Red Sky at Night Ain't No Delight

Smoke has returned to northern Utah and with it degraded air quality and reddish sunrises and sunsets.  Below is last night's sunset showing a deep orange sun and the reddish hue of the smoke.  

Many people believe that the sun is yellow, but in reality, the sun is white.  It emits all visible wavelengths of light at sufficient intensities that the sun in space appears white.  

NASA photo of the sun and International Space Station in May 2011

So, it is a myth that Superman's powers derive from the yellow sun.  It's a good story, but the sun really isn't yellow.

On the other hand, to those of us on the Earth's surface, it certainly appears yellow or, in some instances, orange or red.  This is because of the selective scattering of light by the Earth's atmosphere and constituents, especially aerosols. 

Molecules in the Earth's atmosphere scatter shorter wavelength light, such as blues and violets, more readily than longer wavelength light like reds, oranges, and yellows.  As a result, as sunlight moves through the atmosphere, it shifts from white to yellow if the skies are relatively clear.  If there are additional particles such as smoke aerosols, the scattering is more intense and the shift stronger, resulting in the sun appearing orange or red.  

This process also is why the sky appears blue.  The blue hue results from the preferential scattering of shorter wavelength light in all directions by the atmosphere.  For more gory details, see this Scientific American article.

Perhaps the best situation for spectacular sunsets are situations in which there are aerosols aloft in the atmosphere, but not near the surface.  In such instances, the sunset is redder, but you can breath clean air.  We are not in such a situation right now.  We have smoke at the surface, and it, some degradation of air quality.  

Thus, red sky at night, ain't no delight and red sky at morning is an air quality warning.