A moderate wind (at least 10 miles per hour) disperses the cloud after 4 rounds. A strong wind (at least 20 miles per hour) disperses it after 1 round.
I would assume that it refers to the strength of any wind that might come along (such as from gust of wind and could disperse it. But in that case, there should be a can somewhere in there; ie:
A moderate wind (at least 10 miles per hour) can disperse the cloud...
The way it's written makes it sound like a strong wind emerges after a round and blows the cloud away; in that case it might as well be an instantaneous effect rather than 1-minute concentration spell.