Eyes see colors through a fascinating aspects of the perceptual mechanism of the eye and brain. People often wonder why the sky is blue, especially in Northern Colorado where we are blessed with the beautiful dark blue hues of mountain skies. White light is a combination of all of the visible light spectrum together. Visible light is a small section of the electromagnetic spectrum that includes x-rays, microwaves, and radio waves. The human eye us only capable of perceiving light in 400 to 700 nanometer region, a very small band. Light travels like a wave with distance between wave tops just like in the ocean. Red light has the largest waves and blue light the smallest.
Because blue light is the close to the smallest wavelength it is much more prone to bump into molecules of oxygen, nitrogen, and other gases. A scientist in the 19th century first documented the scattering of light by small particles. Lord Rayleigh expanded on the scattering so today it is usually referred to as Rayleigh scattering. Generally gas molecules become more spread out with altitude and the effect decreases. At 100 kilometers (62 miles) space is generally defined as starting. The oxygen molecules are too spread out to have significant impact on light and if you were gazing up at this altitude the sky appears black. At lower elevations, during the interaction the gas molecules absorb the blue light and re-emit it slightly changing the color and scattering it in all directions.
Since the sun is so far away light rays approach the earth as a flat wave front and are not refracted (bent) significantly since air is so close to the light bending properties of space. Now an environment has been created where the blue light has been scattered in every direction, including away from you. In contrast, the remaining wavelengths (other visible colors making up the white light from the sun) continue towards you with very little scattering. If you look at the moon at night there is not enough illumination on it from the sun to illuminate the sky so it is black. in contrast, during the day their is enough illumination to create widespread scattering of blue light everywhere you look. Only looking at the sun shows the other colors due to the direct view of the source. Since the radiation from direct sunlight literally burns a hole in the retinal tissue lining the back of the eye don’t try it!
As the sun starts to set the angle light traverses through the atmosphere becomes closer to parallel to the earth instead of being perpendicular. This causes a much longer pathway for light to pass through. Multiple scattering of blue light increases the amount going away from you to the point that the other colors start to dominate. Due to the higher altitude in Colorado there is less of a distance for scattering and our beautiful darker royal blue skies are the result.
Water droplets exceed the size of all visible light waves so clouds appear white. When the water droplet density becomes high enough it is similar to sunset except all colors are scattered multiple times. This results in more light being scattered away from you. The result is less light passing through and the darker nature of rain clouds.
One other quirk is the fact that the sky is not indigo, since it is a shorter wavelength and according to Rayleigh’s scattering equation should predominate. The eye has receptors for blue, green, and red light. These are not specific to one wavelength but each one covers a range with peak sensitivity in the specified color. Since indigo is received by blue and red receptors, our perception is altered depriving us of seeing purple all day. Color perception is extremely complex as is the interaction of light with the atmosphere. But the basic concept of Rayleigh scattering is easy enough to explain why the sky is blue. And just in case you were wondering;, no, the blue sky does not make the ocean blue, that is a whole different story.