When can a cloud appear dramatically dark due to the sun's position relative to the observer?

Answer

When the cloud is opposite the setting or rising sun, and the sunlit side faces away

When the sun is low, light travels through more atmosphere; if the sunlit side of the cloud faces away from the observer, the cloud will appear dark because the remaining ambient light is heavily filtered.

When can a cloud appear dramatically dark due to the sun's position relative to the observer?
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