Rain by Sam Usher

Sam wants to go out but it's pouring with rain, so he and Grandpa decide to stay inside until the rain stops. Sam drinks hot chocolate and reads his books and dreams of adventures while Grandpa gets on with his important paperwork. Grandpa seems to have a VERY important letter to write. Then that very important letter has to be posted, despite the rain and floods. As they finally go outside, Sam and Grandpa have a magical adventure. Rain is the follow-up to the acclaimed Snow; and is the second title in a four-book series based on the weather.