anyone know , how to implement that? 
You can do this with a texture that will shift along the vertical axis. I’m not sure it’s going to be good, but you can try it. Take a good texture with the alpha channel and try this. I hope you can do it, because it’s really amazing
I worked this out, sort of. It’s using a particle material.
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Take any entity you want to add rain to, nothing special needs to be done to this entity.
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Create a particle library for your falling rain effect. As many emitters as you need. Probably two to recreate the video, one for the long drops and one for the round drops.
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Make a simple material for your water drops, use a normal map to give the droplets some relief from the entity’s surface. Set this material’s shader property to Particles.
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In your particle emitters, in the Particles section, set the facing type to Decal.
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Make a new entity for your particles, and add your particle system.
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Place the particle entity in the same physical space and the entity containing the surface you want the rain to appear on.
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This is the most detailed description! Thank you!