simple way is just take a glass panel and drop a few drops of water...
as you can see, normally rain is accompanied by dark weather, so when you have fast shutter, you end up with dark image, to counter this, you need flash, but only expensive flash can do fast shutter unless its always ON, but den again, the flash cannot capture the whole scenery. and the water droplets move so fast, you can't even move your camera to follow the droplet & focus it... unless the AF is damn good.