In Asterix and the Magic Carpet, Cacofonix's singing produces rain. (Previously, this had never happened.)
Why is it that when Cacofonix sings, it rains in #28 (Magic Carpet, 1987), but not in Books #1-27 (1961-83)? (Others may disagree, but I find this problematic, puzzling, and in need of explanation.)
(This effect also continues in the next book Asterix and the Secret Weapon.)
Four of the five current answers simply quote from Wikipedia's (unsourced) assertion that there is a French saying that when one sings poorly, it rains.
This could very well be an explanation. But absent either in-universe (i.e. within the comic books, perhaps cleared up in later books) or out-of-universe (e.g. interviews with Uderzo) evidence, this is just speculation. Moreover, it is unsatisfying because it fails to address my above question (in bold).
Edit: In case it isn't clear, I am not disputing the existence of such a French saying. I am disputing that the existence of such a French saying is a satisfactory explanation to my question. In particular, it fails to explain why Cacofonix's singing had not previously produced rain in the previous 27 books.