Tuesday, August 12, 2014

The Red Sunset

As the sun sets in the sky, its color often transitions from yellow to orange or even red.  Why?

Sunlight is comprised of visible light of all colors and wavelengths.  Superman really didn't get his powers from the "yellow sun" because the sun isn't yellow, it's white.

Source: http://www.stellarplanet.co.uk/
The white color reflects the fact that sunlight is comprised of light across the entire visible spectrum, which we can break down using the Roy G. Biv acronym where R=Red, O=Orange, Y=Yellow, G=Green, B=Blue, I=Indigo, and V=Violet.  This also orders the color by decreasing wavelength, with red having the longest wavelength and violet the shortest.

As sunlight passes through the atmosphere, the shorter waves are scattered more easily than the longer waves.  This has two important effects.  First, it shifts the color of the sun from white toward the longwave or "ROY" part of the spectrum.  Second, it shifts the color of the sky, which derives primarily from scattered sunlight, toward the shortwave or "BIV" part of the spectrum.

The strength of this shift depends on both the depth of the atmosphere and its constituents.  When the sun is high in the sky, it's usually yellow.  As the sun sets, sunlight follows a longer and longer path through the atmosphere, resulting in a gradual shift toward orange or even red.  The shift is stronger when there are smoke or pollution particles in the atmosphere, which are stronger scatterers than gas molecules.

A really horrible illustration of the longer path taken through the atmosphere by sunlight as the sun sets.
Geometry allows the transition to be especially rapid in the last few degrees of a sunset.  Here's a smoke-enhanced version of the orange to red shift as the smoke-enhanced sunset last night over Puget Sound.  Apologies for the change in magnification and somewhat crooked photos.

Here's a view of the orange sun between those two photos, which also shows a nice example of crepuscular rays higher in the sky.  Crepuscular rays are produced when scattered clouds produce alternating columns of shadowed and sunlight air.  The rays appear to radiate from the sun, but that's an illusion produced by perspective, much like train tracks to appear to converge at a distant point.  

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