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Storm clouds are made of the same thing as white fluffy clouds: water droplets. Then why are storm clouds dark gray instead of white?

Answer:

The clouds look dark because there is less light traveling from the cloud to your eye. OK, so why is there less light?

The water droplets in a cloud scatter light in all directions. Since they scatter all wavelengths of light, clouds tend to appear white. Now imagine light entering the top part of the cloud. The light, which would have traveled to the ground is scattered in all directions, which means that less light makes it to the ground, since some of it is being scattered upwards and to the side. As light travels through a cloud, more and more is scattered upwards and to the side, and less and less makes it through to the bottom. The further the light travels through the cloud, the darker the cloud will look. Notice in the photo that the tops of the clouds are white, since the light has not traveled through very much of the cloud.

The width of the clouds also plays a part, especially when the sun is at an angle. In the areas where the sunlight can hit the bottom of the cloud from the side, it can appear white, even though the cloud is very tall.

The size of the water drops also plays a part. Water is not perfectly clear. It reflects some of the light, refracts some of the light, and absorbs some of the light. Larger drops mean that the light has to pass through more water, which means more of the light is absorbed, and less passes through the cloud. Since storm clouds tend to be tall, wide and have large water drops, they are usually dark.

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