I'm confused about the relationship between air density, air pressure and gravity. Does water evaporate faster under low air pressure simply due to the air density being lower? If the air pressure was lower due to lower gravity rather than lower air density, how would that affect evaporation? I'm imagining an alien planet with 75% of earth's gravity, but the same air density.
4
-
$\begingroup$ Do you only want to keep the surface density the same, or do you want to change temperature so the exponential decay in density also has the same length scale, so the density is the same as in the real world regardless of altitude? $\endgroup$– J.G.May 25, 2022 at 21:56
-
$\begingroup$ J.G. - I want to keep the surface density the same. But I don't undersatnd the second part of that sentence. What do you mean " so the density is the same as in the real world regardless of altitude"? Do you mean mimicking the atmospheric density gradient of Earth, so it's the same as earth at every altitude? I assume this wouldn't happen because my atmosphere would be expanded due to weaker gravity. Also how does temperature affect the exponential decay in density? $\endgroup$– ElhammoMay 26, 2022 at 16:12
-
$\begingroup$ See here. $\endgroup$– J.G.May 26, 2022 at 16:14
-
$\begingroup$ J.G. - It says, "In these equations, g0, M and R* are each single-valued constants," so they're based on earth's gravity, right? Also, honestly I'm not great with math (nor do I have anything better than a simple phone calculator), so if you have an idea what the answer to this question might be, lmk: If surface gravity was lower, but surface air density was the same, do you think that would affect evaporation? And how? Thanks so much for the help, btw! $\endgroup$– ElhammoMay 26, 2022 at 17:45
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
2
Simple answer A phase diagram shows how the phase of water varies with pressure and temperature. Reduced pressure, regardless of cause, increases evaporation at the surface temperatures common on Earth (excluding those that form ice).
Complication On your low-gravity planet, each extra kilometre of altitude gives less density reduction (the length scale for an $e$-fold reduction is proportional to $1/g$) and more temperature reduction (this time proportional to $g$), so the atmosphere is colder and thicker than Earth's at a given above-surface altitude. These have opposing effects on how readily high air holds evaporated water. If we try to calculate the overall effect, we encounter further complications. For example, at reduced temperature the air thins more slowly, and moist air has a more complicated rule for how temperature falls at increasing altitude.
-
$\begingroup$ Thanks for that explanation! Hm maybe there are too many factors involved here. What if I'm okay with less surface air density... So the air density is less at surface, changing more slowly w/ increase in altitude. How would that affect precipitation? I know air at lower pressures holds more vapor before condensing, so would that mean the clouds would condense higher up than Earth clouds? Also, normally I think of cold air as being drier. I'm a little confused because when you lower the pressure, temperature is also lowered - but in this case, even with cooler air, you get more humidity? $\endgroup$– ElhammoMay 26, 2022 at 20:27
-
$\begingroup$ "the air density is less at surface, changing more slowly w/ increase in altitude" - the lower pressure would increase evaporation further, but the relative change would be the same. I'm not sure about the rest of your questions. $\endgroup$– J.G.May 26, 2022 at 20:31