Комментарий к Берешит 8:6

מקץ ארבעים יום AT THE END OF FORTY DAYS from when the tops of the mountains were seen.

ויהי מקץ ארבעים יום. It was at the end of a period of forty days. How did Noach know when the time had come to safely open the window? Was he not scared the surrounding waters would flood the ark? Perhaps G'd had told him that the deluge would not last longer than twelve months and he had prepared a food supply to last for that period of time.

ויהי מקץ ארבעים יום, these words refer to the end of forty days after the waters had begun to recede; personally, I feel that the period described dates back to the day when the ark had come to rest on Mount Ararat. How else would Noach have been able to determine that the waters had begun to recede, seeing that all around him there was only water? Once the ark had come to rest on Mount Ararat Noach could measure how much more of the mountain was becoming visible beneath the ark. Noach knew that the ark had come to rest on something solid. He then waited another 40 days to give the waters a chance to recede still further, before he saw any point in sending out a bird (raven) to determine if that bird could locate a tree which was exposed above the water.