calm air vs air current

calm air and air current both are nouns.

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

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

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

Other hypernyms of calm air include current of air, wind.

calm air (noun)	air current (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