Why is the Sky Blue?
Photo by user "guitrento" on Flickr
When I was a little kid, I was often told, along with many others, that the sky is blue because it reflects the color of the ocean. However, this couldn’t be farther from the truth!
When you stare at the sky during the daytime, the sky appears blue with white clouds; however, when you look at the sky at night, it appears black, showing only the stars and the moon. Why is it that? Why does the sky turn to bright blue and the stars seemingly disappear during the day?
The sunlight we see from earth takes about 8 minutes to travel the 93,000,000 miles from the sun to our eyes and provides the necessary heat and energy to make life possible on earth. Contained in that sunlight are all of the wavelengths of so-called visible light, and each wavelength is viewed as a different color (red, green, blue, etc. …invisible infrared and ultraviolet light are at the outer ends of this visible spectrum). The molecules in earth’s atmosphere, mostly nitrogen and oxygen, inherently scatter some of those visible wavelengths to varying degrees.
Because those atmospheric molecules are small compared with the wavelengths of visible light, they only end up scattering the shorter wavelengths of light, sending the light beams in all directions, knocking them out of the direct path of light on its way to your eye. This “selective” scattering of wavelengths (colors!), is called Rayleigh scattering (named after the British scientist Lord Rayleigh, 1842-1919). Rayleigh scattering is a physical phenomena that causes light to scatter when it passes through particles that have a diameter one-tenth that of the wavelength (color) of the light. The sky is blue during the daytime because the wavelengths for violet and blue are the shortest in the entire spectrum of visible colors to the human eye. They are therefore scattered more than any other colors in the light, and as these wavelengths scatter across the sky, the sky appears blue.
To put it differently, the blue color of the sky comes about because the tiny air particles are too small to make good “antennas” for that visible light. It has been discovered that an antenna works best when it is one-quarter as long as the wavelength of the electromagnetic wave it is trying to receive. Thus the air particles make particularly bad antennas for the long wavelength of red light, therefore very little “red sunlight” undergoes. Rayleigh scattering on its way through the atmosphere. Because red light, yellow light, green light and the other colors aren’t scattered nearly as well, you see the sky as the beautiful side-effect of light blue (again, the shortest wavelength of all the visible colors). Ultimately, the blue sunlight is being scattered tremendously and reaches our eyes from all directions.
Rayleigh scattering not only makes the sky appear blue; it also makes the sunrises and sunsets appear reddish. As the sun rises or sets, its light must travel long distances through the earth’s atmosphere in order to reach your eyes because of its wide angle. Its path is so long that most of the blue light that the Rayleigh effect scatters is already bounced many miles away to your east or west and all you see is the remaining red light. As an interesting addition, sunrises and sunsets are particularly colorful when extra dust or ash is present in the atmosphere to enhance the long-distance Rayleigh scattering even further. Air pollution, forest fires, and volcanic eruptions tend to create unusually red sunrise and sunsets.
In regard to clouds: Clouds are made up of very large collections of very tiny droplets of water or ice crystals. And while these droplets are tiny, perhaps, they are still large enough to scatter the light from all seven visible wavelengths (red, orange, yellow, green, blue, indigo, and violet). Thus, because of their high rate of reflectance (between 70-95%) of visible light wavelengths, clouds often appear white. Clouds appear dark or colored only when caught in shadows or enveloped by a sky that already appears a certain color or colors – or when there are so many clouds in the sky that sunlight is simply blocked out completely!
In regard to the ocean: The varying blue color of the ocean is due to the fact that water absorbs wavelengths in the red part of the visible color spectrum, and thus leaves behind colors in the blue part of the spectrum which the human eye sees. Thus, the ocean also sometimes appears greenish in shallow water because there is a wider spectrum of blue-green colors being transmitted, while in deep water, the only colors being transmitted are a small range of blues, and that’s about it – thus the almost black appearance. Hardly any light penetrates deeper than 200 meters, and at the depth of 2,000 meters absolutely no light can penetrate. The reason why the ocean seems to take on the color of the sky at times, such as during a sunset, is actually due to the flat surface of the water acting as a mirror, and not due to the color of the water itself. This can be further understood when viewing rough water during a sunset as it becomes less reflective of a colorful sky.
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