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Photoshop Tutorial - Create a Cloud Brush
It is fairly easy to create new, custom brushes in Photoshop, and a brief guide can be seen here.
One of the more imaginative brushes you can make is based on an image of clouds.

Start with an image of clouds.  Download one from Google Images or photograph some clouds yourself.

To create my brush, I've selected just part of the image and cropped it:

Cropped Cloudy Sky

 

Cloudy Sky

The first step is to 'Destaurate' (i.e. convert to black and white). 

Go to IMAGE...Adjustments...Desaturate.

Destaurated Cloudy Sky

Next, we invert the colours in our image.  IMAGE...Adjustments...Invert.

In Photoshop, brushes work by a black to white transparancy gradient. This means that everything that is black will be opaque, but white will be transparent. Greys become transparent in relation to their darkness – so a darker grey will be less transparent than a light grey.

Inverted Cloudy Sky

In our image so far we can see that the background is a dull grey colour, which means it will not be fully transparent. To help remedy this, we will adjust the levels. Go to IMAGE...Adjustments...Levels.

What I'm trying to achieve is lightening the sky - not 'bleaching it out' completely (Doing this will lose some of the detail, and you will end up with a harsher, less natural looking cloud).

Levels
Here is the result.  Obviously, every single image of clouds will vary so you will have to adjust your image accordingly. Cloudy Sky Levels

As you can see, the background is still a light grey, which will stop it from being fully transparent. To fix this, we will use the Dodge Tool Dodge Tool to lighten the areas around the cloud, without affecting the actual ‘wisps’ and detail in the cloud.

Set the dodge tool to work on the ‘highlights’ (the lighter areas) of the image, or it will just wash out the black colours. It is also a good idea to set the opacity down to 20-40% so that you can get more detail with the dodge.
Note: This is not the same as bleaching out the background in Levels.

Dodge Tool Options
Cloudy Sky Dodged
Nearly There!  Now that the cloud is surrounded by white, we can finally make it into a brush. However, there is still a piece of cloud visible in the bottom right of the image. We want to choose just the central cloud to make into a brush, so we use the Polygonal Lasso Tool Polygonal Lasso Tool to select it. Marqueed Cloudy Sky
We then go to EDIT...Define Brush to create the brush. Remember that in photoshop 7 and higher, the maximum brush size is 2500 x 2500px. In photoshop 7 and below it is 999px. If your image is a higher resolution than this, you will need to resize it.  Name your brush suitably: Name Brush
Select your Brush Tool Brush Tool and there is your new brush - at the very end of the selected brushes. Cloudy Brush
By creating a few of these brushes, you can quickly build up very rich sky compositions, and achieve a great variety of moods. Collection of Cloud Brushes