What is the collective noun of rain?

There isn't really a specific collective noun for rain. We usually just say "rain" or "a downpour" or "a shower" or "a storm" to describe rain as a group.

You could be creative and say something like:

* A curtain of rain

* A deluge of rain

* A torrent of rain

* A sheet of rain

But these are more poetic descriptions rather than official collective nouns.