Genesis 1:6-7 notes:

1:6 firmament. The Hebrew word speaks of something spread out. God commanded the water to separate, and He placed an expanse, or a “firmament,” between the water that remained on the earth and the water that now rose above the expanse. The imagery is that of a vast expanse, a protective layer that overlays the earth and divides the waters below from the waters above. The expanse in-between includes the earth’s breathable atmosphere.

1:7 under the firmament. This refers to subterranean reservoirs (cf 7:11). above the firmament. This could possibly have been a canopy of water vapor which acted to make the earth like a hothouse, provided uniform temperature, inhibited mass air movements, caused mist to fall, and filtered out ultraviolet rays, and thereby extending life.