in what sense is the air of the gulf uncertain?

This adds to the mysticism of the story. The people of the Gulf have a connection with the land that is rooted in the unseen as well as the seen. Their belief in the spiritual mirrors the misty Gulf, “thus it might be that the people of the Gulf trust things of the spirit and things of the imagination" This also reflects Kino and his countrymen’s belief that that riches may lie somewhere unseen beneath the terrain.