Does Monk Unarmored Movement work on snow?

Question

Monk's Unarmored Movement says that at 9th level

[...] you gain the ability to move along vertical surfaces and across liquids on Your Turn without Falling during the move.

Is there anything in the rules or otherwise saying, if this should work over deep snow, same way it would work over liquid water?

I guess moving on quicksand or canvas tent/awning roof would be other similar cases, so anything covering this would help too. They're not liquid and not vertical, so by the above quoted text alone, the monk would sink/crash/fall.

Is there anything better than "well, thematically it would fit and be so cool and not really unbalancing" to say to the DM when asking them to allow running on snow the same way as on water?

My DM is generally reasonable and likes to run a consistent world, so any official or semi-official material, where this or similar ability allows moving on snow (or quicksand or something) without sinking should be enough for them to want Unarmored Movement to follow suit.

Answer 1

This works, the spell water walk defines snow as a liquid for mechanical purposes.

Water walk says:

This spell grants the ability to move across any liquid surface--such as water, acid, mud, snow, quicksand, or lava

Here snow is described as a liquid for mechanical purposes. I argue that this demonstrates that for mechanical purposes, the rules are respecting snow as a liquid.

Thus, I conclude that Unarmored Movement does work on snow, as snow is elsewhere described as a liquid in a mechanical context.

In the comments Mark Wells raised an objection worthy of consideration. He writes,

[Water walk] provides that list for the purpose of the spell it's describing. The context is that it's explaining how water walk works, not providing a general definition of the word 'liquid'.

My argument relies on an important assumption, based on the "Spells only do what they say they do" principle: a spell only changes the mechanical nature of something if it says it does. Else, we assume the nature of the thing is used consistently within and without the spell's description.

An example of this can be seen in the spell storm of vengeance. The effect during rounds 5-10 of storm of vengeance state:

Gusts and freezing rain assail the area under the cloud.

Under normal circumstances, environmental phenomena such as wind and rain do not require a caster to make concentration checks, unless the DM determines they should. Storm of vengeance specifies a change in the nature of this mechanic when it says:

The wind and rain count as a severe distraction for the purposes of maintaining concentration on spells.

The spell description specifies that we treat the nature of wind and rain differently for the purpose of the spell.

Therefore, since water walk does not specify that we are respecting snow (or any of the other terrains listed) as mechanically unique for the purposes of the spell, we conclude that the rules are consistent in their treatment of snow both within and without the spell description of water walk.

Answer 2

No

RAW, this lets you move across liquids. As you note yourself, snow is not a liquid. Quicksand could be argued to be a liquid. A canvas tent isn't a liquid.

The feature doesn’t really say why you can move across liquids and vertical surfaces so depending on what kind of internal logic you ascribe to it you might argue with your DM one way or the other (as the feature mostly relates to speed the implication is that you just move very quickly, which isn’t entirely implausible even though of course this isn’t possible for real world humans, but this is never made explicit), but the rules as written are pretty clear.