  The integro-differential formulation of the RTE and its solution by
iterations on the source has been extended here to handle anisotropic
scattering. The iterative part of the method is O(N ln N ), thanks to an
efficient use of H-matrices. The precision is good enough to evaluate the
effect of sensitive parameters for the study of contrails. Most of the time the
stratified 1D approximation should suffice, but in complex cases with high
relief the 3D formulation is needed.
