That's no moon, that's a potential adventuring location!
Actually, it's both.
The world that Felltower is on is very much like our own, and its climate and location is very much like northern NJ because that's where I'm situated. If I was running it remotely from Japan, it would have been a heck of a lot snowier in the winter and hotter and more humid in the summer.
Because of this, in general, the players can rely on real-world facts and real-world impressions of the world to determine how things are in the area in and around Felltower.
The moon, however, may be slightly different.
A long while back the PCs heard this rumor:
"There used to be a guy who swore he went down in the dungeons and through a magic door and came out on the moon and spat down on Stericksburg. He’s gone, now."
This either says something about this guy, or it says something about the moon and the physics of the atmosphere in the world of Felltower.
Either way, the moon is a potential adventuring location. I've designed it as such. It may be accessible through a "magic door," or by one of the gates that the moon phase is said to affect, or by direct flight (by Flight spells or otherwise), or by other means. But it's a place the PCs can go.
This is unapologetically thanks to Gary Gygax and his Up on a Soapbox columns from Dragon magazine. He had an NPC bamboozle a PC out of a lot of money for a moonshot catapult. As a result, the player gave up on the moon and no one went . . . and Gary had apparently stocked it up for adventure. Oh well.
It's also inspired by Wallace & Gromit, old bad movies that MST3K riffed on, H.P. Lovecraft's writings, and The Adventures of Rocky & Bullwinkle. It might not be as silly as all four of those, but they did play into my desire to put a PC on the moon by the end of this campaign and return some of the survivors home safely.
I'm not sure I'll ever get to use my moon location, or how my PCs will get there, but it's a potential adventuring location. Because, why not?