Biblical Creationism holds that the Book of Genesis is scientifically inerrant, and therefore a universal flood actually occurred in Noah's time. The Institute for Creation Research, the largest and most active of the creationist organizations, teaches that the source of a large part of the flood waters came from a "vapor canopy" cloud cover.
The legitimacy of ICR's claim can be evaluated by calculating the thickness of a cloud cover required to supply a given depth of flood waters. This can be done by referring to well established data on the absolute value, in terms of grams per cubic meter, of saturated humidity at different temperatures. This information has been repeatedly confirmed for at least a century and is therefore incontrovertible.
At 77 degrees F. the amount of water vapor in air at 100% humidity is 23 grams per cubic meter. A cubic meter of liquid water contains 1,000,000 grams. These figures are equivalent to 0.043 cubic meters per gram of water vapor, and 0.000001 cubic meters per gram of liquid water. Therefore the volume ratio of water vapor to liquid water is 0.043/0.000001 = 43,000. If all of the moisture in air at 77 degrees F. and 100% relative humidity were converted to liquid water one mile deep, it would require an atmosphere 43,000 miles in thickness to hold that amount of water!
The creationist position is that the vapor canopy was not merely a region of high humidity, but instead a dense cloud layer comprised of fog droplets. There are two flaws in that argument: (1) The amount of water that can exist as suspended fog droplets is limited, and (2) a fog layer dense enough to hold the amount of water sufficient to cause a universal flood as shallow as 1/2 mile would block virtually all sunlight, leaving the world in perpetual darkness. We will deal with each of these in turn.
Fog and/or clouds are formed when warm air at 100% humidity is chilled. Taking an extreme example, if air at 110 deg F and 100% relative humidity is chilled to 35 deg F., the amount of water present as fog droplets will be the difference in the amount of water vapor at 110 deg F and 35 deg F.
The humidity values for 110 deg F and 35 deg F are 60.5 and 5.4 gms per cubic meter, respectively. The difference is: 60.5 - 5.4 = 55.1 grams of fog droplets per cubic meter, or 1/55.1 = 0.018 cubic meters per gram of fog droplets. The value for the liquid flood water is 0.000001 cubic meters per gram. The volume ratio is 0.018/0.000001 = 18,000. It would require a cloud 18,000 miles thick in order to account for a flood water thickness of one mile!
In a partial eclipse of the sun, even when more than half of the sun's disk is covered, the visual intensity is only slightly less than normal. The decrease in light intensity is not nearly as much as that occurring under heavy cloud cover. In order to give the creationist vapor canopy theory the benefit of the doubt, we will stipulate that a dense fog layer one mile thick will reduce the light intensity of the sun by 50%, even though all indications are that it would reduce it much more than that amount. As was calculated above, a flood water depth of one mile would require a cloud cover 18,000 miles thick.
Since the intensity would be reduced by 50% for every mile of cloud thickness, it would be reduced by one-half 18,000 times. The factor would therefore be 0.5 to the power 18,000. This calculates to a figure so small that my calculator can't handle it. However, 0.5 to the power 200 equals 6.2 times 10 to minus 61 power. This is a fraction equivalent to a decimal point followed by 60 zeros and the figure 62. From this we can conclude that a cloud cover 200 miles thick would block all sunlight, leaving the world in perpetual darkness, but nevertheless could not account for a flood more than 200/18,000 = 0.0112 miles thick, or 0.0112 X 5280 = 59 feet flood depth!
The vapor canopy theory suffers from a fundamental contradiction: it's impossible to postulate a cloud cover sufficient to provide significant flood water depth without placing the world in perpetual darkness.
Additional information is available in talk.origins faq: The Water/Vapor Explanation for the Deluge