Katty,
I'm going to attempt to offer a suggestion, but it's fairly complicated and I'm new at answering these things, but when I saw this method used, I was really impressed. So here it goes.
I'm going to assume that you want clouds without a background so that you can put your own picture behind it and have that show through as a background or something like that. I tried doing this on PS 5.5 and it worked okay.
1. With a new document open, choose the foreground and background colors you want to use. Goto Filter> Render> Clouds.
2. Go to the Channels pallette and look at each of the R, G and B channels separately. Try to pick the one that has the most contrast.
3. Duplicate that channel either by dragging it down to the New Channel icon on the bottom of the pallette or by some other way. This should produce a black and white channel just like the one you chose.
4. Now go to the Levels dialog box by hitting ctrl+L. Here is a brief(!) explanation of levels: the little triangles at either end of the slider indicate where pure white and pure black are. When you drag those to the inside (ie. closer to each other) you set a new pure white or pure black point. Everything outside of those little triangles switches to pure white or black. There are 256 shades of gray between pure white and pure black. By dragging them in closer to each other you take out a few shades of gray and make them either white or black--the lightest grays turn white and the darkest grays turn black. (Anyone can correct me on that if I'm wrong.)
5. Now that you have the levels dialog open, you want to use it to define the sections of the artwork that you will eventually make transparent--the background that you will get rid of. Do this by sliding the triangles toward each other until you have white sections (parts of the clouds you want to keep), gray sections (parts of the clouds that will be, well, cloudy), and black sections (parts of the clouds you will delete). Notice that since you used render> clouds to make the first cloud pattern you don't have a lot of say about where the black sections are and where the white sections are. Render> Clouds is random. You can use a soft paintbrush and gently add white or black in this channel if you want to keep a certain part or delete a certain part (white--keep, black--delete).
6. With this channel selected and visible on the screen, go to Select> Color Range. Make sure you're selecting "Shadows" and click Okay.
7. Here I would feather the selection a little bit.
8. Go back to the layers pallette and select the clouds layer.
9. With the selection still active, hit Delete on the keyboard and clear out the selection.
10. Now you can add your background picture behind the clouds layer and have the buildings, trees or whatever show through a cloudy layer.
I understand if this post is confusing. It's hard to explain some of this stuff if you're not watching someone do it (don't know how the tech support does it!) PLEASE ask for any clarification at all. I can probably post some screenshots, too if you want.
I first saw this method used to remove the background a track runner's body. It's a good method to use to select odd shapes, expecially if they are darker than the background.
--RJ
I will try , and will inform you the result soon.