There are several myths in Glorantha pertaining to when rain disappeared.
However, are there any myths about when rain first came into being? It strikes me as a pretty world-altering event, introducing a natural phenomena that nowadays pretty much the entire world is reliant on, but at some point apparently did not exist.
I know there are some quick mentions of Water invading the sky and air in the Sourcebook, and I know Jeff has mentioned that at some point Heler was king of the world, possibly before he was pacified by other gods, but that's about it.
I'm mostly conflating Heler and rain here, although I'm open to the process/event being more complex.