Major image of a dense cloud

[Peelee]

I question whether you've actually engaged with real fog. Real fog is undifferentiated greyness. IT doesn't visibly swirl or anything in real life.

Even a several-thousand-pound car moving at 45 mph isn't enough to make fog react in any noticeable way.

[Willowhelm]

Even a several-thousand-pound car moving at 45 mph isn't enough to make fog react in any noticeable way.

If it’s dense enough to obscure vision and bounded by a 20ft(?) cube… it absolutely would be affected by a car driving through it at 45mph. Think dry ice on the floor of a room, not a fogbank blanketing an entire town.

[Segev]

If it’s dense enough to obscure vision and bounded by a 20ft(?) cube… it absolutely would be affected by a car driving through it at 45mph. Think dry ice on the floor of a room, not a fogbank blanketing an entire town.

Except that dry ice swirling on the floor of a room isn't a fog bank 15-20 feet high. An airplane passing through a cloud doesn't create visible disruption that can be easily seen. An arrow certainly wouldn't. People moving around within it DEFINITELY don't.

You're looking for the boundary effects, where the fog bank swirls about. This won't cause people inside to be able to see their movements as (not) affecting the interior. It generally won't even interact enough with the wind of people entering and exiting to be noticeable compared to other wind effects. Especially with magical fog, such as generated by fog cloud, which has a strict boundary. It just STOPS at its edge.

[Willowhelm]

Except that dry ice swirling on the floor of a room isn't a fog bank 15-20 feet high. An airplane passing through a cloud doesn't create visible disruption that can be easily seen. An arrow certainly wouldn't. People moving around within it DEFINITELY don't.

You're looking for the boundary effects, where the fog bank swirls about. This won't cause people inside to be able to see their movements as (not) affecting the interior. It generally won't even interact enough with the wind of people entering and exiting to be noticeable compared to other wind effects. Especially with magical fog, such as generated by fog cloud, which has a strict boundary. It just STOPS at its edge.

I didn’t say it was the same as dry ice. I used dry ice to provide an easily visualised similarity. 45mph car through 20ft cube is going to disrupt it.

[Segev]

I didn’t say it was the same as dry ice. I used dry ice to provide an easily visualised similarity. 45mph car through 20ft cube is going to disrupt it.

Is it? There's no provision for the fog extending beyond the radius, nor failing to extend to the edge of the radius. So there's no billowing. It's just...a gray wall of mist.

[Willowhelm]

Is it? There's no provision for the fog extending beyond the radius, nor failing to extend to the edge of the radius. So there's no billowing. It's just...a gray wall of mist.

I have no clue what point you’re trying to make.

Real fog would move (perceptibly or not) in all the situations. The physical matter of the fog moves around the objects passing though it. I happen to think in the case of a car it would be very easily perceptible.

Real fog behaves according to the rules of
fluid dynamics. A real car travelling through a real 20 ft cube of pea-soup dense fog is going to displace a large volume temporarily, even if it can’t leave the 20ft cube. Air will be sucked in behind it and there will be a clear difference. This is not a theoretical person moving inside the fog. It’s half the volume of the cube of fog being violently displaced.

An illusion of a fog would not. It would remain a obscured cube and the car would just vanish and reappear as it barrelled through.

My contention is that the illusion spell’s use of “pass through” doesn’t apply to the common usage of “passing through” fog. Physical matter does not pass through other physical matter. With an illusion it does. Thus your physical interaction with the cloud would reveal it to be an illusion (by the text of the spell) regardless of what it looks like. The illusion passes through your body, not around it. Regardless of what it looks like and whether some other magical effect would look exactly the same or not.

Other spells have different effects and don’t seem relevant. Some can be dispelled by a gust. Some roll along the floor. Some stay in a single space. If you’re just going to imagine that an illusory fog can’t be distinguished from a magical fog because a magical fog doesn’t follow physics then great… the illusion won’t look any different to this theoretical spell of yours. It will look the same.

But that other magical fog doesn’t pass through your body. The illusory one does. There is physical interaction and it reveals the illusion as such.

