Rendering cloud needs more time than smoke because the internal scattering in
cloud is very much. It seldom absorb light. So the high level scattering is
frequent.

The attached diagram is about the spectral radiance(light strength) vs level.

first level collect most light because of the sky part of the image. The
scattering of sky is much weak than cloud. Another point is that after level 4 I
using reduce coefficient to calculate the cloud as an approximate. If calculate
complete condition(no approx.) the high level are even more important.

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