The Rain Door (Children's book, 1986)

Harry hears the rag-and-bone man calling "Rainy numbers up! Any thunder up?" and goes through the rain door to a fantastical world where the weather is made to find an old iron tank used by the rag-and-bone man as a cloud catcher. To scare off the lion that is frightening the man's horse, Lightning, Harry begins to build a mechanical dinosaur. It takes lots of hard work and ingenuity on Harry's part before the rag-and-bone man is able to coax Lightning, who is pulling his junk wagon, to roar across the sky, bringing the relief of rain to the parched earth below.