PROCEDURE:

1.) Open an image file ( Note: use an image which is gloomy.Rain effect on a sunny picture doesn't work.)   and move it to a new document in photoshop.

2.)Create a new layer with, Foreground color as- Black and Background color as- White. Select the layer and :

3.)Select, Filter menu > Render > clouds

4.)Select, Filter menu > Noise >add- 135⁰ and click ok.

5.) Select, Filter menu > Blur > Motion Blur.

6.) In the layer box, select the mode- 'screen' instead of 'normal'.

7.)Select, Image menu > Adjustments > Levels and make changes if you like.

8.) Use CTRLl + T (free tranform) and scale (increase the size) the layer as higher as possible.

9.)Select, Windows > Animation, and create many frames.

10.) In the first frame, move the layer(where you applied the above effects, not the picture layer) downwards in slanting position,

slowly and steadily.

11.) In the second frame, move further downwards.

Note:

Move little by little. The amount of movement depends on the size of layer (resizing using CTRL+T in step no.8).

12.)Follow the procedure for about 4 frames and play.

You would have rainy effect.

Move carefully in the same direction(use grids and guides if you must) or else the rainfall would be a little awkward as though falling in crooked directions.