“Then God said, “Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.” 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 (Genesis 1:6,7)
The basis of the Canopy Theory is that the firmament represents the Earth’s atmosphere and the waters above are in the form of a canopy which surrounded the Earth before the Flood. It is thought that this canopy would have contributed to the waters of the Flood, and that the pre-Flood canopy would have caused conditions of environment and weather before the Flood to be very different from today.
The actual nature of the canopy differs in various interpretations, with some assuming it to be a canopy of water vapor, and others suggesting a crystalline canopy.
The late Dr. Henry Morris, father of modern creationism, in his book The Genesis Record writes: "The “waters above the firmament” thus probably constituted a vast blanket of water vapor above the troposphere and possibly above the stratosphere as well, in the high-temperature region now known as the ionosphere, and extending far into space."
Dr. Morris went on to list ways in which it might have made pre-Flood conditions different from today:
The world temperatures would have been uniform and warm. The canopy would have prevented cosmic radiation which would have lead to longer pre-flood lifespans. The Bible states that there was no rain prior to the Flood, but that there was a mist that was on the earth each morning. Therefore there was a uniform worldwide moisture with no deserts or ice caps. In effect, we would have seen a green house effect.
A worldwide rain lasting forty days would be quite impossible under present atmospheric conditions; so this phenomenon required a completely different source of atmospheric waters than it now obtains. Could this be the “waters above the firmament?" The vast thermal blanket of invisible water vapor that maintained the greenhouse effect in the antediluvian world?
It has been suggested that the longevity of humans before the Flood was due to greater atmospheric pressure. This is possible, but another explanation of this longevity is the comparative absence of mutations among the human gene pool. The level of mutations would have greatly increased after the Flood, so this could also explain the rapid decrease in longevity.
Dr. Russell Humphreys (PhD. Creationist Physicist) has suggested a cosmology based on the stretching of space. In this cosmology, he suggests that the firmament, which divides the waters above from the waters below represents the stretching of the universe.
To be more exact, the Hebrew word translated firmament is râqîya (רקיע). This word has a similar root to the concept of ‘stretching’ metal, by hammering it. However, a side effect of his theory is that the “waters above” would therefore be beyond the stars (since we can see them), rather than at the edge of the atmosphere.
We read in Genesis 1 that the Sun, Moon and stars were made in the firmament (Genesis 1:15). If there were a canopy at the edge of the atmosphere, this would mean that the stars were above the canopy, which is not what Scripture says. However, Psalm 148:4 shows that the “waters above” are still actually in place. That lends support to Humphreys’ idea that the “waters above” are beyond the last galaxies, not at the edge of the atmosphere. We know there is water in space, but this doesn’t necessarily prove that this is the water canopy of Genesis one.