Rain Rain

You could do that, but you have to remember to make everthing darker as well. Unless your figs are in ponchos, they won't be glossy. Also, you might want to experiment with some less glossy finishes, since there would be somewhat subdued lighting. If you have puddles, you'll need to invest in either epoxy or a specific water replicating product since Future will not build up enough to create puddles. Woodland Scenics Water Effects is a thick Elmer's-like product you could use for spalshes and dripping effects.

Very easy to make a rainy diorama, you just need to install a sprinkler system above it

You could do that, but I don't think it will look too realistic. I never tried this, but hope to some day because it seems that it should work. Consider it as dry brushing with future and a tooth brush. Just put a little future on the bristles and either pull down, or flick it onto the model. If

you do it that way, let me know who it comes out. 

I seen a diorama a few years back that a gentleman had made of the final moments of an F1 race that finished in a hard rain.The way he finished the diorama looked very convincing.

What he did was similiar to all the statements above with the exception that on the plexiglass box lid that covered the diorama..... he scribed different lengths of perfectly vertical lines in the plexi on all sides except the top.Most of these lines were the heighth of the cover (top to bottom),some were not,but the others were over half the height of the cover.Some on the upper part of the box,some on the lower part.The lines were scribed close together.Like on a ruler,but in very random patterns.

(Imagine,in a very hard downpour how close together raindrops are side by side and you get the idea)

To finish the effect he then took clear sheet plastic,(like Squadron Thermaform) and scratchbuilt the rooster tails coming off of the tires.Lightly fogged with (and I'm guessing here) Model Master clear flat.Just remember back to the time your clearcoat got on your canopy and you will know what I mean.

You might even go a step further by adding a random flashing strobe light along with a 'rolling thunder' sound to complete your storm.....

Just a couple of ideas that I hope help you in some way...

How about doing it like a shadow box? so you'd look at the scene through one opening, that would be covered in glass or plexiglass, and painted/etched/tinted or whatever so it'd be like you were looking through the rain to the scene. Just a thought. Sounds like a neat idea.