Many young earth believers believe that there was a canopy of vapor—basically a permanent cloud cover—over the whole earth prior to the flood. They argue that this allowed animals, including man, to live a long life, protected from the sun's rays, and that at least some of the flood waters came from this vapor canopy. This theory was proposed in the famous book, The Genesis Flood, by Henry Morris and John Whitcomb, which is still in print.
I have been unable to determine even if the Institute for Creation Research, of which Henry Morris was president, still holds to a belief in a vapor canopy. I do know that most staunch young earth believers that I know speak of the vapor canopy like it's an obvious teaching of Scripture. However, Answers in Genesis and Walter Brown both oppose the idea. On the other hand, Kent Hovind still pushes it.
There are numerous problems with the idea, and it is scientifically impossible. The links for Answers in Genesis and Walter Brown in the previous paragraph both outline the scientific arguments against it. I, however, want to discuss the Scriptural problems.
On the second day God made a firmament to divide the waters above from the waters below (Gen. 1:6-8). This one passage is used to support the vapor canopy theory. It simply doesn't hold water, however (sorry for the pun, couldn't resist).
The firmament is said to include the sun, moon, and stars, so if there was a vapor canopy based on Genesis One, then it had to surround the whole universe. Such a vapor canopy would not protect the animals from the sun's rays, nor would it be available as rain for the flood. Even at the speed of light, most of the rain would still be billions of years away from reaching the earth. If you say the rain started falling several billion years before the flood, then you're no longer a young earther and there's no point in the vapor canopy.