Rain clouds are dark because they are thick and contain a large amount of water droplets or ice crystals. When light from the sun or moon shines on a cloud, it is absorbed or scattered in many different directions. The water droplets or ice crystals in the cloud are much larger than the molecules of air that make up the cloud, so they are more effective at scattering the light. This causes the light to be absorbed by the cloud, making it appear dark.

Additionally, clouds form when moist air rises and cools, and the water vapor in the air condenses into water droplets or ice crystals. These droplets and crystals are much larger than the air molecules in the cloud, so they are more effective at scattering light. This can also contribute to the clouds appearing dark.

Storm clouds, in particular, can be very dark because they often contain a lot of water and are thick. Thunderstorms, for example, are characterized by heavy rain and lightning, and the clouds associated with thunderstorms are often very dark. This is because the clouds contain a large amount of water droplets, which are effective at scattering light and making the clouds appear dark.

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