I was looking at the big cloud picture. The cloud itself is awesome!  As a former professional meterologist and weather observer I noticed that the rain is not so realistic looking. It seldom comes down in straight line sheets. The air is turbulent with the downdrafts so the sheets wave a bit. Also the color has a greenish hazy tint. Maybe use 3-4 sheets, each wavy in a different way and add a hit of mist so I looks more like rain and not an object.

Just some thoughts! I really think you are on to something!!