* Light Scattering: When sunlight hits a cloud, the tiny water droplets or ice crystals scatter the light in all directions. This scattering is called Rayleigh scattering if the particles are much smaller than the wavelength of light (like air molecules), and Mie scattering if the particles are roughly the same size as the wavelength of light (like water droplets).
* White Appearance: Because all colors of light are scattered roughly equally, we see clouds as white. This is similar to why we see a white t-shirt – it reflects all colors of light.
* Darker Clouds: When clouds get thicker, more light is absorbed and less is scattered back to our eyes. This makes the clouds appear darker or gray.
* Colors at Sunset/Sunrise: When the sun is low on the horizon, the sunlight travels through more of the atmosphere. Blue wavelengths are scattered away more, leaving the longer wavelengths (red, orange, and yellow) to reach our eyes. This is why we see colorful sunsets and sunrises, and why clouds can take on reddish hues at these times.
In essence, clouds are visible because they scatter sunlight, making them appear bright against the darker sky.
