Rain is notoriously difficult to sketch and paint, so Deviant Art user amorphisss’s fantastic free rain Photoshop brushes might save you a lot of time. This brush set includes four brushes. You may choose the way the rain should fall for each one and use the Motion Blur filter to emphasise the motion impression.
amorphisss is the designer.
Usage: Personal and commercial use are both free.
Rain is available for download on DeviantArt.