Sforno on Genesis 1:6/ספורנו על בראשית א׳:ו׳

1יהי רקיע בתוך המים, let there be טבע, i.e. guidelines, rules governing the function of what is below and what is above the primordial waters. Just as there are such parameters surrounding our world, i.e. the globe we call “earth,” so there should be similar guidelines governing what is below the atmosphere and earth itself. The region in which air exists should become clearly defined. [the gaseous nature of air should not be allowed to escape into all directions of the universe beyond our globe so that creatures on earth would die from the lack of air. In other words, the atmosphere should be “enclosed” whether by gravitational or other forces is immaterial. Ed.] At the same time, air should be given enough space within which “to breathe,” so to speak.