Flavor Text or Rules: Magical Manifestations and How They Disappear

Question

In many different RPGs, magic is defined many ways, especially when it comes to creating or modifying parts of the physical world such as a wall of earth or a fog bank. In D&D 5e specifically, when creating magical darkness, magical fog, or even raising a small wall of dirt, what happens when the spell expires? Is it defined in the rules to instantly dispel all effects and return to its previously untouched state, or does it act naturally and slowly dispel the fog/darkness or cause the dirt wall to crumble and leave a small mound of dirt?

Many of the spells only explain how the spell is created:

You create a 20-foot-radius sphere of fog centered on a point within range. The sphere spreads around corners, and its area is heavily obscured. It lasts for the duration or until a wind of moderate or greater speed (at least 10 miles per hour) disperses it.

I get that a wind can disperse a fog cloud, but what happens when the caster decides to end his concentration on the spell? Is this all flavor text left to the DM or are there rules related to this?

Answer 1

TL; DR: It is not defined in the rules. Most of it should be gone almost immediately, though some unusable physical traces might be left behind. It is up to the DM though. Talk to the DM before assuming anything.

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One thing the spells shouldn't do is to allow for unintended side effects that might eclipse the function of another spell. So if you want lots of water to drink or to irrigate your crops, summoning a water elemental won't be the best use of a spell slot. There is no consensus if parts of a creature remain after detached from a druid wild shape ref, even less for summoned creatures.

If allowed, lingering spell remains should be completely innocuous/harmless after the effect ended. So even if wisps of a fog linger after the spell is over, nobody should have any problem seeing or breathing where the fog was. It is merely cosmetic.

It is an ambiance problem then, and ambiance is entirely the DM's playfield. Should a spell leave traces? Yes, if it adds flavor to the world, or even maybe if it is an investigative story, it would be good to see some clues of spells that took place:

In a corner of the woods, you see that the ground and grass are flattened in a circle about 20 feet in diameter. Roll intelligence(arcana)... the wizard you are tracking probably rested here the previous night; it seems he used Leomund's Tiny Hut.

Or:

Upon investigation of the room, you see signs that a fight took place. In a the middle there is a circular stain on the ground, with bits half-digested meat on it. When you approach, you pick a faint smell of rotten eggs. Roll arcana... Stinking cloud was used here.

So talk to the DM on how it should be handled for your campaign.

Answer 2

When you end your concentration on the spell, the spell ends.
This is explained in the rules on Concentration:

Some spells require you to maintain concentration in order to keep their magic active. If you lose concentration, such a spell ends.

If the spell ends, its effects end as well. Everything in the spell description is a spell effect.
This is explained in the rules on Casting a Spell:

Each spell description begins with a block of information, including the spell's name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell's effect.

In the case of fog cloud, one of the effects is that there is "a 20-foot-radius sphere of fog centered on a point within range." With the end of your concentration, there is no longer a 20' radius sphere of fog. Another effect of the spell is that "..its area is heavily obscured." With the end of the spell, the area is no longer heavily obscured. Both of these effects are ended immediately when the spell ends.

What happens next is up to the DM
This is explained by Rule 1, "The DM describes the environment"

The DM tells the players where their adventurers are and what’s around them, presenting the basic scope of options that present themselves (how many doors lead out of a room, what’s on a table, who’s in the tavern, and so on)...[and moving into Rule 3]...Often the action of an adventure takes place in the imagination of the players and DM, relying on the DM’s verbal descriptions to set the scene.

As a DM, if you want some ambience from a spell to linger, great! Strong descriptions of lingering effects add verisimilitude and atmosphere. As a DM, if you want all traces of a spell to immediately vanish, great! Asserting that it is 'magic' and not 'real', will emphasize how powerful casters are, in imposing their will on a recalcitrant universe.

If you do choose to have lingering effects, though, I would strongly suggest that you either make these non-mechanical, or be scrupulously consistent in applying them. For example, the turn after a fog cloud ends, you will not have "a 20-foot-radius sphere of fog centered on a point within range." But you might want to describe how there is now a 'wet and sparkling mist in the general area, and a light dew on the objects where the cloud was'. That is purely atmospheric and shouldn't cause problems.

The turn after a fog cloud ends, you will not have "its area is heavily obscured." But it might be tempting to have 'half its area lightly obscured' as a lingering vestige of the spell effect slowly resumes normalcy. The danger in a lingering condition having a mechanical effect is that your players may come to rely on that, and be frustrated if you do not always apply it. How come the evil druid gets an extra turn on his fog cloud and we don't? But you didn't do that last time! How fast does the wind need to go to disperse the cloud immediately? If you set up the lingering effects so that they matter, expect the players to be invested in the outcome and to react accordingly.