Creating a Raindrop
First and foremost, we're going to create a rain brush. This is incredibly easy, even if you've never created a brush before. We're going to assume that the rain is falling straight down in this method, and the brush can be used for anything from a rain storm to a light, spring drizzle. Besides, by learning to create your own Photoshop brush, raining, snowing, and even starry sky effects are never more than a few seconds away!
Open up Photoshop and create a new document. You're going to want something around 200 by 200 pixels, which is a good size for a rain drop brush. Make sure your background color is set to white!
Tap D on your keyboard, which is the Photoshop shortcut to set your foreground and background colors to their default black and white.
Tap B on your keyboard, which selects the brush tool. Go into your brush panel and select a round, hard edged brush, about 3-5 pixels, depending on how long and thick you want your raindrop to be. I'm just going to use a 3 pixel brush here, though.
Create a new layer and place a black dot in the center of it with your paint brush.
Now press CTRL+T (Mac users, you'll press CMD+D) - this will bring up your free transform box which looks like this:
Holding down ALT and SHIFT, grab the top of your free transform box and pull your raindrop into a more elongated shape, like I've done. You do this by simply pulling straight up!
Press enter when you're finished, and here's what your result will look like:
Now simply save your rain drop brush as "rain" or "raindrop" by going into Edit > Define Brush Preset and entering the name into the box.