Genesis 7:24

And the waters prevailed upon the earth a hundred and fifty days.

ויגברו...חמישים ומאת יום. These 150 days are counted from the beginning of the 40 days of rainfall. The level of water on the surface of the earth kept increasing for 150 days, concluding on the 17th of the seventh month (Nissan). On that date the ark came to rest (on the waters) seeing there was no pressure exerted on it from any direction. The pressure on the ark to move was exerted from the first day of the rain when the Torah simultaneously reported the fountains below opening and adding their water to that of the rain. (verse 11). The ark resisted this pressure until the waters on earth were so high that they lifted the ark from its base. When the days of rainfall had concluded, the pressure from the subterranean waters kept up until the end of 150 days. When that pressure ceased the ark finally came to rest on Mount Ararat.

ויגברו, the Torah now describes for how many days this overwhelming force of the water continued to increase, i.e. for 150 days. Moses knew all this due to his holy spirit, [which included not only prophetic visions of the future, but knowledge of the past that no living man had seen. Ed.] We cannot assume that Noach, personally, was aware that the waters kept rising for 150 days and that he passed on this information throughout the ages. What instruments did Noach have to arrive at such a conclusion? All he knew for sure was that the rain had stopped after 40 days. No doubt he had implicit trust that G’d had told him the truth when He had announced that here would be continuous rain for 40 days. Still, he had ways of confirming this by his own observations. If there is a question, the question is what way did Noach have to determine that precisely 40 days had elapsed since the beginning of the deluge? Surely, inside the ark with its windows closed, he could not be sure when it was daylight outside and when it was night? This is assuming that even if he had been outside he could have told the difference between day and night, with the sun blocked out through the clouds all that time. Furthermore, how could he determine that after 40 days the waters had begun to recede? How could he know for sure that seven days had elapsed between the dispatch of the raven and the pigeon on two occasions? We must conclude that Noach was an exceedingly smart man who had prepared [during the preceding 120 years, presumably, Ed.] instruments, such as an hour glass, to measure the progress of time, so that he could measure hours and therefore day and night by means of these instruments, even if he could not see either the sun or the sky. There is no question that lamps were burning inside the ark by means of which one could find one’s way around, serving food, preparing meals and gaining access to where supplies had been stored. The 150 days that our verse speaks about were after the 40 days of continuous rain, which had concluded on the 27th day of Kislev. By adding 150 days from that date on we arrive at the 1st of Sivan, counting the months on the basis of the solar year of 365 days, allocating approximately 30 days to each month. [I know that the author’s arithmetic is problematic. Ed.]