Making Noise (1999) (noisemachine.com)

Perlin actually improved his original algorithm by creating the Simplex Noise http://en.wikipedia.org/wiki/Simplex_noise which runs much faster. As a matter of interest, Voronoi noise is also quite popular

The cloud animation (linked from http://noisemachine.com/talk1/24b.html), derived from the noise calculations, looks pretty good. I'll bet that it is computationally much less involved than, say a physics-based model, such as http://www.markmark.net/dissertation/

Stunningly cool; worth reading in full, including the sidebar discussion. It has math equations, code, sun coronas, clouds, and FPGA design schematics! May I suggest this soundtrack while reading: http://www.makingthenoise.com/music/