# If gravity was lower but air density was the same, how would evaporation be affected?

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.

• 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?
– J.G.
May 25, 2022 at 21:56
• 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? May 26, 2022 at 16:12
• See here.
– J.G.
May 26, 2022 at 16:14
• 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! May 26, 2022 at 17:45

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.