Does the Gust of Wind spell nullify the "poisonous gas" from a green dragon's Poison Breath?

Question

I think my question is similar to Can Warding Wind block the effect of a Green Dragon's Poison Breath?, but the wording of the warding wind spell seems more conservative than that of the gust of wind spell.

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I just ran a combat where a druid used gust of wind against a green dragon. I had the dragon retaliate by flying (slowly) towards the druid and using its breath weapon.

The players were delighted, immediately declaring that the monster's attack should have been nullified because the young green dragon's Poison Breath action states:

The dragon exhales poisonous gas in a 30-foot cone. [...]

While the description of the gust of wind spell says:

The gust disperses gas or vapor...

However, I ruled that the breath weapon worked as normal through the line of the wind, using similar logic to the most upvoted answer on the linked question: that "dispersing gas" is for handling cloudkill, fog cloud, and other lingering effects that explicitly describe being dispersed, and that breath weapons (or other instantaneous area effects) do not get cancelled through dispersal. Instead, I would argue that breath weapons may only get blocked by spells that say it more directly, such as wind wall, which says that it "keeps gases at bay".

It would not have been a big deal if the players were correct in my case, as the dragon had other tactical options - moving out of the of the gust of wind to attack from the side, for instance - so the ruling did not change much about the outcome (although it disappointed the players because they were convinced they had outsmarted the dragon and made it waste an action). And in the end, the battle was won by the PCs anyway.

Should I have allowed gust of wind to nullify gas-based breath weapons used within it, according to the rules as written? What exactly does "dispersing" mean in the spell description?

Answer 1

Gust of wind nullifies the green dragon's breath weapon

I would side with the players on this one.

It is quite clear from the wording of the spell that the "gust" is quite strong. From the description of the gust of wind spell (emphasis mine):

A line of strong wind 60 feet long and 10 feet wide blasts from you in a direction you choose for the spell's duration. Each creature that starts its turn in the line must succeed on a Strength saving throw or be pushed 15 feet away from you in a direction following the line.

Any creature in the line must spend 2 feet of movement for every 1 foot it moves when moving closer to you.

The gust disperses gas or vapor, and it extinguishes candles, torches, and similar unprotected flames in the area. It causes protected flames, such as those of lanterns, to dance wildly and has a 50 percent chance to extinguish them.

As a bonus action on each of your turns before the spell ends, you can change the direction in which the line blasts from you.

The gas dispersal section details specific effects of the spell, but even without that, it is quite clear that this is a forceful wind being created.

I would compare the particular wording used in the description of the spell with that of the breath weapon. The spell is very clearly designed as a powerful, continuous burst of wind, while the dragon's breath weapon is simply described as being "exhaled", which doesn't stand up to any vigorous scrutiny as being of comparable strength.

Answer 2

The gust causes the gas to vanish in the 10-foot-wide column.

The gust of wind spell lists the following effects, among others:

A line of strong wind 60 feet long and 10 feet wide blasts from you in a direction you choose for the spell's duration. Each creature that starts its turn in the line must succeed on a Strength saving throw or be pushed 15 feet away from you in a direction following the line.

[...]

The gust disperses gas or vapor, and it extinguishes candles, torches, and similar unprotected flames in the area. [...]

A strong wind is defined in the Dungeon Master's Guide as follows (p. 110; emphasis mine):

A strong wind imposes disadvantage on ranged weapon attack rolls and Wisdom (Perception) checks that rely on hearing. A strong wind also extinguishes open flames, disperses fog, and makes flying by nonmagical means nearly impossible. [...]

As "disperse" isn't a defined game term, and D&D 5e is written using natural language, we need to refer to the normal meaning of the word. Merriam-Webster includes the following definition:

transitive verb

1.

• a: to cause to break up (see break up sense 1a)
  • police dispersed the crowd
• b: to cause to become spread widely
  • disperse the troops
• c: to cause to evaporate or vanish
  • sunlight dispersing the mist

In particular, 1c is the most relevant definition, dealing with "disperse" as it relates to an environmental effect (mist). So we have to conclude that the meaning of "disperse" in the spell description, as it applies to a gas in this context, is to cause the gas to "vanish or evaporate".

Conclusion

As a result, the gust of wind spell causes the gas to vanish in the 10-foot-wide column, and thus has no effect on the creatures behind the line of wind. This does not mean that the breath weapon has no effect on creatures on either side of the column (if its cone is appropriately wide).

This DM would take account of the dragon's positioning to determine just how much of the breath weapon is dispersed. For example, if the dragon releases its breath weapon while its mouth is directly in the gust, then this DM would rule that the gust nullifies the whole breath weapon (as it vanishes before it has a change to "fill" the cone). Alternatively, were the dragon to release its breath weapon from above the gust, angled downwards, then only the central line of the cone would be dispersed (protecting creatures behind the gust).

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As an aside, if the dragon is flying, within the column of the strong wind (as you indicate), then it must either land at the end of its turn or fall (and take falling damage) due to the Strong Wind rules. It also must spend 2 feet of movement for every 1 foot it moves toward the caster (in effect, difficult terrain) due to the gust of wind spell (on top of the Strong Wind effects).