Is the sky a naturally occurring terrain?

Question

This question is in regards to the effect of Hallucinatory Terrain which states:

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

in combination with the illusionist's 14th level ability Illusory Reality which states:

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.

So the question is: Can a wizard replace a 150-foot cube of mountain with sky by using the spell and ability? and could a wizard replace 150-foot cube of sky with mountain (in the same way as above) assuming the mountain didn't fall on top of anyone?

Answers should be made considering that the way in which Illusory reality manifests the illusion is by weaving shadow magic into reality (therefor it cannot create a vacuum as noted in other question on this site i.e. no using plain terrain to create a pit) and that the fantasy nature of many campaign settings means that what constitutes a "Land" does not always mean it is on the ground (i.e. the "Land" of the cloud giants being literal clouds); however that does not always mean that those lands and their terrain are naturally occurring which is why I pose the question. Address the two points above as well as either of the questions and receive a +1 guaranteed.

Answer 1

No, the sky isn't terrain. “Terrain” is a word that means “land of a particular kind”, which obviously doesn't include sky.

However, you don't need to replace a piece of mountain with sky. Replace it with another terrain instead: a bit of plain. Same effect, without the need to abuse the English language to justify it.

Note also that the 150-foot cube that Hallucinatory Terrain alters is really quite tiny compared to a mountain, so it will only change a vanishingly small portion of the mountain, looked at as a whole. Basically, you could replace a little bumpy part on the mountain with a smooth alpine meadow; the mountain itself is far, far too large to disappear. Alternatively you could make a tiny cave somewhere under the mountain, but in no way would that endanger the structural integrity of the mountain itself (even assuming for the sake of argument that Illusory Reality has that effect), because the amount of mountain affected would be like poking a pinhole into a boulder.

Similarly, you can't put a mountain in the air by replacing the sky “terrain” with mountain terrain. That doesn't work because sky isn't terrain, and a mountain floating in the sky isn't “naturally occurring terrain” anyway. You could add a stumpy, 150-foot spur of rock attached to the land, at best.