I used clumps so it would look like sheets of rain. Now I just motion blurred it in the direction I wanted the rain to come down. (To get the effect to stand out, you might want to duplicate your layer so it’s twice as opaque, otherwise it could be too subtle).
I did two layers like this at slightly different angles. Then I just set these layers to the opacity I wanted, drew in a few random strokes in straight lines (they look best at slightly different angles from the rest of the rain) that’s it! Here’s a detail of the finished picture:
Softening effects
Motion blur works great to soften things a bit, much the way you’d use a soft directional stroke of a brush in Oils or Acrylics.
In this picture, I wanted soft wings. First, on a new layer, painted in strokes of color where the feathers would go, getting the basic form correct and worrying about light and dark, but didn’t care about modeling the form at all. Then, selecting sections of wings that were similar angles, I used motion blur to soften them, blurring in the direction of the feathers.
On top of this I simply painted dark and light areas to bring out the feathers. I thought this made a pretty nice “feathered” look!
For one recent picture I did (”Spear Maiden“), I wanted a nice, soft look to a sand dune texture. I painted in the rough “sand ripple” shapes on their own layer, which didn’t look too great. But then I Motion Blurred them and it made all the difference. Here’s a close-up of the effect:
