Fromdown- +‎pour.

• (UK)IPA^(key):/ˈdaʊn.pɔː(ɹ)/

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downpour (pluraldownpours)

1. A heavyrain.

   Synonyms:cloudburst,rainstorm,(rare)pourdown

   They got caught in adownpour without an umbrella and came back soaked.

   • 1951 January, H. A. Vallance, “Kyle of Lochalsh Revisited”, inRailway Magazine, page14:

     I left Inverness on a Saturday afternoon in July in a steadydownpour of rain.

downpour (third-person singular simple presentdownpours,present participledownpouring,simple past and past participledownpoured)

1. (intransitive) Topour down;rain heavily.
   • 2002, Patricia Koretchuk,Chasing the comet: a Scottish-Canadian life - Page 211:

     It started todownpour, so Billy and I made our way back to our house, with Scotty following—or so I thought.

• downpouring

• pour down