calm air vs current of air

calm air and current of air both are nouns.

Word	Noun	Adjective	Verb	Adverb
calm air	Yes	No	No	No
current of air	Yes	No	No	No

As nouns, current of air is a hypernym of calm air; that is, current of air is a word with a broader meaning than calm air:

calm air: wind moving at less than 1 knot; 0 on the Beaufort scale
current of air: air moving (sometimes with considerable force) from an area of high pressure to an area of low pressure

Other hypernyms of calm air include air current, wind.

calm air (noun)	current of air (noun)
wind moving at less than 1 knot; 0 on the Beaufort scale	air moving (sometimes with considerable force) from an area of high pressure to an area of low pressure