Welcome To My Rain Tutorial In Photoshop
Here you will learn how to make a sweet rain using one brush and some filters.
Now what we are looking to make in this tutorial is this.

Open up a document of any size. For this tutorial I will be using a 500 x 500 document
Fill your first layer with black and create a new layer on top of that.
Now for this you will need some star brushes which can be easiy obtained from DeviantArt. The ones I'm using are called Sha Star Brushes. Find a brush that does not have too many stars in it and yet not to little.
Now just dot y our selected brush around your new layer about three times. until you have something like this:

Now with that done go to Filter-> Blur-> Radial blur and use these settings:

Now keep repeating that step on the same layer. Brush a few times, blur etc. When you have done that a good few time you should have something similar to this:

Okay were not done just yet, we still need to add in the storm clouds.
Now go back to your black layer and press D to set your colors to the default Black and White.
Go to Filter-> Render-> Clouds and keep doing that until your satisfied with your cloud layout.
This is something like what you should have so far:

Now add some color to your clouds.
Go to Layer-> New Adjustment Layer-> Hue/Saturation press okay and use these settings:

Once you have that done got to your blurred brush layer and duplicate it 2 or 3 times or until you get your desired effect.
.: Finished product :.

Now you have a pretty decent rain effect fom just using filters and 1 brush. Yuo could enhance the effect even more by making a new brightness/contrast adjustment layer on top or your cloud layer and using these settings:

There you go a fully finished Rain effect using only filters and 1 brush.