Why is the sky blue?

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Why is the sky blue?
rock formation in shape of half a dome, under bright blue sky

Half Dome (in California’s Yosemite National Park) under a bright blue sky.

To understand why the sky is blue, we need to understand a little about light. Light is a form of electromagnetic energy, which can propagate through empty space. We describe this energy by wavelength – the distance between successive crests of the wave. Our eyes are sensitive to light with wavelengths between approximately 0.4-0.7 microns (one micron is a millionth of a meter or one one-hundredth the diameter of a human hair). Blue colors have wavelengths between about .455 and .492 microns, while red colors have longer wavelengths between .622 and .780 microns.

When light beams interact with particles suspended in air, the energy can be absorbed or scattered. Scattering causes the light path to change direction. The amount of scattering is a function of the size of the particle relative to the wavelength of the light falling on it. Particles that are tiny compared to the wavelength of the light scatter selectively according to wavelength.

While air molecules scatter all colors, they scatter violet and blue the most. That’s why, no matter which direction we look, it’s more likely that blue light has been scattered back toward us. Why not violet? Two reasons. The sun emits more blue than violet light, and our eyes are also more sensitive to blue.

At sunset and sunrise, sunlight must pass through more atmosphere than during the day when the sun is higher in the sky. More atmosphere means more molecules are scattering violet and blue light. If the path is long enough, all of these colors are redirected out of our line of sight, while much of the yellow, orange and red colors continue straight from the sun to the eye. This is why sunsets often are composed of yellow, orange and red light.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.