downfall vs rainfall

downfall and rainfall both are nouns.

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

As nouns, rainfall is a hyponym of downfall; that is, rainfall is a word with a more specific, narrower meaning than downfall:

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

Other hyponyms of downfall include fine spray, hail, rain, sleet, snow, snowfall, virga.

downfall (noun)	rainfall (noun)
a sudden decline in strength or number or importance	water falling in drops from vapor condensed in the atmosphere
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)