And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. 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. And God called the firmament Heaven. And the evening and the morning were the second day.
Prior to the second day, it seems that the earth was completely surrounded by a thick layer of water, perhaps in the form of a heavy vapor. On the second day, God divided this layer, part covering the earth with water and part forming clouds, with the atmospheric layers between. God called the firmament Heaven, the expanse of space immediately above the earth. This area was prepared by God for the fowl of the earth.