Raining cats and dogs

The English idiom "it is raining cats and dogs", used to describe particularly heavy rain, is of unknown etymology and is not necessarily related to the raining animals phenomenon. The phrase (with "polecats" instead of "cats") has been used at least since the 17th century. A number of possible etymologies have been put forward to explain the phrase; for example:
In addition to at least one folk etymology:
There may not be a logical explanation; the phrase may have been used just for its nonsensical humor value, like other equivalent English expressions ("it is raining pitchforks", "hammer handles", etc.).
Other languages have equally bizarre expressions for heavy rain: