D&D 5E Question about "moving effect" spells like Cloudkill

[jayoungr]

Possibly a dumb question, but I've never used these spells so far.

Cloudkill creates a sphere of poisonous gas that moves away from the caster at a rate of 10 feet per round. What do you do if someone casts this spell in an enclosed area? When the gas sphere reaches the wall, does it dissipate or change direction? Or something else?

 

[Staffan]

At least in the particular case of cloudkill, I'd have it stop moving at the wall. Cloudkill is described as having a physical presence, with vapors that "sink to the lowest level of the land" and being able to be dispersed by wind. If it was more of a pure energy thing, like a moonbeam that moved automatically instead of as directed, I'd probably have it fizzle out.

 

[Stalker0]

If the wall is somewhat porous (this is gas so even a small hole can do the job) I would have it move into the wall.

But if it’s truly a solid wall than just leaving it there makes sense