Create or Destroy Water can create 10 gallons per upcast-level of rain in a 30 ft cube, up to a total of 90 gallons. It can also be cast so it falls as rain.
It looks like that would produce a rainfall of about 100 mm/hour or twice the violent shower listed here.
I've calculated that the water would reach terminal velocity almost instantly, and would fall at 0.01 m/s for the whole 30 ft height. That means the rain would last for ~15 minutes.
Would it count as difficult terrain, or some other effect?