The complete lighting equation involves doing a diffuse calculation with the surface of the cloud defined by a normalmap. Added to the diffuse term is the illumination of the light transmitted through the cloud from any light sources behind it. This is approximated by the dot product of the eye and light vectors, and modulated by the transmissivity map of the sprite.

Illum = Transmissivity * ( -Eye . Light ) + diffuse

The diffuse term is calculated from a normal map in the standard per-pixel method.

Wrap lighting is used for the transmissive term, which maps the values generated by the dot product from the range [-1,1] to [0,1]. This results in the hilight "wrapping" around edges to emulate subsurface scattering and keep the cloud looking good when the light is at a 90 degree angle to the eye.

So the final equation with the scale and bias for the warp lighting ends up as:

Illum = Transmissivity * ( ( -Eye . Light ) * 0.5 + 0.5 ) + diffuse