Potential combat with a Green Dragon. The most damaging item in a Green Dragon's arsenal is its Poison Breath. The text of poison breath is some variant of this for Green Dragons:
The dragon exhales poisonous gas in a xxx-foot cone. Each creature in that area must make a DC YY Constitution saving throw, taking ZZ (AAd6) poison damage on a failed save, or half as much damage on a successful one.
The text of Warding Wind states that one of its effects is:
It hedges out vapor, gas, and fog that can be dispersed by strong wind.
Can this effect stop the Poison Breath from getting into the creatures within the Warding Wind Spell?
Best Answer
No.
The key factor is:
So if the effect description of said gas includes a note that it can be dispersed by a strong wind, it will be.
The green dragon breath has no such statement, therefore it is not warded by Warding wind.
OP asked in comments if a stronger wind would affect the Green Dragon's Breath:
It doesn't matter the wind. The breath of the green dragon would spread out to its cone-shaped area of effect even inside a hurricane. Unless an effect's description states that no gas can get through or something amounting to that, it doesn't matter. That spell cannot stop the breath because the breath cannot be dispersed by wind (and the spell states that it only affects what "can be dispersed"). The dragon entry would have "can be dispersed" if it would.
Regarding wind as weather in the DMG, a strong wind would not affect the Dragon's breath, because it is gas, not fog:
User Doval in the comments below brought an example of a higher (3rd) level spell without the that can be dispersed limiter:
An effect that stated "blocks gases" without the "that can be dispersed" remark of Warding Wind would be effective to block the Green Dragon's Breath.
Example of an effect that blocks gases without the "that can be dipersed" clause:
Example of an effect that is hedged out by warding wind:
Fog Cloud