The conventional flood story states that the flood waters came from rain that lasted 40 days and 40 nights.
[2][3] Rain appears when the air can no longer support water in the vapor phase and it becomes saturated. Normally, the atmosphere is on the brink of saturation, and the variations in temperature and pressure caused by weather fronts are capable of altering the threshold at which precipitation will form quite easily. What about the amount of water vapor suspended in air needed for the 4.5 billion cubic kilometers of water needed for the global flood? The water vapor currently in the air is only around 2-3% on average, with a maximum of 4% limited by temperature and pressure.
[4] The change in atmospheric conditions required to support enough vapor for 112 million cubic kilometers of rain per day - about 120,000 times more than the current daily rainfall worldwide
[5] - would have rendered the air unbreathable.
Indeed, the atmosphere really couldn't sustain that much water even under the most extreme temperature and pressure conditions the planet can produce. If the conditions were right for that much water to be in the atmosphere, humans and virtually every other animal would have drowned through the simple act of breathing, as well as turning the earth into the equivalent of a pressure cooker with atmospheric pressure at nearly a thousand psi instead of the standard 14.7 or so that we have today. Barring the
goddidit escape hatch(a tried and tested fallback for creationists everywhere), this is impossible.