I repeated the couplets my mother used to say:
"mackerel in the sky,
three days dry"
"mackerel sky, mackerel sky,
never long wet, never long dry"
I asked, "Have you heard of buttermilk sky?" He said that he had (naturally, I began singing Ole Buttermilk Sky; listen below to the composer Hoagy Carmichael's version).
I told him, "Buttermilk sky is the same as mackerel sky; it's called that because of the curdled look of the clouds."
He said, "Well, I've seen a mackerel and I've seen buttermilk and they don't look the same."
I guess it's poetic license.