July 12, 2002
Q: What Factors Contribute to the Brilliant Colors of
Twilight and Sunset?
A: Everyone at one time or another has marveled at the
beautiful red and orange colors of a sunrise
or sunset.
Although colorful sunrises and sunsets can be seen anywhere,
certain parts of the world are especially famous for their
twilight hues (i.e., deserts and tropics). Eye-catching sunrises
and sunsets also seem to favor certain times of the year. In the
middle latitudes and over the eastern half of the United States,
for example, fall and winter generally produce the most
spectacular low-sun hues. Why do some parts of the world enjoy
more beautiful sunsets than others, and why do they favor certain
months? What are the ingredients for truly memorable sunrises and
sunsets?
What dust and pollution don't do
It is
often written that dust and man-made pollution are responsible for
colorful sunrises and sunsets. If this were true, however, large
cities—such as New York, Los Angeles, London or Mexico City—would
be celebrated for their twilight hues, which is not the case. The
truth is that tropospheric aerosols—especially in large
quantities—do not enhance sky colors, but instead subdue them.
Clean air is, in fact, the main ingredient common to brightly
colored sunrises and sunsets.
To understand why this is so, one only needs to recall how
typical sky colors are produced. Color in the form of pigment does
not exist in the atmosphere. Instead, the color we see in the sky
results from the scattering, refraction, and diffraction of
sunlight by particles in the atmosphere, especially small
particles such as air molecules. If there were no particles in the
atmosphere, then sunlight would travel straight down to the Earth
and the sky would be black.
Specifically, sunlight travels thought the solar system in
straight, invisible waves (unless something sends it off in a
different direction) and consists of a mixture of all colors
(i.e., red, orange, yellow, green, blue, indigo, and violet) in
the visible portion of the electromagnetic
spectrum. Furthermore, each color in this spectrum is
associated with a different wavelength: red and orange have the
longest wavelengths—while blue, indigo, and violet have the
shortest (i.e., 0.47 um for violet to 0.64 um for red). Thus, when
sunlight first enters the Earth's atmosphere, air molecules are
typically the first to scatter the colors in sunlight—one by one,
beginning at the violet end of the spectrum. Specifically, when
the sun is high
in the sky (and there is a relatively short pathway to the
Earth), violet, indigo, blue, and a little green are scattered,
producing a blue sky. However, when the sun is low in the
sky (i.e., sunrise or sunset), its path through the atmosphere
is longer and yellow, orange, and red
colors are scattered near the ground. Thus, as a general rule, the
farther light travels through the atmosphere, the redder it
becomes. The longer trip means more and more light at the blue end
of the spectrum is scattered. This leaves red, yellow, and orange
light to reach our eyes or reflect off clouds. This notion is
perhaps best illustrated by example: a beam
of sunlight that at a given moment produces a red sunset over the
Appalachians is at the same time contributing to the deep blue of
a late afternoon sky over the Rockies.
Now what happens when airborne pollutants enter into
the picture?
Typical pollution droplets—such as those
found in urban smog or summertime haze—are on the order of 0.5 to
1 um in diameter. Particles this large do
not scatter sunlight as effectively as
smaller air molecules since they are comparable in size to the
wavelength of visible light (0.47 to 0.64 um). Furthermore,
because such aerosols normally exist in a wide range of sizes, the
overall scattering produced is not
strongly wavelength-dependent. As a result, hazy
daytime skies, instead of being bright blue, appear grayish or
even white. Similarly, the vibrant oranges and reds of "clean"
sunsets give way to pale
yellows and pinks when dust and haze fill the air.
Airborne pollutants, however, do more than just soften sky
colors. They also enhance the attenuation of both direct and
scattered light, especially when the sun is low in the sky. This
reduces the amount of light which reaches the ground, robbing
sunrises and sunsets of brilliance and intensity. Thus, twilight
colors at the surface on dusty or hazy days tend to be muted and
subdued, even though purer oranges and reds persist in the cleaner
air aloft. This is most noticeable when taking off in an airplane
on a hazy evening: a bland sunset near the ground suddenly gives
way to vivid color as the plane ascends beyond the haze.
Furthermore, because air circulation is more sluggish during
the summer, and because the photochemical reactions which result
in the formation of smog and haze proceed most rapidly at that
time of the year, late fall and winter are the most favored times
for sunrise- and sunset-viewing over most of the United States.
Pollution climatology also largely explains why the deserts and
tropics are noted for their twilight hues. Air pollution in these
regions is, by comparison, minimal.
The role of clouds
Although the twilight
sky can certainly inspire awe even when it is devoid of clouds,
the most memorable sunsets tend to be those with at least a few
clouds. Clouds
catch the last red-orange rays of the setting sun and the
first light of the dawn. But certain types of clouds are more
closely associated with eye-catching sunsets than others. Why? To
produce vivid sunset colors, a cloud must be high enough to
intercept "unadulterated" sunlight (i.e., light which has not
suffered attenuation and/or color loss by passing through the
atmospheric boundary layer. The boundary layer is the layer near
the surface which contains most of the atmosphere's dust and
haze). This explains why spectacular shades of scarlet, orange and
red often grace high level cirrus
and altocumulus
layers, but only rarely low clouds such as stratus
or stratocumulus.
When low clouds do take on vivid hues, it is a clue that the lower
atmosphere is very clean and therefore more transparent than
usual.
Some of the most beautiful sunrises and sunsets feature a solid
deck of middle or high clouds that covers the entire sky except
for a narrow
strip near the horizon. Skies like these are often associated
with well-defined mid- and upper-level jet streams (i.e., they
mark the zones of transition between large-scale regions of
atmospheric ascent and descent). When viewed at sunrise, a sky of
this type implies that the weather is likely to deteriorate as the
mid- and upper-level moisture continues eastward. At sunset, of
course, the opposite is true, hence the saying "Red sky at night,
traveler's delight; Red sky in morning, traveler take warning."
Sunsets like this are perhaps most notable for the "bathed in red"
effect which they produce. The entire landscape takes on a surreal
saffron hue as the cloud deck reflects the fading sun's red and
orange glow, allowing very little blue (scattered) light from the
upper levels of the atmosphere to reach the ground.
