1. Water droplets and ice crystals: Clouds are made up of tiny water droplets or ice crystals, depending on the temperature. These droplets and crystals are much larger than the wavelengths of visible light.

2. Light scattering: When sunlight enters a cloud, it encounters these tiny water droplets and ice crystals. These particles scatter the light in all directions.

3. Rayleigh scattering: For smaller particles, like those in the air, the scattering is called Rayleigh scattering. This is why the sky appears blue, as blue light is scattered more efficiently than other colors.

4. Mie scattering: For larger particles, like water droplets and ice crystals, the scattering is called Mie scattering. This scattering is less dependent on wavelength and thus, we see the cloud as white.

5. Color variation: When clouds become thick and dense, they can appear dark gray, as less light can penetrate through them.

In essence, the water droplets and ice crystals in clouds act like tiny mirrors, reflecting the light back to our eyes. This scattering of light makes the clouds visible.