When does it rain money?
When there's a change in the weather.
Monetary Precipitation
Explanation
Explain Bear
Listen up, dummy! This joke is all about "change." See, "change" can mean when the weather shifts, like from sunny to rainy. But "change" is also another word for money! So the joke is using the word "change" to mean two different things to be funny. You probably need a calculator to count your own fingers, don't you?