WRAL WeatherCenter Blog

Wolfpack Red sunsets in a Carolina Blue sky

Posted March 6, 2015
Updated March 7, 2015

Sky

The sun emits energy across the most of the electromagnetic spectrum (gamma through radio waves), with much of the energy concentrated about in the visible range. The combination of all colors of light gives the sun a white appearance to astronauts. So why does the sun look yellow to us mid morning and afternoon, more white midday, and orangey-red at sunset? The answer lies in our atmosphere; specifically in it’s ability to scatter blue light.

As sunlight filters through our atmosphere, tiny molecules of water vapor and other gas molecules scatter light. This scattering, called Rayleigh scattering is most efficient at the blue end of the spectrum where wavelengths are shorter. Referring to clear skies as “Carolina Blue” turns out to be pretty reasonable, scientifically.

According to the University of North Carolina’s branding and visual identity guidelines, “Carolina Blue” has an RGB equivalent of #56a0d3 that roughly converts to a wavelength of 475 nanometers (nm). This is right around the size of a lot of gas molecules causing that (Carolina) blue light to be scattered in all directions. This is also known as Rayleigh scattering for the British physicist who described the phenomenon. On a clear day, about 10 percent of the light reaching Earth is diffused by this process. The darker Duke Blue appears after sunset I suppose.

Wolfpack fans aren't left out. Those larger waves are able to pass unaffected through those small molecules. The color of the sun changes to a (Clemson, #F66733, 630nm) orange, then to Wolfpack Red (Pantone 186C, #CC0000, or around 650 nm), and finally to toward the end of the spectrum (on very special days) to Chicago Maroon (Virginia Tech, #660000, 780nm). As the sun sinks lower on the horizon, light must pass through more and more atmosphere. Less high frequency blue light makes it though creating a redder appearance, especially around the sun itself.

Sunsets on other terrestrial planets differ with their atmospheric make-ups. Mercury really doesn’t have an atmosphere but rather has an exosphere made up of atoms blasted off the surface by solar wind. The sun would appear about 3 times larger and 10 times brighter in Mercury’s sky and a brilliant bright white (take your pick of ACC school with white in their official colors) due to that lack of atmosphere. Mercury’s slow rotation would keep the sun above the horizon for nearly an Earth month.

When the Soviet Venera 13 probe landed on Venus 53 years ago this week, it transmitted images of an deep yellow sky, closest to Wake Forest Gold (#9E7E38). The mostly carbon dioxide atmosphere of Venus is so thick the sun would be hard to even make out in the sky. It takes Venus nearly as long to rotate on its axis as it does to orbit the sun. Sunup to sundown on Venus lasts about 88 days but that golden sky would likely be lit much longer due to diffusion of sunlight from below the horizon in that thick atmosphere.

The skies of Mars have a red tinge that would make any Wolfpack fan proud, but not for the reason you might expect. Mars' thin atmosphere is filled with fine particles of red dust from the planet. These dust particles are so tiny that they do a very good job of absorbing high short wavelength blue light and scattering red. Skies have a red cast throughout the day but at sunset something beautiful happens. Light around the sun, having lost its red components has (Carolina) blue color producing brilliant sunsets as seen by the Curiosity, Spirit and Opportunity rovers.

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