By Meghan Oliver
posted
Nov 29, 2012
It seems each autumn, I start noticing sunsets more. They are so
pink, so orange, so bright. I've always chalked up my autumnal
sunset attention to my mood shifting with the changing season;
perhaps I'm feeling a little wistful at summer's end and reflecting
on nature's splendor more than usual. But according to the National
Oceanic and Atmospheric Administration (NOAA), the colors we see
during sunsets really are more vibrant in fall and winter than they
are in spring and summer - seasonal melancholia has got nothing to
do with it.
The intensity of sunset and sunrise colors has to do with
Rayleigh scattering. I spoke with meteorologist Chris Bouchard of
the Fairbanks Museum and Planetarium in St. Johnsbury, Vt., who
kindly dropped some knowledge on this scattering business.
Bouchard started off by explaining that the light we receive
from the sun here on Earth is white - meaning it is composed of an
even mix of all visible colors. But this white light is transformed
as it passes through our atmosphere and encounters nitrogen and
oxygen molecules, which causes the light beams to "scatter," or
change direction. It is this scattering that "paints the sky with
color," Bouchard explained. "Because of some fairly complex
physics, these gases cause light beams of shorter wavelengths to
scatter more than those of longer wavelengths."
Light beams of shorter wavelengths include violet, blue, and
green; those with longer wavelengths include red, orange, and
yellow.
"Violet has the shortest wavelength in the visible spectrum, and
so it scatters the most," Bouchard said.
Which begs the question, why isn't our sky violet then, and not
blue?
"Well, in fact it is!" said Bouchard. "It's just that the human
eye is more sensitive to blue light than purple, so the sky appears
blue."
At high noon, when the sun is directly overhead, the direct
sunlight travels through a relatively small amount of air to reach
your eye. During sunrise and sunset, the sun's direct light comes
in at an angle, which means that it has to travel through much more
air than at noon, explained Bouchard. "Therefore, more cool colors
are scattered away from the light beam, causing the tint of what's
left of the beam to shift strongly toward the warm end of the
scale, causing oranges and reds."
What accounts for sunsets being more stunning in autumn and
winter, as opposed to spring and summer? Pollution. Though, if you
grew up thinking (as I did) that more car exhaust and smokestack
spewage meant better sunset colors, you've got it backwards… for
the most part.
"The truth is that tropospheric aerosols [that's pollution] … do
not enhance sky colors, they subdue them. Clean air is, in fact,
the main ingredient common to brightly colored sunrises and
sunsets," states NOAA's website.
The particulates of atmospheric pollution (like dust, smoke, and
vehicle exhaust) are much larger than molecules of gas (like oxygen
and nitrogen), making them poor Rayleigh scatterers of color. "In
other words, they scatter all colors fairly evenly, much like a fog
does," said Bouchard. "Since most air pollution sources are at
ground level, particulates tend to collect in the lower atmosphere.
Bright sunset colors passing through these hazes tend to be muted
and dulled as a result." However, while pollution has a dulling
effect on the intensity of colored light, the smog itself can
become saturated with that colored light, expanding the coloration
of the sunset.
During our summers, particulate concentrations peak because a)
winds are lightest, and b) the sun is strongest, which contributes
to faster photochemical smog production. Photochemical smog
develops when primary pollutants (oxides of nitrogen and volatile
organic compounds created from fossil fuel combustion) interact
with each other under the influence of sunlight, producing a
mixture of different and hazardous chemicals known as secondary
pollutants.
"In the colder months, winds stir with more fervor, leading to
less concentrated pollutants," Bouchard explained. "Photochemical
pollutants are also at a minimum because of the low sun angle and
shorter days."
So while you may not welcome the bitter cold of late autumn and
winter, you can take some comfort in the seasons'
stop-dead-in-your-tracks-and-stare sunsets.
Meghan Oliver is the assistant editor at Northern Woodlands
magazine. The Outside Story is assigned and edited by Northern
Woodlands magazine and sponsored by the Wellborn Ecology Fund of
New Hampshire Charitable Foundation.
