sky vs mackerel sky

sky and mackerel sky both are nouns.

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

As nouns, mackerel sky is a hyponym of sky; that is, mackerel sky is a word with a more specific, narrower meaning than sky:

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

Other hyponyms of sky include blue, blue air, blue sky, wild blue yonder.

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

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