Thank you for telling how to do snow, I will give it a go Ok this how to my rain Make layer and use the paint bucket to true the layer black.
Then you Run Filter>Noise>Add Noise And put it on Gaussian and Monochrome, then you put the amount of around 65. on the Noise filter Depends on how big your picture is and how much rain you want. When you done that then you go to Filter>Blur>Motion Blur You put the angle at 60 and the distance to 25
Change the layer blending mode to "Screen" (That makes all the black transparent) Now do this Image>Adjust>Levels Slide the central gray arrow to the right, till the rain looks right.
This how to make rain, I know it is long to do, that is how I learnt how to make rain.
Are I can make a tutorials On DA showing how to do rain, it would not take me so long to, It would take me at least 30mins to do it.