Create a new document (size doesn't really matter here), fill the background with black colour. Create a new layer and grab a 1px, hard round brush:

On the new layer, choose a point, hold down the Shift key and use your mouse/or tablet pen, flick it upwards:

Rotate this layer slightly to the right via Free Transform Tool (Ctrl + T):

Then turn off the background layer, and go to Image and Trim and apply the following trim setting:

After the trim, you will have only the brush area left. Hit Ctrl + A to select the brush layer:

Go to Edit and Define Brush Set and define a brush set, name it "rain brush" and hit ok:

Press F5 and bring up the brush dynamics window, apply the following brush dynamics:
Shape Dynamics

Scattering

Other Dynamics

and tick the "Airbrush", "Smoothing", "Protect Texture" options.
That's it! Now you can use this brush to paint on a new layer and you will have this rain drop effect below:
Hint: you can also adjust the flow and opacity of the brush to create more depth.

Make sure you save this brush so you can use it in the future.
That's it for this tutorial! Hope you enjoy this tutorial and find it useful! Till next time, have a great day!

