Clouds appear white or gray due to the way they scatter light. Cloud droplets are large enough to scatter all wavelengths of light (colors) almost equally, unlike the selective scattering that makes the sky blue. When sunlight hits water droplets in clouds:
- White clouds: All colors of light are scattered back to our eyes equally
- Gray clouds: Thicker clouds allow less light to pass through or reflect back
- Dark gray/black clouds: Very thick clouds block most sunlight
The appearance can also vary based on the cloud’s position relative to the sun and the observer, as well as the presence of pollution or other particles in the cloud.