‘And God said “Let there be an expanse in the midst of the waters, and let it separate the waters from the waters.'
Up to this time there has been no atmosphere, for creation is seen as being one blanket of ‘primeval water'. All is ‘liquid'; all is primeval, unshaped, formless matter, but now given body by ‘light'. And now God acts to produce an atmosphere with ‘water' below and clouds above.
The word for ‘expanse' or ‘firmament' is raqia which originally indicated ‘something trodden on and stamped out', and then ‘to make thin like a piece of metal beaten into shape', and thus ‘to spread out, to expand'.
The ancients saw the water come down through the atmosphere from the heavens, but we know from later descriptions that they recognised that this came from the clouds (e.g. Deuteronomy 11:11; Judges 5:4; 2Sa 22:12; 1 Kings 18:45; Job 36:27). And people then as now had climbed mountains and found themselves above the clouds and above the rain (we must stop thinking of them as stupid).
Thus the writer is not suggesting that there is a physical cupola somehow holding up the water. He is using a vivid metaphorical description to describe a reality, water held above by something ‘stretched out' by God, and water below. He does not pretend to understand the mechanics of it, he does not try to explain it. He simply describes what he sees. He just knows that God has made some way of holding the water up. He sees that it is so, and He knows that it is so at the behest of God.
The Bible writers give many descriptions of this ‘firmament'. It is described in terms of being like a transparent work of sapphire stone (Exodus 24:10), in terms of a molten mirror (Job 37:18), in terms of the curtains of a tent (Isaiah 40:22; Isaiah 54:2), but all were vividly descriptive, not an attempt to explain the universe.
We must not over-literalise the descriptions of poetic minds and make them hold views that they did not hold, however simple minded we make them to be. They saw things as an artist sees them, not a scientist. Their very ‘simplicity' and practicality of mind prevented them from trying to formulate scientific theories, but that did not prevent their ideas from being profound. This writer was not investigating world phenomena, he was taken up with what God was doing. He was not analysing ‘how', he was asking ‘Who?' and ‘Why?', profounder questions far. The how he left to God.