I’m stepping out of this thread. I really just don’t care. I know how I’d rule this as a DM. As a player I’d go with whatever the DM decided. The discussion here isn’t uncovering anything worthwhile.

[Segev]

I have no clue what point you’re trying to make.

We're discussing magical spells. The spell whose effect the major image is mimicking is fog cloud. Fog cloud has a defined radius, and no provision for the fog to extend beyond that radius in billows, nor for billows of non-foggy air to extend into that radius. Therefore, it is a grey hemisphere. And there's no visible "swirling" effect at the boundary. There's certainly no visible swirling within, just as there isn't inside a cloud of real fog.

Real fog would move (perceptibly or not) in all the situations. The physical matter of the fog moves around the objects passing though it. I happen to think in the case of a car it would be very easily perceptible.

It does move, but no more perceptibly than air does. And thus the car would not move it perceptibly. A car barrelling through the fog doesn't create swirls and eddies you can see. Your dry ice example only creates billows at the boundaries of it with non-foggy air, which, again, the magical sharpness of the boundary forbids swirls and eddies at the boundary, so the car barrels in and out, and the fog is not visibly disturbed at the boundaries any more than it is within. And again, you're just plain wrong if you claim the fog is visibly disturbed within; when it's foggy, cars don't create visible swirls and eddies or anything of the sort; they just get obscured.

Real fog behaves according to the rules of fluid dynamics. A real car travelling through a real 20 ft cube of pea-soup dense fog is going to displace a large volume temporarily, even if it can’t leave the 20ft cube. Air will be sucked in behind it and there will be a clear difference. This is not a theoretical person moving inside the fog. It’s half the volume of the cube of fog being violently displaced.

Except there's no rule backing that up. The fog cloud extends around corners. Unless you use the wind effects that can blow it away, it is undisturbed.

[Spiritchaser]

Things don't pass through fog or smoke. They move it aside and otherwise interact with it. Colloquially we may refer to it as that, but it's not what's actually happening.

I appreciate the difference you are talking about, but:

Generally we are to use ordinary language to interpret rules, this wouldn’t be a problem for ordinary language. To create a problem we need to read this a certain way (I’m aware that to avoid a problem we must also read this a certain way) I find one reading clearly more compelling, you find the other more clear, but both are reasonable.

And

On a more logical level, based on my experience of fog, it’s pretty easy to move in and out of the fog without noticing it swirling around you. Now, if you spent a few seconds (perhaps… 6?) looking closely, moving your had in and out you might notice something odd, though I’d leave a distinction between “odd magical fog” and “probably an illusion”, but because magic we can probably choose to interpret that however we want.


Anyway

The rules do not define the result clearly, and it’s not logically necessary for players to notice the fog immediately…

What works best from a play balance point of view?

Probably not requiring an action to recognize the illusion even if you’re touching it. I suspect that would lead to a degenerate play style. Even with the action required to move the illusion, concentration and the third level slot, I see this as being strong and coming up a lot.

As a starting point creatures probably need to get a free chance to save the first time they physically interact with the illusion.

[Peelee]

I didn’t say it was the same as dry ice. I used dry ice to provide an easily visualised similarity.

The problem is its not similar, though. The amount isn't the issue. If you had a square mile of dry ice and drove a van into it, it would still have the telltale swirling effects that fog does not.

[Segev]

The problem is its not similar, though. The amount isn't the issue. If you had a square mile of dry ice and drove a vna into it, it would still have the telltale swirling effects that fog does not.

You'd also suffocate.

[The_Jette]

So... if the enemy saw you casting a spell, and then they were surrounded by fog that didn't act like fog... wouldn't they think it was magical fog, therefore it wouldn't be strange that it isn't swirling around? I mean, sure if a wizard, lich, or illithid saw you do it they'd pretty instantaneously understand it was an illusion. But, if we're talking about a group of orcs, or harpies, or... heck, even a group of demons (like Dretches or Vrocks), wouldn't they assume that it doesn't function like real fog because it isn't real fog? It's magical fog?

