Less light reaching below a dark cloud can indicate what?

Correct answer: Greater cloud thickness or density

Direct answer: A. A cloud appears dark from below when it prevents a relatively large amount of sunlight from passing through. A thick cloud contains a longer path of water droplets, ice crystals, or both, so light is scattered and absorbed before reaching the ground. Greater optical thickness or density can therefore produce a darker appearance and reduced illumination below the cloud. A is correct because less transmitted light is a useful indication of a thick or optically dense cloud, although darkness alone does not prove exactly how much rain will fall. B is wrong because clouds commonly contain water droplets or ice; lack of water would not explain strong light blocking. C is wrong because a cloud is not a rock. D is wrong because cirrus clouds are generally thin and feathery, so they often allow more light through than thick lower clouds. Memory cue: dark underneath usually means light has been blocked by a thick optical path, not that the cloud is solid.