The easiest to thing to do is to be in a dry place, and a good option is a window. We all have a window, well, if you are not homeless, but the thing that you are reading this let me think you are not. Anyway, as soon as you notice that it rains, go to a window and wait for a nice pattern on it : the drops.
Here in Paris at CDG, Terminal 2 waiting for a plain to Vienna, Austria :
You can get amazing lights contrats when the sun is still shinning at the other side of the horizon.
And I love the stormy late afternoon moof of this shot (witch was actually shot early in the morning in St Priest, France) :