BBC: Genesis 8:6-12

At the end of forty days Noah opened the window of the ark that he had made and sent forth a raven. It went to and fro until the waters were dried up from the earth. Then he sent forth a dove from him, to see if the waters had subsided from the face of the ground. But the dove found no place to set her foot, and she returned to him to the ark, for the waters were still on the face of the whole earth. So he put out his hand and took her and brought her into the ark with him. He waited another seven days, and again he sent forth the dove out of the ark. And the dove came back to him in the evening, and behold, in her mouth was a freshly plucked olive leaf. So Noah knew that the waters had subsided from the earth. Then he waited another seven days and sent forth the dove, and she did not return to him anymore. – Genesis 8:6-12

Returning to the interesting chronology here in brief, the end of these forty days is counted from the landing on the mountain. The 150 days start and end in contrast with the covering of the mountains. This extra set of forty can be seen from Noah’s perspective as the perfect amount of time for the sanctification of the messengers he will send out.

First Noah sends out the unclean raven for fear of losing a clean animal. But the sending out of the dove is necessary for the completion of the purification process. The inspection for life and growth stands in contrast to the Leviticus process of inspecting for leprosy (Lev 13-14). In practice, Noah is waiting for the declaration of the land as “clean”. He is waiting for things to once again, in a shallower sense, be declared “very good” (Gen 1:31).