I'm looking to achieve an effect in AE that makes it seems as though rain is pouring down a clear window pane placed before the composition effect. Essentially, I'm trying to replicate what is happening in this video around 22:50 :
I'd suggest two things. First check out this series of tutorials: Weather Replacement.. He does a real nice job of creating raindrops falling/distorting the lens.
To make it look like they are running down lens, check out CC Glue Gun. Make a copy of your layer/video, and apply Generate CC Glue Gun to the top layer. Experiment with Brush Position, Stroke Width, Style, Time Span(if you want the water to run trail to follow the head). Pretty easy to get running water drops with it.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.