Well, fluids aren't really designed for small particulate matter such as rain.
Fluids are best left to things like larger volumes of things like clouds, smoke, lava, fire, etc.
Simple emitted particle streaks, or blurred point particle types are your best choice for rain.
For a few suggestions on how to better achieve rain, I answered your other rain post also btw, in case you missed it
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS