water is wet; it's obvious
Idiomatic expression meaning something is self-evident or stating the obvious, like saying 'water is wet' in English. The literal meaning is 'on days when it rains, the weather is bad'.
Saying 'water is wet' is so obvious it makes me laugh.
A fixed idiomatic phrase; the literal meaning is 'on days when it rains, the weather is bad', used figuratively to point out something obvious.