Behind the Forecast: Science behind sunsets

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Science Behind the Forecast 4/12/19

LOUISVILLE, KY (WAVE) - Sunrises and sunsets are phenomena that we experience every day. Have you ever stopped to think about why they look the way they do?

It's all because of scattering. Scattering is defined by the National Weather Service as "The process in which a beam of light is diffused or deflected by collisions with particles suspended in the atmosphere." While there are quite a few types of scattering, the ones that are most seen in the meteorological field are Rayleigh and Mie. Rayleigh scattering occurs when the particle scattering the light is small in relation to light's wavelength. On the other hand, Mie scattering occurs when spherical particles are around the same size as the wavelength it is scattering.

Sunlight is made up a range of colors from reds to violets. The wavelengths in this spectrum stretch from .64 micrometer for red to .47 micrometers for violet. Air molecules are about 1000 times smaller than the color wavelengths so they scatter them well.

When the sun is high in the sky, the sky appears blue because small particles Rayleigh scatter mostly blue light. Blues and violets, which are shorter wavelengths, get scattered out as sunlight travels through more of the atmosphere since the sun is much lower on the horizon during sunrise and sunset. With the blues and violets gone, reds, oranges, and yellows are vibrant; these are the longer wavelengths. Since light has to travel a longer distance at sunrise at sunset, the longer path means the blue and violet are scattered out well before the light ever reaches our eyes. Think of it this way, a bright blue sky over California fades into a beautiful red and orange sunset over Kentucky and Indiana. Fun fact: Since the sensitivity of human eyes peaks in the middle of the color spectrum at green, the sky appears blue to us. Otherwise, the sky would actually look violet (blue is closer to green on the spectrum than violet.)

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Contrary to popular belief, pollution actually has a negative effect on the colors of sunrises and sunset. Particles found in smog or haze are too large in comparison to the wavelengths of visible light; they are usually on the order of .5 to 1 micrometer. Due to this, they are not good Rayleigh scatterers and can even mute the colors in sunrises and sunsets. The varying sizes of particles also mean that they scatter light in a way that's not wavelength sensitive. Pollution can even reduce the amount of light reaching our eyes and in turn the brightness of a sunrise.

Louisville sunset
Louisville sunset

Clouds can actually improve a sunrise and sunset. Upper-level clouds catch sunlight that hasn't been reduced in intensity/brightness by traveling due to traveling through the atmosphere.

Late fall and winter are the best times to catch a sunrise or sunset in the United States according to Meteorologist Dr. Stephen Corfidi. This is because air circulation is more stagnant during the summer months which helps with the formation of haze and smog. During the winter and fall, the air tends to be drier and cleaner leading to more vivid sunsets and sunrises.

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