See commentary on verse 6.
Targum Onkelos: And the Lord made the expanse, and distinguished between the waters which were under the expanse, and between the waters which were above the expanse: and it was so.
Rashi: And God made the expanse: He fixed it upon its base, which is [what is meant by] “making” it; as [in the verse] (Deut. 21:12):“and she shall ‘do’ her nails,” וְעָשְׂתָה.
above the expanse: It does not say, “on the expanse,” but “above the expanse,” because they [the waters] are suspended in the air (Mid. Ps. 19:4). Now why does it not say, “that it was good” on the second day? Because the work involving the water was not completed until the third day, although He commenced it on the second day, and an unfinished thing is not in its fullness and its goodness; and on the third day, when He completed the work involving the water and He commenced and completed another work, He repeated therein“that it was good” twice (sic): once for the completion of the work of the second day and once for the completion of the work of that [third] day. — [Gen. Rabbah 4:6]
Rambam: God set the expanse in its proper setting, giving forms to the upper prime matter. It set the measure between the waters above the expanse and the waters below the expanse; and it is so forever.
Ibn Ezra: Once God’s wind had dried the land, God made the sky together with the air and they divide between the seas and the rain clouds. When it was so,