Step 1

Start with a bright high resolution image

Step 2

Next create a new layer, fill the new layer with white or black and then add noise to the image. You can use guasian or monochrome it really doesn't matter. Make sure the level is fairly high.

Step 3

Once the noise has been added create a motion blur on the noise in the direction that you want the rain to be travelling in.

Step 4

Then we will set the blending mode to screen. This will get rid of the black.

Step 5

Set the levels of the layer to whatever you desire , the closer the sliders come together the less rain activity you will have.

Step 6

Create a new layer and put it inbetween the background image and the rain layer. Create a black to white gradient in this layer and only drag the gradient about half way down the image. Then apply the Multiply mode and you can reduce the opacity if you like.

Step 7

Go back to the background image that has the picture of the car and lower the brightness to match the gradient that you applied. It should darken the whole picture giving it a gloomy look.

Step 8

This is how it should turn out.