Why is the sky blue?

2011-09-26 19:41:11.000 – Kevin Cronin,  Summit Intern

Tonight’s Sunset from the MWO

Before I jump into my explanation I want to give a background of electromagnetic radiation. Every object emits and absorbs radiation. You can think of radiation as the output of energy that an object produces. The greater the energy, the smaller the wavelength an object generates. The sun emits within the shorter wavelengths due to the immense amount of energy it radiates. The human body emits at a much lower energy level and creates longer wavelengths called infrared radiation. Infrared radiation cannot be detected by the human eye but visible radiation which is given off by hotter objects such as the sun can. It is for this reason why infrared satellites are handy at night because they can detect cloud tops without sunlight.

As I mentioned earlier the sun emits within the shorter wavelengths due to the large amount of energy that it releases. The sun emits in the visible light spectrum which the human eye is most sensitive to. The sun’s radiation must pass through the atmosphere in order of reaching the surface. The radiation that passes through the atmosphere comes into contact most frequently with particles in the 474-476 nm range. This range is most effective at scattering the blue wavelength of the visible light spectrum. Larger wavelengths within the visible light spectrum such as yellow, red and orange do not scatter as easily as blue light. They require a greater amount of and slightly larger particles in order of being scattered as effectively as blue light. The scattering of light from particles that are smaller than the wavelength of light is called Rayleigh scattering. It is this phenomenon that gives the sky blue color. Our eyes detect this scattering because it happens within the visible light spectrum.

You might also be questioning why the sky turns red and orange as the sun sets. During the daytime the angle between the sun’s rays and earth is greater than the angle at sun set. This contributes to a shorter distance the solar radiation must travel during daytime and therefore less scattering. As the sun reaches the horizon the sun’s radiation must travel a greater distance through the atmosphere. The sun’s shortwave radiation must pass through larger particles of dust, pollution, and water vapor as it sets on the horizon. The copious amount of these particles allow red and yellow light of the visible light spectrum to be more effectively scattered which contributes to a spectacular range of colors during sunset. The picture I took above was of tonight’s sunset from the Mount Washington Observatory deck. As you can see the reds and oranges stick out due to Rayleigh scattering of those wavelengths. Now you can let others know that not only do you know why the sky is blue during the daytime but you understand why the sky turns orange and red during a sunset!


Kevin Cronin,  Summit Intern

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