Special Effects - The Photoshop: Professional Retouching and Compositing Tips, Tricks, and Techniques - page 242

Figure 4.37
5. To turn this rain effect into a brush, go to Image > Adjustments > Invert so that the
rain effect is now black against a white background. Photoshop uses the dark and
gray areas to form the brush, ignoring the white areas. Go to Edit > Define Brush
preset, name the preset rain , and click OK.