That's definitely one way to look at it and I'm not claiming it's a RAW thing at all. Just a ruling on this particular situation that I feel is feasible.

[Segev]

So... if the enemy saw you casting a spell, and then they were surrounded by fog that didn't act like fog... wouldn't they think it was magical fog, therefore it wouldn't be strange that it isn't swirling around? I mean, sure if a wizard, lich, or illithid saw you do it they'd pretty instantaneously understand it was an illusion. But, if we're talking about a group of orcs, or harpies, or... heck, even a group of demons (like Dretches or Vrocks), wouldn't they assume that it doesn't function like real fog because it isn't real fog? It's magical fog?

That's definitely one way to look at it and I'm not claiming it's a RAW thing at all. Just a ruling on this particular situation that I feel is feasible.

Where do you get the idea that it doesn't act like fog? Fog doesn't swirl around, for one thing, when you're inside it. At best, it swirls around at the edges. And arguably, fog cloud doesn't do that, either, because of how the radius is defined. Though that's very much open to interpretation and fuzzy gameplay rulings for dramatic effect. Certainly, though, you can enter a fog bank and not cause visible disturbance to its edges.

Smoke, on the other hand, tends to behave differently. Very light amounts of dense fog, such as caused by small amounts of dry ice, can have such swirling effects. But natural fog thickens gradually such that, while you can see the boundary from a distance, you are hard-pressed to even say when somebody has actually entered it just by watching.

Now, magical fog is denser, so maybe its edges do behave more like smoke or dry ice. But even tehn, I challenge you to toss a rock into a thick bowl of dry ice fog and tell me you can actually detect the disruption. Sure, if you blow on it, you can see that, but that's not at all the same thing, and arrows, rocks, and even people entering will tend not to have the same effect. If you're blowing on it, that probably counts as your Intelligence(Investigation) action.

[Thunderous Mojo]

TLDR-

If you want to argue that an illusion isn't revealed because you've created something people normally move 'through' then I just don't agree with that, I believe the second statement is just explaining that if they physically move through the illusion, it is revealed to whatever moved through it.

Such an argument strains my credulity.🕊
Major Image can make visual "phenomena, which can be something like sunlight....the illusionary sunbeam even feels like sunlight, with warmth etc.

Unless one is a Vampire, walking unharmed through a ray of sunshine shouldn't in any way cause someone to automatically know that the phenomena is an illusion.

This is a case when the creature involved will need to spend an Action to Investigate.

Also, as someone that has experienced Tule Fog...not only do Semi Trucks not visibly push Tule Fog...sometimes Tule Fog does not feel wet.
https://en.wikipedia.org/wiki/Tule_fog

[Melphizard]

So if we create Fog with a mirror image somebody could theoretically see through it just but moving in it. What if I just make a Major Image of darkness? Darkness (the 2nd level spell) is a 15-foot-radius sphere which enemies can't see through. Major image can create a big sphere of darkness (not the spell) that's smaller but "reacts" in the same way the 2nd-level spell darkness does not react to light and creatures with darkvision. Couldn't it be plausible that enemies wouldn't suspect an illusion of a bunch of fog a spellcaster creates when Fog Cloud is a 1st level spell with the same concept?

[CapnWildefyr]

So... if the enemy saw you casting a spell, and then they were surrounded by fog that didn't act like fog... wouldn't they think it was magical fog, therefore it wouldn't be strange that it isn't swirling around? I mean, sure if a wizard, lich, or illithid saw you do it they'd pretty instantaneously understand it was an illusion. But, if we're talking about a group of orcs, or harpies, or... heck, even a group of demons (like Dretches or Vrocks), wouldn't they assume that it doesn't function like real fog because it isn't real fog? It's magical fog?

That's definitely one way to look at it and I'm not claiming it's a RAW thing at all. Just a ruling on this particular situation that I feel is feasible.

