C&C welcome
I like this. The cloud in the foreground looks very realistic; while the rain doesn't quite look convincing, it's still a great image. The ground could use a little detail though.
I agree that the cloud looks really good, but I also agree the rain isn't that convincing. I guess you made the rain in post?
If you look in real life at raining clouds at this distance you'll see a kind of cloudy curtain from the cloud till the ground.
I know it is VERY difficult to simulate this but you could try with an image-map and very thin clouds :)
Martin