Clouds Break the Rules

Cloud water droplets are much larger than the wavelengths of visible light — typically 10-20 micrometers versus light's 0.4-0.7. When particles are this big, a different effect takes over: Mie scattering, which bounces all wavelengths roughly equally. Every color scatters the same and reaches your eyes together, so the cloud looks white. The same physics makes milk, snow, and sea foam white. Storm clouds look dark gray only because they are so thick that most light scatters back upward before it reaches the bottom.