This is an important point that's been hinted at upthread. Until you touch the illusion of fog, it's hard to justify someone thinking it's an illusion when they live in a world full of magic, magic effects, and spellcasters. Whether you know a lot or nothing about magical spells and spell effects does not matter -- there are always more spells than any wizard/druid/bard/whatever knows. Your knowledge, even at L20, isn't complete by a long shot. So, from a distance, no reason to automatically get a free save or free anything with respect to an illusion of magical fog.

The problem is that RAW the illusion fails immediately if you walk into it. (That's where I might deviate from RAW, depending on circumstances, but that's me.)

But I agree that when dealing with illusions from a distance -- even if someone sees you casting, unless they literally identify your spell as you cast it, why would they not think it's a "regular" magical fog, instead of an illusory one?

This thread underscores why illusions are hard to adjudicate. What one person thinks should be obviously a flaw that grants instant relief from the illusion, others strongly disagree, and there's not a lot of RAW you can write to handle it IMO.

[Tanarii]

Fog doesn't swirl around, for one thing, when you're inside it.

Wait, what? It absolutely does.

[Thunderous Mojo]

This thread underscores why illusions are hard to adjudicate. What one person thinks should be obviously a flaw that grants instant relief from the illusion, others strongly disagree, and there's not a lot of RAW you can write to handle it IMO.

This line seems fairly clear: "Physical interaction with the image reveals it to be an illusion, because things can pass through it"

Placing your hand through a stone wall, while meeting no physical resistance is enough to inform a creature in a D&D World that they are dealing with an Illusion.

Now if the Illusion is of a Phenomena or Creature that one would expect thing to pass through we have the rest of the paragraph to guide us:

"A creature that uses its action to examine the image can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC. If a creature discerns the illusion for what it is, the creature can see through the image, and its other sensory qualities become faint to the creature."

The difficulty seems to me that some parties are fixating on one sentence and ignoring the holistic impact of the entire text.

[Chronos]

Quoth Segev:

But natural fog thickens gradually such that, while you can see the boundary from a distance, you are hard-pressed to even say when somebody has actually entered it just by watching.

Well, sometimes. I have in fact seen and interacted with small natural fog clouds that did have well-defined boundaries (a boundary thickness on order of a centimeter or two). I think that they were related to the vegetation in the area, as the boundary of the fog was very close to the boundary of some bushes, but in any event, it's possible.

(and on seeing such an interesting natural phenomenon, I of course proceeded to spend several Actions worth of time Investigating it)

[Segev]

Wait, what? It absolutely does.

I have walked in, driven through, and otherwise experienced fog directly many times in my life. It does not visibly move nor react. It just makes everything look like it has a low render distance. Things fade to fuz and blur into the background quickly and the world looks very small around you. The thicker the fog, the smaller the world around you. There are no wisps, no swirls, no reaction to any amount of motion, except as the motion brings things closer or further away and thus emerge from or are swallowed by the fog. The fog itself is undifferentiated.

[Keltest]

I have walked in, driven through, and otherwise experienced fog directly many times in my life. It does not visibly move nor react. It just makes everything look like it has a low render distance. Things fade to fuz and blur into the background quickly and the world looks very small around you. The thicker the fog, the smaller the world around you. There are no wisps, no swirls, no reaction to any amount of motion, except as the motion brings things closer or further away and thus emerge from or are swallowed by the fog. The fog itself is undifferentiated.

In the case of the illusion, the fog wouldn't even be low render distance, it would be like having a brick of grey in your face/around your arm/whatever youre using to probe the illusion. Real fog reacts perceptibly as you move through it, even if it isnt a big stereotypical swirling fog cloud. The illusion wouldnt react in the same way, it wouldnt back off to allow even the minimal render distance. Even beyond that, real fog is wet, which the illusion would not be.

[Telok]

So... thread tldr: 20 years on and still no decent general illusion rules from WotC. Take an hour to Q&A the DM before thinking about anything but invisibility & mirror image.

I really wish they'd just drop the whole "turns transparent on knowing its an illusion" thing.

