After creating the cycles of day and night, God began transforming the world.
"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." Genesis 1:6-8
Modern translations of this text call this firmament an expanse or dome. Significantly, the expanse is not described as being around the earth; it is above it. This implies that the authors believed that the world was a flat expanse of oceean--not a sphere. In creating this expanse, God divided waters in the atmosphere from surface waters. This act provides the primitive explanation for precipitation.
Anticipating the account of the catastrophic flood described in Genesis chapters 6-9, some modern interpreters assert that the waters above the heavens constituted a water vapor canopy. It was the collapse of this water canopy that created Noah's flood. This enables believers in a world wide deluge to explain the source of the waters. In addition, these commentators suggest that water canopy explains the incredible longevity of human life described in the early chapters of Genesis.
The authors of Genesis, however, probably intended nothing more than an attempt to account for weather. The authors--and their readers--understood that the waters above the firmament accounted for both Noah's flood and all subsequent rainfall. The Psalmist believed that the waters above the heavens--or at least some of them--remained there (Ps. 148:4). With the primitive state of knowledge about meteorology and the earth's surface features, the authors had no "explaining to do."
Below, another well-produced video from The Bible Skeptic: