threatening vs lowering

threatening is not a noun while lowering is a noun.

Word	Noun	Adjective	Verb	Adverb
threatening	No	Yes	No	No
lowering	Yes	Yes	No	No

As adjectives, threatening and lowering are synonyms defined as:

Other synonyms of threatening include heavy, sullen.

threatening (noun)	lowering (noun)
	the act of causing to become less
	the act of causing something to move to a lower level

threatening (adjective)	lowering (adjective)
threatening or foreshadowing evil or tragic developments	darkened by clouds
darkened by clouds