For a recent project I found I needed to create some realistic looking clouds in the background of an illustration, but I had to keep the whole thing in Illustrator. Moving to Photoshop wasn't an option as the image could potentially be used at any size, from 10cm wide to two-feet wide or more, so vector format was the order of the day.
A quick search online for "realistic clouds in Illustrator" threw up plenty of results, but none that would give the effect I was looking for. Most seemed to rely on using gradient meshes or symbols, custom brushes and the like. After a little experimentation I found a solution that worked well, and I've since used for a couple of other projects, so I thought I'd document it in case anyone else might find it useful.
This is my first stab at a tutorial, so hopefully it all makes sense outside my head. If people do find it at all useful then maybe I'll post some other tips and tricks.
1. In Illustrator create a new document (File > New), draw a rectangle to cover the background and then draw an oval as the base for your cloud.
2. Apply the Feather effect (Effect > Stylise > Feather).
3. Apply the Scribble effect (Effect > Stylise > Scribble). Some of the preset settings can be a good starting point to get the cloud shape you're looking for.
4. Apply the Gaussian Blur effect (Effect > Blur > Gaussian Blur).
5. At this point it's probably not looking too good, now that all the effects are applied we can tweak them to get a better looking effect. To edit an effect that's already applied select the shape on the artboard and double click the effect you'd like to edit in the Apearance Palette.
That's a bit more like it!
6. One cloud on its own looks a bit lonely so you're probably going to want to duplicate it and resize/reposition. Alternatively, once you have a basic cloud with all the effects in place you can drag it to the Graphic Styles palette to create a style that can be applied to any other shape/object in one click.