From Wakapon
Preetham et al.
http://www.cs.utah.edu/~shirley/papers/sunsky/sunsky.pdf
Javascript implementation for O3D : http://www.patapom.com/O3D/patapi/skydome.js (and http://www.patapom.com/O3D/patapi/skydome_SHCoefficients.js for SH coefficients)