Genesis7:12

Translation

And it came about that the rain [was] upon the earth forty days and forty nights. (See comment below.)

Paraphrase

It happened that the earth was subjected to a complete period of judgment by water. (See comment below.)

Footnotes

1: How did Noah know that a change happened after day 40?

It is reasonable to assume that Noah kept track of these things and wrote them down. He noticed on day 41 that the rain had stopped, then it started up again and a new pattern was established involving rain that came and went.

The Importance of the Number 40

It is clear from this verse that the rain fell non-stop for 40 days and 40 nights. After the 40 days were over, we are not told if it rained or not. I believe that it did rain off and on (more on than off) until close to day 150 (see Genesis 8:1-3).

While there was a literal reality about what happened on day 40, it would have been seen by the ancients in a symbolic way as well. God sent a full and complete number of days of non-stop rain as a fulfillment of the judgment He had foretold. We know that there was much more involved than 40 days of rain, but those forty days and nights served to confirm what God had said. Even if the 40 days did not comprise the full extent of the flood, they were symbolic of the reality that God’s judgment on the wicked people on the earth was complete, unstoppable, unhindered.