Interesting discussion:
'Again, notes Augustine, someone (Basil in his homilies on the six days of creation (Basil, homil. in Hexaemeron, III, 8.) has interpreted the firmament to mean the air that separates the rain clouds from the condensed water on the surface of the earth. [my first Interpretation] It is firm and supportive inasmuch as it holds up the clouds after the fashion of a cushion. ( De Gen. ad litt., II, 4.) Augustine admits that the air below the clouds can be called the heaven (cf. "the birds of heaven": shamayim [Gen 7:23]), but he is unconvinced that this lower air can be called a firmament. ( De Gen. ad litt., III, 1.) In his opinion the firmament (raqia) of heaven is located in the space stretching downwards from the top of the fiery spheres through the thinner air to the top of the heavier air which can support the clouds. It is called a firmament because of its tranquility, by which it resembles the truth, "for nothing is more firm and sure than the truth": thy truth (reaches) even to the clouds (Ps 35:6; 56:11). Hence, in Augustine's opinion the creation of the Four Elements is presented in another way: all together without their distinctive forms under the word ground in verse 1, and then as distinctly formed during the second, third, and fourth days. Air as a distinct element is presented under two names in verse two: the separating out and formation of the dry upper air is expressed under the name firmament, while the moist lower air is presented under the name waters below the firmament. The heavier air is called water in that it contains moisture: it is both vaporous water and condensed air (.Imperf. lib., 14).'
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Ok, here is another opinion. The lord used the term waters below and the waters above, to prevent any thinking that space was infinite, a heresy some on Earth held and hold.