Why is the sky blue?

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That picture-perfect blue sky punctuated with clouds is compliments of Rayleigh scattering. ©iStockphoto.com/Dean Turner
That picture-perfect blue sky punctuated with clouds is compliments of Rayleigh scattering. ©iStockphoto.com/Dean Turner
Blue skies, smiling at me / Nothing but blue skies do I see ...
-- Irving Berlin

If you've ever wondered why, like Irving Berlin, you see "nothing but blue skies," you're in good company. It took many centuries and a lot of smart people -- including Aristotle, Isaac Newton, Thomas Young, James Clerk Maxwell and Hermann von Helmholtz -- to puzzle out the answer, in part because the solution encompasses so many components: the colors in sunlight, the angle at which solar illumination travels through the atmosphere, the size of airborne particles and atmospheric molecules, and the way our eyes perceive color.

Let's take the sky out of the equation for a moment and begin by looking at color. From a physics standpoint, color refers to the wavelengths of visible light leaving an object and striking a sensor, such as a human eye. These wavelengths might be reflected, or scattered, from an external source, or they might emanate from the object itself.

The color of an object changes depending on the colors contained in the light source; for example, red paint, viewed under blue light, looks black. Isaac Newton demonstrated with a prism that the white light of the sun contains all colors of the visible spectrum, so all colors are possible in sunlight.

In school, most of us learned that a banana appears yellow because it reflects yellow light and absorbs all other wavelengths. This is not accurate. A banana scatters as much orange and red as it does yellow, and scatters all of the colors of the visible range to some degree or other [source: Bohren]. The real reason it looks yellow relates to how our eyes sense light. Before we get into that, however, let's look at what color the sky actually is.

We'll do that next.

Blue Skies: The Size’s the Limit

Like bananas, atoms, molecules and particles in the atmosphere absorb and scatter light. If they didn't, or if the Earth had no atmosphere, we would perceive the sun as a very bright star among others in a sky of perpetual night. Not all wavelengths in the visible light spectrum scatter equally, however. Shorter, more energetic wavelengths, toward the violet end of the spectrum, scatter better than those toward the longer, less energetic, red end. This tendency is due in part to their higher energy, which allows them to ping-pong around more, and in part to the geometry of the particles that they interact with in the atmosphere.

In 1871, Lord Rayleigh derived a formula describing a subset of these interactions, in which atmospheric particles are much smaller than the wavelengths of the radiation striking them. The Rayleigh scattering model showed that, in such systems, the intensity of scattered light varies inversely with the fourth power of its wavelength. In other words, shorter wavelengths -- like blue and violet -- scatter a lot more than long ones when particles -- such as oxygen and nitrogen molecules -- are relatively small. Under these conditions, scattered light also tends to disperse equally in all directions, which is why the sky appears so saturated with color [source: Bohren].

If we were foolish enough to look directly at the sun, we would see all wavelengths, because light would be reaching our eyes directly. That's why the sun and the area around it look white. When we look away from the sun, at the clear sky, we see light mostly from shorter, scattered wavelengths like violet, indigo and blue.

So why doesn't the sky appear violet instead of light blue? The eyes have it. Your peepers perceive color using structures called cones. Your retinas bristle with about 5 million cones each, made up of three types that specialize in seeing different colors [source: Schirber]. Although each kind of cone is most sensitive to certain peak wavelengths, the ranges of the cone types overlap. As a result, different spectra and spectral combinations can be detected as the same color.

Unlike our auditory senses, which can recognizing individual instruments in an orchestra, our eyes and brains interpret certain combinations of wavelengths as a single, discrete color. Our visual sense interprets the blue-violet light of the sky as a mixture of blue and white light, and that is why the sky is light blue.

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  • Bohren, Craig F. and Eugene Edmund Clothiaux. "Fundamentals of Atmospheric Radiation: An Introduction with 400 Problems." Wiley-VCH. March 21, 2006.
  • Boyd ,Padi. "Why is the sky blue and why is the Sun red at sunrise and sunset (taking into account the properties of interstellar dust)?" NASA Goddard Space Flight Center: Ask an Astrophysicist. June 11, 1997. (June 16, 2011) http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970611f.html
  • Encyclopedia Britannica. "Scattering (Color)." Encyclopedia Britannica Online. 2011. (June 14, 2011) http://www.britannica.com/EBchecked/topic/126658/colour/21861/Scattering
  • Fitzpatrick, Richard. "Rayleigh Scattering." Feb. 2, 2002. (June 13, 2011) http://farside.ph.utexas.edu/teaching/em/lectures/node97.html
  • Gibbs, Philip. "Why is the Sky Blue?" Usenet Physics FAQ. May 1997. (June 13, 2011) http://www.desy.de/user/projects/Physics/General/BlueSky/blue_sky.html
  • NASA's The Space Place. "Why is the Sky Blue?" May 12, 2011. (June 14, 2011) http://spaceplace.nasa.gov/blue-sky/en/
  • Nave, R. "Blue Sky." Georgia State University Physics and Astronomy: Hyperphysics. (June 15, 2011) http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html
  • Schirber, Michael. "Why Skies are Blue Instead of Purple." MSNBC. July 19, 2005. (June 16, 2011) http://www.msnbc.msn.com/id/8631798/ns/technology_and_science-science/t/why-skies-are-blue-instead-purple/
  • Stern, David P. "The Many Colors of Sunlight." Goddard Space Flight Center. Sept. 23, 2011. (June 15, 2011)http://www-istp.gsfc.nasa.gov/stargaze/Sun4spec.htm