[Segev]

In the case of the illusion, the fog wouldn't even be low render distance, it would be like having a brick of grey in your face/around your arm/whatever youre using to probe the illusion. Real fog reacts perceptibly as you move through it, even if it isnt a big stereotypical swirling fog cloud. The illusion wouldnt react in the same way, it wouldnt back off to allow even the minimal render distance. Even beyond that, real fog is wet, which the illusion would not be.

Er, no. Real fog does ont react visibly to you moving through it. And thick enough fog will obscure the hand in front of your face. If you just use fog cloud as a model, it lets you see five feet, and then nothing. There's no reason a major image of fog couldn't do the same thing. Fog is a phenomenon, which major image can explicitly create.

[CapnWildefyr]

So... thread tldr: 20 years on and still no decent general illusion rules from WotC. Take an hour to Q&A the DM before thinking about anything but invisibility & mirror image.

I really wish they'd just drop the whole "turns transparent on knowing its an illusion" thing.

Yeah. And the magic aspect gets ignored, too, ironically. I mean, it's a spell, it's not a hologram, it's not VR, it's not crappy VGA graphics rendering. It impacts your brain, not your senses (well, I guess since there aren't tight rules on illusions we don't really know that, but in a better-framed rule set, it should be more in the mind than in physical existence). At 3rd level, an illusion spell ought to seem real to the senses it mentions in the spell description. The assumption should be that it's real, or in this thread's case, "real magic." If the rules were different, walking into an illusion of a fog cloud should make no difference to how you perceive it except maybe to grant a passive save, since you're in contact with it. A spellcaster is concentrating on it; it is magical; it can adapt, and it should be using the minds of the creatures in the AOE to make itself seem real.

But that's not RAW. The PH hints at it, but doesn't provide actionable rules.

[Tanarii]

I have walked in, driven through, and otherwise experienced fog directly many times in my life. It does not visibly move nor react. It just makes everything look like it has a low render distance. Things fade to fuz and blur into the background quickly and the world looks very small around you. The thicker the fog, the smaller the world around you. There are no wisps, no swirls, no reaction to any amount of motion, except as the motion brings things closer or further away and thus emerge from or are swallowed by the fog. The fog itself is undifferentiated.

I have walked in, driven through, and otherwise experienced fog may times in my life. It visibly moves on its own, and reacts to things moving through it. Whisps, swirls, reacting to (a reasonable) amount of motion.

If you're lighting it up at night (ie headlights) it can be very hard to see it. But otherwise it's very visible.

[Spiritchaser]

I have walked in, driven through, and otherwise experienced fog may times in my life. It visibly moves on its own, and reacts to things moving through it. Whisps, swirls, reacting to (a reasonable) amount of motion.

If you're lighting it up at night (ie headlights) it can be very hard to see it. But otherwise it's very visible.

I’ve been inside fog, and not been able to see swirling. At the transition, I can see it being noticeable if you’re looking for it. It’s not a given

I can recall skiing down into a temperature inversion with very thick cloud/fog and I never noticed it swirl, now in fairness mostly I was thinking along the lines of CANT SEE! STOP… no wait DONT STOP! someone will run into me! Where’s the side!

Looking for swirling patterns on my transition was not exactly the first thing on my mind, it was mostly just shock at how dense it was. I was not prepared.

I’m going to submit that as with real life, it may be possible in game, to notice fog interactions with physical objects at the boundary of the fog, it is certainly not something to take for granted, and once you’re in it? No. No I don’t think so. At least not really thick stuff. Thinner wispy stuff probably isn’t useful in game for this task anyway.

[Darth Credence]

I am unclear as to why this is an argument. RAW says that physically interacting with a major image reveals it to be an illusion. It says why that is the case, but I don't think it really matters here. Regardless of whether or not anyone thinks they can tell if a real life fog bank reacts to them, it absolutely does, simply because the fog cannot pass through the person's body. Walking into a major image of fog involves physically interacting with the fog, which leads to the person knowing that it is an illusion. We can say that it's because the person recognizes that it doesn't interact normally, or we can say that it's just a mystery of magic, but RAW pretty clearly shows that walking into the fog or just being inside of it makes it no longer effective.
So cast it on the party, not the enemy. The party can see through it, the enemy cannot, and you get the advantage without the argument about it.

