How did the book Roll of Thunder Hear My Cry get its name?

The title for the book "Roll of Thunder, Hear My Cry" (which is a wonderful book) is appropriate for the book mainly because in the last portion of the book. The main character, Casey, is in the middle of a fire occurring in her father's cotton field. But when it seems that all hope is lost, a large thunderstorm comes in with hundreds of gallons of water pouring down from the heavens. This obviously puts out the fire. So the idea of the thunderstorm coming to save Casey and her family is the basis for the title of this book.