mackerel sky vs sky

mackerel sky and sky both are nouns.

Word	Noun	Adjective	Verb	Adverb
mackerel sky	Yes	No	No	No
sky	Yes	No	Yes	No

As nouns, sky is a hypernym of mackerel sky; that is, sky is a word with a broader meaning than mackerel sky:

mackerel sky: a sky filled with rows of cirrocumulus or small altocumulus clouds
sky: the atmosphere and outer space as viewed from the earth

mackerel sky (noun)	sky (noun)
a sky filled with rows of cirrocumulus or small altocumulus clouds	the atmosphere and outer space as viewed from the earth

mackerel sky (verb)	sky (verb)
	throw or toss with a light motion