‘And God made the firmament and separated the waters which were under the firmament from the waters which were above the firmament, and it was so. And God called the firmament Sky (or Heaven). And there was evening and there was morning the second day.'
So by His word the waters were separated to produce atmosphere, and the waters above were held up by His ‘sky'. And it was all done by His word. As we have already seen the writer knew about clouds and rain. He is using metaphorical language to describe what he sees.
The first ‘ yom ' has established light as the basis of the positive aspects of the universe, and has established light and darkness and called them ‘day' and ‘night'. The second ‘ yom ' has established an atmosphere above the waters so that fish and birds might enjoy their benefit, and He has called the upper canopy Sky (or Heaven). The giving of names by God is an indication of His authority over them. Man will have no control over them. They are outside man's control.