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    Literally there have been times where the sunset took my breath away.  The colors can be so extreme and vivid they can actually take you by surprise.  Without an expectation, you suddenly look up to see an amazing visual display.  What causes sunsets?  Why are some so spectacular others are just fair?

Atmosphere

    To understand a sunset, you need to have a basic understanding of the atmosphere.  The atmosphere is the main factor between a brilliant display and a dull display.  The majority of the atmosphere is made up of oxygen and nitrogen.  The remaining 1% is composed of trace gases, water vapor, and solid particles.  Solid particles include soot, ashes, pollen, salt, and dust.

Light

    As discussed in a previous discussion on rainbows, light appears white but is made up of many different colors.  The sun gives off electromagnetic radiation with a wide spectrum of wavelengths.  Of those many wavelengths, our eyes only see the visible wavelengths.  Of the visible wavelengths, different colors are associated with different size wavelengths.    

    A prism can split the colors up.  The colors react to the prism differently based on their wavelength. Red for example, has a long wavelength.  Blue has a short wavelength.  As the wavelengths come in contact with particles, they are refracted (change direction).   It is this scattering process that creates the independently visible colors.

    Rayleigh scattering, causes shorter blue and violets wavelengths to be scattered at a higher rate than longer red wavelengths.  Gas molecules are about a thousand times smaller than a visible light wavelength but they are closer in size to the blue wavelengths than the red wavelengths.  Small molecules tend to refract or absorb wavelengths that are closer to them in size.

    As the sun dips lower into the horizon, the suns rays are forced to pass through more of the atmosphere to reach you.  The particles in the atmosphere refract light.  When wavelengths of light are refracted, they scatter and drop out from your line of site.  The lower the sun gets, the more particles the wavelengths must pass through and different colors begin to drop out.  Think of a rainbow from the bottom up.  First the blue and violet wavelengths scatter leaving behind the rest.  Next the green wavelengths drop out. Then the yellows and oranges.  In the long run all that is left is magnificent display of reds.

From Your Perspective

    Similar to a rainbow, what you see is dependent on your perspective.  The blue wavelengths (shorter wavelengths) scatter first while the red wavelengths (longer wavelengths) remain visible.  It is not that the blue wavelengths are not there, they just do not reach your eyes based on your location.  The colors you see depends on the path that the light had to take on it's way to your eyes.  

The Sun has already dropped below the Horizon

    The actual sun has likely already dropped bellow the horizon.  The sunset you are gazing at is actually the light rays which have been bent by the atmosphere so you can still see them. The sun is physically gone from your line of sight but the light is still visible.

Does pollution create a "better" sunset?

    Added particles in the upper atmosphere, such as that from a volcanic eruption, can enhance a sunset. This is because the particles that make it to the upper atmosphere tend to be of the same size and thus filter out specific colors.  

   Added particles in the lower atmosphere, tropospheric aerosols, reduce the chances of a colorful sky. Pollution particles are of various sizes so they do not refract a certain color.  The wide range of particle sizes causes all of the colors to be refracted.  The result is regions with high levels of pollution having "hazy" skies and dulled sunsets.  Spectacular colors are a result of clean clear air.

Whey do some regions have "better" sunsets then others?

As discussed the brilliance and vibrant colors of a sunset can be diluted by pollution in the lower atmosphere.  Regions with low levels of pollution, such as deserts, have amazing sunsets due to their relative lack of pollution.

Do certain times of the year produce "better" sunsets?

The season definitely can affect the sunset.  As we know from air quality reports, air pollution is at the highest levels in the summer.  This is due to the reduced air circulation and smog formation.  Times of the year with higher air circulation, such as the fall and winter, will produce "better" sunsets.

Clouds

Clouds catch the color wavelengths that bypassed the lower atmospheres particles and then reflect the light to the ground.  Typically this is seen with higher elevation clouds such as cirrus and alto cumulus.