Is there a rain effect in Optics?
There are no rain particle effects at this time. However, there are some Rain patterns in the Light filter located in the Light category. You may want to check out the free Particle Illusion standalone application here: https://borisfx.com/products/particle-illusion/ You will find rain particle effects.
Sorry to miss this. Sometimes you can use filters that were not designed for a specific purpose to get near your desired results.
I started with a black color effect and then created a pretty decent “Rain” effect in Optics using the NightSky filter in the Render catagory with the streaks setting set to 1 and then rotated and sized.
If you then play with the top Location sliders, you could make simple variations to generate something to simulate rain. I would then use 2-3 variations of this on their own layers PS or LR to layer the rain in for depth and a transfer mode to comp it all together.
You can download my Optics setup here.
Here is a screen shot:
Or as Marco suggests, standalone Particle Ilusion is free and has tons of emitters.