[Segev]

I have walked in, driven through, and otherwise experienced fog may times in my life. It visibly moves on its own, and reacts to things moving through it. Whisps, swirls, reacting to (a reasonable) amount of motion.

If you're lighting it up at night (ie headlights) it can be very hard to see it. But otherwise it's very visible.

You and I have vastly different experiences with fog, it seems.

[Spiritchaser]

I am unclear as to why this is an argument. RAW says that physically interacting with a major image reveals it to be an illusion. It says why that is the case, but I don't think it really matters here. Regardless of whether or not anyone thinks they can tell if a real life fog bank reacts to them, it absolutely does, simply because the fog cannot pass through the person's body. Walking into a major image of fog involves physically interacting with the fog, which leads to the person knowing that it is an illusion. We can say that it's because the person recognizes that it doesn't interact normally, or we can say that it's just a mystery of magic, but RAW pretty clearly shows that walking into the fog or just being inside of it makes it no longer effective.
So cast it on the party, not the enemy. The party can see through it, the enemy cannot, and you get the advantage without the argument about it.

The rule does not state “physical interaction with the image reveals it to be an illusion”

What the rule states is “physical interaction with the image reveals it to be an illusion because things can pass through it”

The second part of the rule stipulates the logical condition which causes the application of the rule.

Because the logical conflict, that being passing through solid objects, does not exist for objects which are not solid, it is not a given that the illusion is revealed when an object passes through it.

(Though for play balance reasons, perhaps it should be!)

In any case, if passing through a solid object reveals the illusion, it’s reasonable to conclude that observing the fog/dust/smoke behave in obviously wrong ways when you move your hand through it, or rather “through it” would also reveal the illusion (though perhaps not without a roll, since an assessment of incorrect swirling is a whole heck of a lot less obvious than shoving your arm through a supposedly solid object…)

[Mellack]

The rule does not state “physical interaction with the image reveals it to be an illusion”

What the rule states is “physical interaction with the image reveals it to be an illusion because things can pass through it”

The second part of the rule stipulates the logical condition which causes the application of the rule.

Because the logical conflict, that being passing through solid objects, does not exist for objects which are not solid, it is not a given that the illusion is revealed when an object passes through it.

(Though for play balance reasons, perhaps it should be!)

In any case, if passing through a solid object reveals the illusion, it’s reasonable to conclude that observing the fog/dust/smoke behave in obviously wrong ways when you move your hand through it, or rather “through it” would also reveal the illusion (though perhaps not without a roll, since an assessment of incorrect swirling is a whole heck of a lot less obvious than shoving your arm through a supposedly solid object…)

The rule says “physical interaction with the image reveals it to be an illusion because things can pass through it”. I tend to go with spells do what they say. It says that physical interaction reveals the illusion. Fog, or other phenomenon do still have people pass through it. It fulfills the statement. The spell doesn't say that if things normally would be able to pass through, that it changes the rules. You are adding that which the spell does not.

[Segev]

The rule says “physical interaction with the image reveals it to be an illusion because things can pass through it”. I tend to go with spells do what they say. It says that physical interaction reveals the illusion. Fog, or other phenomenon do still have people pass through it. It fulfills the statement. The spell doesn't say that if things normally would be able to pass through, that it changes the rules. You are adding that which the spell does not.

In this case, it really depends on whether you read "because things can pass through it" as actually mattering. If you read it such that those words are wasted ink on the page, then yes, things entering an illusory fog cloud reveal it to be illusory. In fact, you can read it in a literal way and say that anything physically interacting with it in any way, including simply having air be present where the image is located, causes all creatures to immediately recognize it as an illusion.

I hope it is obvious to all of us that such an extreme interpretation is not RAI, even if you can read the RAW to justify it.

Personally, I read "because things can pass through it" to indicate that the whole sentence is contingent upon that. Put another way, that sentence is indicating that you don't need to make an Intelligence (Investigation) check to determine the obvious.