Why is the sky blue, and why are clouds white?
(Lansing State Journal, May 22, 1996)
Not all light waves are scattered equally. Light that has a longer wavelength than the particle it meets can get around the particle and not be scattered. If the light has a shorter wavelength than the particle it hits, it cannot get around the particle and will be scattered. Blue light has the shortest wavelength of light we can see and is most likely to get scattered. When we see the atmosphere, we see the blue light scattered off of it.
This also explains why sunsets look red. Near the sun, light is not scattered very far from its original path, and the blue light is scattered farther than the red light. We see the sky near the sun as red because red is the longest wavelength of light we can see.
Clouds are made of water droplets, which are bigger than the wavelength of red light. This means they can scatter red and blue light equally well, and clouds will be the same color as the white sunlight shining on them.