A cloud is a visible mass of water (tiny water droplets and/or ice crystals) suspended in the atmosphere at a considerable altitude. When the clouds are soaked with water particles, they do not allow the sunlight to pass through them easily. A significant amount of light is absorbed by the cloud, giving it the dark appearance. Contrarily, the clouds that are lightly filled with water droplets are able to reflect light and all the visible colours in that light. That is why these clouds look white in appearance.
Why are some clouds darker in appearance?
