"Firmament" is translated from the Hebrew raqiya, which is derived from raqa which means "to spread abroad, stamp or stretch." Raqiya means "an expanse." Young's Literal Translation of the Bible renders Genesis 1:6 as, "And God saith, 'Let an expanse be in the midst of the waters, and let it be separating between waters and waters.'"
Remember, clouds completely enshrouded the planet, and water inundated the whole earth. Because the sun's light and heat had not been able to reach the earth's surface before God cleared away the debris, the hydrological cycle—the process whereby water evaporates, rises to form clouds and later falls as rain—had ceased.
Now that sunlight could reach the surface, God set about to clear away the fog and mist and reestablish the hydrological cycle. In so doing, He caused the water that was in the fog either to rise and become part of the clouds or to precipitate as rain. He thereby "divided the waters from the waters" by creating an expanse of clear air between the watery surface of the earth and the water-laden clouds. This is plainly stated in verse 7: God "divided the waters which were under the firmament [the oceans] from the waters which were above the firmament [the clouds]."
"And God called the firmament Heaven." This is the first heaven, as verse 20 clarifies: "Let birds fly above the earth across the face of the firmament of the heavens." Since birds fly between the earth's surface and the clouds, the firmament must be the expanse of clear air we call our atmosphere. An observer of this process could now clearly see the expanse of the sky from one horizon to the other. However, as clouds still covered the earth, the sun, moon, and stars could not yet be seen.
On this second day of creation, God probably adjusted the atmosphere's composition to contain the correct amount of nitrogen, oxygen, carbon dioxide, water vapor, and inert gases necessary for photosynthesis and the sustenance of life. This would be vital in preparing for the events of the third day.
Earl L. Henn (1934-1997)
Genesis 1: Fact or Fiction?