Commentary for Genesis 1:7

ויעש אלהים את הרקיע AND GOD MADE THE EXPANSE — He put it in proper condition in its place: this is the meaning of “making” it. Similarly (Deuteronomy 21:12) ועשתה את צפרניה “And she shall let grow (literally, make) her nails”.

"And G-d made the firmament" - and it was that when some of the elemental water was removed from under that part of the waters which became vaporous in nature, as it was when He said, "let the waters be gathered from beneath the heavens" Genesis 1:9, it was fit that the vaporous part would descend to that same place from which the that [non-vaporous portion of] waters was removed. But He made it such that that "firmament" that separates would have the power to stop and prevent the vaporous portion from descending - that's the "waters which are above the firmament", such that the transformed atmospheric portion would descend, while the vaporous would remain in its original place. Because of this, when the moist vapor reaches it [i.e., the firmament], it condenses and gives rise to rain, snow and hail. As they condense they descend, as it is said, "at the sound of His placing an abundance of water in the heavens" Jeremiah 10:13. - what it wanted by saying "heavens": the firmament which condenses the atmospheric part [of the waters], as it is said "And G-d called the firmament "heavens" Genesis 1:8. Now when the cloudy, excited vapor arrives there [at the firmament], it generates thunder and lightning, and it is said: "He lifts up clouds from the ends of the earth, thunder from the rain He makes" [continuation of verse quoted previously from Jeremiah]. Now, seeing as how some of the heavier watery element is above the lighter air, which from our perspectives is against their nature - this without doubt indicates the action of an volitional actor, intended such an outcome, as it is said, "the work of His hands shall the firmament declare" Psalms 19:2

And it was so: that the firmament sufficed to hold up the higher waters, that they should not get mixed up with the lower ones.