פירוש על בראשית 8:3

מקצה חמשים ומאת יום AFTER THE END OF ONE HUNDRED AND FIFTY DAYS they began to decrease: that was on the first day of Sivan. How so? The rains ceased to fall on the twenty-seventh day of Kislev — you have three days left in Kislev, and the 29 days of Tebeth make 32, and Shebat, Adar, Nisan and Eyar have together 118, making altogether 150.

וישובו המים מעל הארץ. The waters retreated from the earth's surface. The waters which had flooded the earth kept retreating towards the oceans. They decreased naturally, starting 150 days after the onset of the deluge.

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.