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Disclaimer: I am not a meteorologist.

Snow forms when supercooled water droplets nucleate onto particles in the atmosphere. After forming, they grow further by both tumbling around within the cloud and by tumbling down through the atmosphere, colliding and mingling with other snowflakes throughout their way. At journey's end, the snowflakes aggregate on the ground or whatever surface they happen across, building snow cover.

From my own understanding, there are a few factors in this process that are dependent on gravity and air pressure.

  • I suppose the first thing to consider is that an atmosphere of greater pressure may have a greater water vapor partial pressure. (This, incidentally, leads to water having a higher evaporation temperature, which is a whole can of worms we don't have to open. For more information, see this question). The takeaway from this is that the boiling point of water increases while the freezing point stays close to its original figure of zero Celsius, and that the atmosphere may retain more water for its greater pressure than otherwise. (Equations for estimating vapor pressures, if it's relevant.)

  • A second thing to consider may be the effect of both lower gravity and greater air pressure on a snowflake after it has formed. With lower gravity, the snowflake takes more time to fall through the atmosphere, probably increasing the rate of snowflake-snowflake collisions. Simultaneously, the greater air pressure is responsible for greater drag on the snowflake as it falls, slowing its descent further (I am unsure how this affects the rate of snowflake collisions). With this, I think it is important to note that, under lower gravity, clouds may tend to form higher up in the atmosphere and may tend to be taller, giving snowflakes even longer travel distances. See this question.

  • A third thing that may need consideration is the effect of a lower gravity on snow cover aggregation. Under lesser gravity, snow might be lesser inclined to compact itself on the surface.

What could snow be like on a low-gravity and high-pressure world?


A general case answer is acceptable, one which describes the snow and its various meteorological changes associated with raising atmospheric pressure and dropping gravity.

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  • $\begingroup$ Can even you have low gravity and high pressure together? Isn't the weight of the atmosphere what causes the pressure? $\endgroup$ – colmde May 3 at 7:54
  • $\begingroup$ You are correct. gravity is the dominant term that determines the atmospheric pressure of a planetoid. Temperature, and gas constants also have a contribution since they establish mean free path and velocity of atoms.molecules. High temp can mean a low gravity planetoid loses atmos because gases achieve escape velocity $\endgroup$ – EDL May 5 at 6:14
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    $\begingroup$ @colmde: The atmospheric pressure is mainly determined by how much atmosphere there is and it's temperature. Venus has about same size and gravity as Earth, but the atmospheric pressure at the surface is about 100 times greater. $\endgroup$ – AlexP May 5 at 9:47
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    $\begingroup$ @EDL Titan has lesser gravity than Earth and yet has a higher atmospheric pressure than Earth. Temperature plays a sizeable role too, like AlexP says. $\endgroup$ – BMF For Monica May 5 at 11:56
  • $\begingroup$ @BMF, you are correct. I only considered the limiting effect of higher temperature in my comment. $\endgroup$ – EDL May 5 at 13:43
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The pressure is not a big factor. From water phase diagram [1] you can see the solid-liquid boundary is at same temperatures in wide span of pressures. You need to reach extreme pressures - gigapascals - to get different behavior.

Most depends on atmosphere temperatures which should dip below zero in the altitude where clouds form, but above zero on surface to allow for plentiful evaporation.

http://www1.lsbu.ac.uk/water/water_phase_diagram.html

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  • $\begingroup$ How would gravity affect the formation of snow? I believe points 2 & 3 of mine are relevant factors, but because I'm not a meteorologist and don't study these things, I can't know for sure. Is gravity not a big factor as well? Why or why not? $\endgroup$ – BMF For Monica May 4 at 13:24
  • $\begingroup$ Did not do the math nor experiment, but I don't see how would ice condensation from saturated water vapor be changed by gravity (in the range usual on planets) in any way. $\endgroup$ – Juraj May 5 at 19:23
  • $\begingroup$ What about the aggregation of snowflake particles in free fall/on the ground? I think this is an important factor. $\endgroup$ – BMF For Monica May 5 at 19:33
  • $\begingroup$ Of course it would be denser or fluffier on average, but there's no thing such as average snow. There are very wide range of snowflake sizes and densities on Earth, this would likely happen with any gravity and pressure. However with high pressure you'd get also high lapse rate, this means higher temperature rise with depth as on Earth. So any snow would melt quickly before reaching the ground. See en.wikipedia.org/wiki/Lapse_rate $\endgroup$ – Juraj May 5 at 19:42
  • $\begingroup$ Definitely important to note. I think you should add it into your answer. Whether the snow melts before it hits the ground is dependent on the air it's in of course. A denser atmosphere can hold more vapor and thus more heat, unless the atmosphere is sufficiently frigid. So, you're probably on the right track there. $\endgroup$ – BMF For Monica May 5 at 20:01
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Your question has a contradiction in it.
The title asks about low g and high P, but in your text you ask about low g and low P.

But its Okay, because I give you Enceladus, the Ocean Moon of Saturn.

It snows H2O there because of its combination of high pressure and low pressure conditions, in a lowish g environment

https://solarsystem.nasa.gov/missions/cassini/science/enceladus/

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  • $\begingroup$ I repeatedly say low g and high p. I can see that on my second point "... the effect of both lower gravity and air pressure..." would imply low p, but this is a typo on my part. Every other mention (six mentions) of p states high p, including the title and question itself, so it would be very safe to assume that I do mean high p, not low p. Edit: I will edit the question to remove the typo. $\endgroup$ – BMF For Monica May 5 at 11:54
  • $\begingroup$ Found another typo in my question. Sheesh, sorry for the confusion, I don't know what got into me when I typed it up. I respect your answer though. I'm almost tempted to ask a separate question about "snow" on a low-p, low-g world. It's very intriguing. $\endgroup$ – BMF For Monica May 5 at 14:07

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