Thinking about what was probably being talked about (it was years ago on another site) it was probably heat a vapor canopy would cause.
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Obviously, you can't get more than a tiny fraction of the flood waters from the atmosphere without running into a host of difficulties, and that includes the latent heat of vaporization, yet another fatal heat problem. It takes a fair amount of heat to boil a quart of water into vapor. Even if that quart of water slowly dries up of its own accord, it still takes the same amount of heat to turn it into vapor. In that case, the heat is gradually drawn from the surrounding environment. Step in front of a fan after getting out of the shower and you will soon appreciate just how much heat water takes with it when it turns into vapor! Well, the reverse must also be true. When vapor condenses into water, it releases the same amount of heat which originally turned it into a vapor. If that were not true we would be losing (or gaining) energy in the cycle, and the first law of thermodynamics prohibits that.
In order to condense that vapor canopy into rain, it would have to release enough heat to raise the temperature of our atmosphere to 6000 degrees! That's a straightforward calculation of the latent heat of vaporization. There is no way to convert the vapor canopy into rain in time for Noah's flood without burning up the Earth! That is to say, the sheer heat would quickly turn that rain back into vapor! You would not be able to get more than a fraction of that water from the vapor canopy until the heat slowly, in stages, dissipated into space. The key word is "slowly," given that the thick vapor canopy would act as a blanket to keep thermal radiation from escaping."
How Good are those Young-Earth Arguments: Additional Topics
So I guess there may be no heat problem for just oceans of falling water..?
http://physicsworld.com/cws/article/news/2012/feb/24/rain-drains-energy-from-the-atmosphere