When sunlight hits a cloud, some of the light is absorbed by the water droplets or ice crystals in the cloud. Some of the light is also scattered in all directions, which is what gives clouds their white color. The amount of light that is absorbed or scattered depends on the wavelength of the light and the size of the water droplets or ice crystals.
Smaller water droplets or ice crystals scatter more blue light than red light, which is why clouds often appear white or gray. Larger water droplets or ice crystals scatter all wavelengths of light equally, which is why clouds can appear dark gray or black.
The amount of water droplets or ice crystals in a cloud can also affect its darkness. Clouds with a higher density of water droplets or ice crystals appear darker because they block more sunlight from reaching the ground.
Finally, the angle of the sun can also affect the darkness of a cloud. Clouds that are closer to the sun appear darker because the sunlight has to travel through more of the cloud to reach the ground. Clouds that are farther from the sun appear lighter because the sunlight has to travel through less of the cloud to reach the ground.
In summary, the darkness of a cloud depends on a number of factors, including the optical thickness of the cloud, the size of the water droplets or ice crystals in the cloud, the density of the water droplets or ice crystals in the cloud, and the angle of the sun.
