In what cases are the clouds in the sky considered terrain?

Question

Based on the question and answer here, regarding the uses of Illusory Reality and Hallucinatory Terrain, the question that popped into my mind is:

Are clouds in the sky "terrain" such that they may be the object of those two spells? In some cases flying creatures might treat them as such.

Hallucinatory Terrain:

You make natural terrain in a 150-foot cube in range look, sound, and smell like some other sort of natural terrain.

Since clouds obstruct vision and have something in them (even if it's only bits of water) it appears that this spell can change part of a cloud.

Illusory Reality:

You can choose one inanimate, nonmagical object that is part of the illusion and make that object real. The object remains real for 1 minute. The object can’t deal damage or otherwise directly harm anyone.

While this application seems mostly related to aerial combat, or detection, do these spells affect clouds in that way (opening up other useful applications of these spells?) Is there a reason to believe these two spells would not so affect/interact with clouds?

Answer 1

There is no definition of terrain in the rules so we fall back on normal English. The OED (both US and UK versions) defines terrain as:

A stretch of land, especially with regard to its physical features: they were delayed by rough terrain

Now, here on Earth, clouds are not land but in D&D, who knows? I mean cloud Giants can build castles on and out of the stuff so those clouds probably are land.

Answer 2

It's unlikely that they would affect clouds by-the-book. I can't see a reading that would support that, given the meaning of the word “terrain.”

But, D&D 5e is fairly toolkit-y, and it would be a plausible table interpretation that aerial terrain counts. In which case, the most sensible interpretation of “other… terrain” in the spell description would be “other… (aerial) terrain”, making it a slightly more flexible spell, but not have uses that are outrageously different than the default reading.

Similarly, it would seem reasonable to be able to substitute one Hellscape terrain for another (“I would like to replace the blasted earth with a lava lake”), or to replace a swathe of coral reef with a kelp forest.

(Though of course, once house rules are involved there's nothing stopping a group from deciding that they can put an upside-down gingerbread forest in place of a hurricane using the spell, since people can houserule how they like. It's obviously a much more extreme adjustment to the spell though, is my point, than just letting it swap one environmental terrain type for some other terrain of the same environment.)