lowering vs threatening

lowering is a noun but threatening is not a noun.

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

As adjectives, lowering and threatening are synonyms defined as:

Other synonyms of lowering include heavy, sullen.

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

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