It's the little things that make this work. The way you've got light patches and dark patches against the sky showing the flow and speed of the rain. The florescent lights reflecting off the splattering rain drops. The dark sky and stormy weather help tell the story too, because these machines appear to be gearing up, or at least standing by in the terrible weather which gives us a clue to their desperate purpose.
The image itself shows so little but tells so much. It's truly gorgeous.