Genesis 8:3

And the waters returned from being on the earth, going and returning. And the waters were minished at the end of a hundred and fifty days.

וישבו המים..הלוך ושוב, this formulation described the gradual nature of the waters abating and the water level on the globe falling. The period this took corresponded to the 40+150 days described during which the waters rose, giving us a total of 340 days. When you add the 3 times 7 days which elapsed for the 3 missions of the raven and the pigeon respectively, you have a total of 361 days, very close to the 365 days described in the Torah as the period which elapsed from the onset of the rain until Noach left the ark. The beginning of the return to normal commenced on the second day of Sivan. As soon as the waters had peaked, they began to recede.

ויחסרו המים מקצה חמשים ומאת יום, “the waters decreased at the end of one hundred and fifty days.” This date was identical with the first day of the month of Sivan. The ark came to rest in the seventh month, the month of Sivan, counting from the month of Kislev which was the month during which the rains had ceased descending. The “seventh” month here could not have been the seventh month after the rains began to descend, as that would have been Iyar, seeing that it had commenced during the second month of the year, i.e. during Marcheshvan. If we were to assume that the Torah referred to the seventh month after the rains had begun, a count of 40 days of rain falling, plus 150 days of the waters continuing to rise, this would bring us only to the 17th day in Iyar. Clearly, we must allow for days during which the waters had begun to recede, else how could the ark come to rest anywhere, seeing that the Torah told us that at its highest, the waters were 15 cubits above the highest peaks on earth? If we accept our interpretation the ark would have come to rest on Arrarat on the sixteenth day after having risen to its highest level. That date had been in the tenth Av, not Ellul. When the Torah, in verse 5, speaks of the tenth month, on the tenth of which the highest peaks became visible once more. According to Rashi’s calculation forty days elapsed (verse 6) elapsed before Noach for the first time lifted the window at the top of the ark to dispatch the raven on its mission, which it did not really carry out. Apparently sixty days passed between the time when the highest peaks became visible until the surface of the earth was no longer covered with water. If we were to assume that the highest peaks had not become visible until the month of Ellul (the tenth month when commencing with Marcheshvan when the rains first descended) we would therefore have to conclude that the waters subsided at a rate of one cubit per four days, 15 cubits during the 60 days commencing with the highest peaks having become visible again. According to this calculation, the bottom of the ark had been in eleven cubits of water at one stage at least.
According to Rashi’s calculation we face the following problem: if as the Torah writes, (verse 6) Noach opened the window of the ark after forty days of it having come to rest on Arrarat and he sent the pigeon out more that 40 days after the ark had come to rest, how come that the pigeon did not find any place to park on? It surely should …