Relating to this question: Explain the exploding pipe in The Shawshank Redemption. In that scene Andy is timing his strikes on the sewage line to the match the lightning strikes (shouldn't it be the thunder he waits for?).
What would he have done if there was no lightning?
He has already followed his plan to a point that he couldn't just slip back into his cell and replace everything. So he needed to crawl out through the pipe, but he hadn't made a hole in it already or even weakened the pipe in any way. I always assumed it was the pressure in the pipe that kept him from making a hole before, like he had in his wall, but "Explain the exploding pipe" shows that it was a drama effect only.
He needed to go out that pipe, he had only a short amount of time to escape and if the storm hadn't come through he wouldn't have been able to make so much noise breaking into the pipe.
Andy is meticulous, so I don't believe he would have left this detail to such a great chance as a lightning storm (even if he checked for a storm before he switched the books there would be no guarantee of lightning); so how else would he have gotten the pipe open within the limited time frame he had to do it?