Volume Primitive Fog.

Well, after running through a few different ideas to make the fog. Including using sprites, as well as textures rendered on geometry planes. I settled on adding back in the very simple atmospheric fog to kind of "wash" the scene. As-well as making use of the volume primitives again. This combined with a 3d texture makes for an acceptable fog effect, taking a cue from the Maya fluid effect, I am going to use an expression to control the timeslider that will animate the fog. This solution has provided both a reasonable aesthetic outcome aswell as a reasonable render time.

Maya Fluids Fog

I tried to use the Maya fluid system to create a fog for the clouds, after a failed attempt trying to use sprites. However this method of creating the fog increases the render times to an unacceptable amount. However I an aware of another technique used by Alex Alvarez in several of his images (Including "Dirge" which was the basis for this project initially).
However here he explains his use for creating some background fog in his image "Gathering"
I hope to utilize the same technique as well as some expressions to get an animated fog that renders quickly.

Lighting Tweaks

Here we have the rain, with its size and lighting tweaked to closer match the reference, as-well as what the the rain would look like on a plain background, which could be used later for compositing. I want to try and find a better solution for the fog as-well, one that can be animated, however fog in the past is what has made the render times impractical, so hopefully I will find a good solution.

Reference and Render Test 02

Here we have some more reference footage and images.As you can see it was raining incredibly heavily when I took these. There is some things that I noticed aswell, Im sure its more to do with the camera I was using but the streaks in these images are a lot shorter than the ones In my current renders.
Secondly, the rain is illuminated a lot brighter than I currently have it, and so in order to match the real rain better, I will need to tweak the render settings and the lighting.
Currently I amen't using any motion blur as it increases render times to an unacceptable degree, I'm using different anti-aliasing filters to blur the result of the image to get a better look for now though.

Render Test 01

The first test render of the rain effect, I'm reasonably happy with how its falling at this stage, though it could benefit from falling faster. However looking at my reference, the lighting is off and I feel that the environment fog is too dark as-well and so the effect is not as visible as I would like.

Building Up The Effect

After a failed attempt to get the scene rendering in vRay (its being funny about rendering nParticles), I rebuilt the scene from scratch in Maya 2014 (it was in 2015 previously). I also went out and shot an IBL to illuminate the scene and provide the rain something to refract.
I managed to create the "streaking" effect by using instanced geometry over the particle system, I had previously tried to create the effect using motion blue, but the result was not as good and it increased the render times too much.
Currently the scene is also using Maya's simple fog environment shader to provide a bit of atmospheric perspective.
I have here a play-blast of the effect as it is currently, as well as the IBL I used.

Fog and Lighting

The way the rain is lit is going to be very important. Rain is hard to pick out a lot of the time because of its transparent nature, especially when situated against a grey sky. Although the size of the rain plays an important factor - its far easier to see bigger drops - the way its lit is still the most important factor in creating a good effect.

I've also been out gathering a bit of reference pertaining to the environment fog. Now, its been quite bright here today and so the fog is a lot lighter here than it will probably end up being in the scene itself. It still serves as good reference for me however.

I am also currently reworking the scene in order to render with V-Ray, this will hopefully lead to a further improvement in render times.