Rain Photoshop rules? Thread

Ok this might sound like a strange question but is there a specific way a 'Rain' texture should be made? I understand the opacity but it's the way the the opacity with the white, black is designed like:

Anyway I was wondering because I'm trying to make sure everything I create is my own work. Anyway if you have any further idea's to improve then feel free to expand on that, I was looking at multiply texture's being used for more detailed rain but I have a hard time understanding of Rain when it come's to Udk, I assume the same method also would apply so similar engine's but Udk has been my main focus for a while.