical).
JeopardyTempest
<meteorology><planetary-science><precipitation><gravity><hypothetical>
How might 25% lower gravity affect precipitation on an Earth-like planet?
Elhammo
# How might 25% lower gravity affect precipitation on an Earth-like planet?

(Asked by: username_0 on 2022-05-25)

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?

**Tags:** meteorology, planetary-science, precipitation, gravity, hypothetical

> **Comments:**
> **username_1**: This question would be better to ask on the [Worldbuilding](https://worldbuilding.stackexchange.com) StackExchange site.  
> **username_2**: I’m voting to close this question because it belongs in https://worldbuilding.stackexchange.com/  
> **username_0**: Please don't. I came here because world building wasn't giving me the answer :(  
> **username_3**: I’m voting to close this question because this was [cross-posted to Worldbuilding SE](https://worldbuilding.stackexchange.com/questions/230487/how-might-a-lower-gravity-planet-sustain-a-warm-earth-like-atmosphere), and they've given you answers there.  
> **username_0**: @username_3 - 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?  
> **username_4**: As others alluded to, Earth Science SE tends to be focused on reality rather than hypotheticals, that's why it's not a good fit here (it's a bit like asking "what if humans had a different immune system" in a medical group, or asking the bus driver how he'd drive if he had a different route.... it can be fun for some to daydream, but most like to focus on more practical realities, so there's a special site for those who like to focus on such abstract theorizing).  
> **username_4**: And StackExchange generally encourages a single post at one site because this means one thread with the full information rather than bits and pieces in different spots, and prevents duplication of efforts (you can post a question link in chat at other sites if it would be topical).  

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## 1 answers

### Answer by username_1 on 2022-05-25. Score: 2

> 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.

> **Comments:**
> **username_0**: 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?  

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