The canopy theory offers an explanation for what happened on the second day of creation and also provides a source for where all the water came from during Noah’s Flood. The model states that when God separated the “waters from the waters” (Gen. 1:6) on Day 2, he left water on the Earth in the oceans and the rest went up into the atmosphere, forming a protective water canopy that encircled the Earth. This would prevent short wave radiation from entering the Earth. Radiation is known to cause harmful effects to our bodies, so people on Earth were protected and lived long, healthy lives. When the flood started, the canopy fell down as rain, covering the earth and killing everything that was not on board the ark.
There are problems with this model, one being that it would create a greenhouse effect that would make the Earth so hot that people could not live on it. Some creationists still hold to this model while others have moved on to other theories and formulated new ideas.
One thing to keep in mind as we formulate ideas on the pre-flood world is this: we don’t know what the world was like then. We can’t say with certainty that all systems we have today were in existence prior to Noah’s flood. The earth back then could have been profoundly different. Some questions may never be answered on this side of heaven.
More information about the Canopy Theory and some of its problems can be found here.