Do creatures in an area of heavy obscurement created by a spell know the contours of the spell's area of effect?

Question

Spells like fog cloud (PHB p. 243) and darkness (PHB p. 230) create areas of heavy obscurement. Because the point of these spells is mostly to obscure vision, ordinarily (at least in my experience) they are only cast in areas that are not heavily obscured already. For example, unless you're facing an enemy that can see in the dark (via darkvision, blindsight, etc.), there is rarely reason to cast fog cloud in an area that is already completely dark.

As a result, creatures caught in one of these spells usually start out being able to see their surroundings before the spell takes effect. A party of PCs marching across a battlefield in broad daylight will have had at least a glimpse of the terrain before that fog cloud hits them and renders them effectively blinded, at least for purposes of seeing "something obscured by it." (See PHB p. 183 and this errata.)

This has raised questions at our table. The DM draws out a basic encounter map, the players place their minis, etc. -- and then one of these spells drops, reducing visibility to zero. The quandary is: Can the PCs respond by moving outside the bounds of the visibility-reducing spell? If so, how? How would they even know where to move?

Clearly the players know what is on the map. And their characters might be able to work with their memory of what terrain features are where, based on what they were able to see prior to the spell taking effect. ("OK, I think the well is over here, and there was a copse of trees somewhere over there...") But, being effectively blinded, are the characters not then unable to determine where the area affected by the spell begins and ends?

This quandary is especially easily observed in -- though hardly limited to -- play on a grid, where any given spell will routinely affect this 5-foot square but not that 5-foot square right next to it. If I'm effectively blinded because I'm standing in a square covered by fog cloud, can I see that the square next to me is outside the spell's area? Or am I forced to wander around blindly, guessing at where the spell's borders are?

(Bonus round: the party's paladin, stuck in the fog cloud, arbitrarily chooses a path of movement he thinks will take him outside the area, but that in fact runs him straight into the square of the party's rogue, who has used the reduced visibility to hide. Even if the paladin doesn't collide with the rogue, moving through the rogue's square is difficult terrain, see PHB p. 190, which costs the paladin the one extra square of movement that would have actually taken him outside the area.)

That has been the ruling at the table so far: being unable to see in front of you means the only way to know where to move to escape the affected area is by trial and error.

My worry is that this ruling, in effect, cranks up every visibility-reducing spell into a combination of itself and confusion, PHB p. 224-225, in that characters have no choice but to move in random directions in the hope of a lucky escape. Lest that seem like an idle worry, consider spells like stinking cloud, hunger of Hadar, or cloudkill, which not only obscure vision but also actively hamper or harm affected creatures every round they remain in the area. If characters have no way to know where the area is (and isn't), they are much more likely to get stuck in it, being affected round after round.

Bonus points for a rules-as-written answer, but any persuasively reasoned answer is acceptable.

Probably related: Does this errata allow vision out of an area of Darkness?

Answer 1

Going by parts

1. Knowing where the spell starts

As from firechant's answer, the characters can be assumed to know where the spell started, assuming they saw the spell being cast.

Every area of effect has a point of origin, a location from which the spell's energy erupts... A spell's effect expands in straight lines from the point of origin.

2. Knowing where the spell ends

If I'm effectively blinded because I'm standing in a square covered by fog cloud, can I see that the square next to me is outside the spell's area? Or am I forced to wander around blindly, guessing at where the spell's borders are?

You can't see that the square next to you is outside the spell's area - you are blinded, you can't see at all, period. But, if you have knowledge of the spell (which could be an Arcana check, or an instant pass if you have the spell yourself - it's up to the DM), and knowing where the spell started (check Section 1), you should know the area affected by that spell. If you don't know the spell, your character can only guess - obviously a not-dumb guess would be that going towards the point of origin doesn't help, instead you should probably move away from it, even if you don't know how much you should move. By that logic, the next quoted statement seems too harsh.

That has been the ruling at the table so far: being unable to see in front of you means the only way to know where to move to escape the affected area is by trial and error.

As I said, there is a smart "trial" and a not-that-smart "trial". You know, at least, where the spell started.

Answer 2

The creatures are effectively blinded, that's really all the rules say.

So if they are blind, they do not know the area and shape of the spell

As a DM I remove all map markers from the map, Including the area of effect of the spell and only give the details on the area of effect of the spell to those folks outside of the effect.

Players inside the effect tell me where they are going and what they are doing privately.

Beyond that, if a player doesn't remember or know where folks were (or are), I allow for perception (or investigation) checks to give them details on what's going on where they can't see.

But your question strikes me as a "How do I run the game?" question, not a rules question, per se.

Answer 3

From the PHB, p. 204:

Every area of effect has a point of origin, a location from which the spell's energy erupts... A spell's effect expands in straight lines from the point of origin.

So, we can assume that our PCs saw the origin point of the Fog Cloud and then saw it spread out to cover the whole area, meaning they know where the origin of the spell was.