rainfall vs downfall

rainfall and downfall both are nouns.

Word	Noun	Adjective	Verb	Adverb
rainfall	Yes	No	No	No
downfall	Yes	No	No	No

As nouns, downfall is a hypernym of rainfall; that is, downfall is a word with a broader meaning than rainfall:

rainfall: water falling in drops from vapor condensed in the atmosphere
downfall: the falling to earth of any form of water (rain or snow or hail or sleet or mist)

Other hypernyms of rainfall include precipitation.

rainfall (noun)	downfall (noun)
water falling in drops from vapor condensed in the atmosphere	a sudden decline in strength or number or importance
	failure that results in a loss of position or reputation
	the falling to earth of any form of water (rain or snow or hail or sleet or mist)