Another example, and some answers:

As many deduced, and Gilles illustrated, the clouds are POV-Ray pigments
billboarded onto semi-transparent objects.  And, as Gilles pointed out, I
first posted an example of this back in Sep 2000 (see
http://news.povray.org/povray.binaries.images/12959/).

I recently sat down with 3.5, though, and attacked the idea again with all
of the new features available.  However, after trying various pattern
functions I discovered my original 3.1 code gave faster, more attractive
results.  These last two postings, then, don't use any 3.5 features - no
function patterns or otherwise.

From my experiments, I would say the technique is best for creating cumulus
clouds - i.e. puffy, medium height "cotton wool" clouds.  The hard edges in
the last post, and the depth of fog, basically replicate what I see when I
look outside my window every day!  Of course, it's very easy to soften the
cloud textures, and to reduce the amount of fog.  Because the clouds are
individual objects, you can have complete control over their placement in a
scene, and they are focal-blur compatible.  I've also created various
animations, flying from below to above the clouds, through the clouds,
around the clouds, etc., and the results are reasonably effective.

Now the downsides - because the clouds are flat objects and designed to look
best from the camera's viewpoint, there can be problems with reflections and
shadowing.  I am working on solutions for this, which I don't think should
be too difficult.  I also want to build in more sun-lighting features, and
possibly combine the cloud generation with other related effects...

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