Genesis 1:7
“And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so.”
KJV
Read in Genesis 1Meaning
The verse reports the fulfillment of God's separating command. The expanse functions as the boundary by which the waters are assigned to different regions.
Read carefully
The point of the sentence is the successful ordering of the waters. It does not provide a modern meteorological or astronomical explanation of the upper and lower waters.
Reflection
The repeated correspondence between command and fulfillment gives the chapter a strong sense of purpose: creation is portrayed as ordered rather than accidental within the story.