(A slightly abridged form of this manuscript appeared in
Weatherwise magazine, June/July 1996)
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. The deserts
and tropics quickly come to mind. Indeed, it is a rare issue of
Arizona Highways that does not include at least one sunset view,
and one could amass a respectable collection of Hawaiian sunset
postcards in just one trip.
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? These and a
few other twilight topics are explored on the next few pages.
What dust and pollution don't do
It is often written that dust and manmade pollution are responsible
for colorful sunrises and sunsets. Certainly the brilliant
twilight afterglows which follow major volcanic eruptions owe their
existence to the ejection of dust and other particles high into the
stratosphere (More will be said on this a bit later). If, however,
tropospheric dust and haze were responsible for brilliant sunsets,
cities such as New York and Los Angeles or London and Mexico City
would be celebrated for their twilight hues. The truth is that
tropospheric aerosols --- especially when present in abundance ---
do not enhance sky colors --- they subdue them. Clean air is, in
fact, the main ingredient common to brightly colored sunrises and
sunsets.
To understand why this is so, one need only recall how typical sky
colors are produced. The familiar blue of the daytime sky is the
result of the selective scattering of violet and blue light by air
molecules. "Selective scattering," also known as Rayleigh
scattering, is used to describe scattering that varies with the
wavelength of the incident light. Particles are good Rayleigh
scatterers when they are very small compared to the wavelength of
the light.
Ordinary sunlight is composed of a spectrum of colors that ranges
from violet and blue at one end to orange and red on the other.
The wavelength of this light ranges from .47 um for violet to .64
um for red. Air molecules are much smaller than this --- about a
thousand times smaller. Thus, air is a good Rayleigh scatterer.
In fact, pure air scatters violet light three to four times more
effectively than the longer wavelengths. Were it not for the fact
that human eyes are more sensitive to blue light than to violet,
selective scattering would make a clear sky appear violet instead
of blue!
At sunrise or sunset, sunlight takes a much longer path through the
atmosphere than during the middle part of the day. Because an
increased amount of violet and blue light is scattered out of the
beam along the way, the light which reaches an observer early or
late in the day is reddened. Thus, it could be said that sunsets
are red because the daytime sky is blue. 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 (Figure 1).
Figure 1
Now what happens when airborne pollutants enter the view? Typical
pollution droplets such as those found in urban smog or summertime
haze are on the order of .5 to 1 um in diameter. Particles this
large are not good Rayleigh scatterers since they are comparable in
size to the wavelength of visible light. If the particles are
uniform in size, they might impart a reddish or bluish cast to the
sky. But since 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.
But airborne pollutants do more than 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. When the pollution layer is
shallow, a similar effect is sometimes evident at the surface, as
shown in Figure 2. The photographs show a sheet of billowed altocumulus
erupting into a blaze of color once the sun has sunk low enough
that it no longer directly illuminates the thin veil of haze below
the clouds. The haze shows up as a dark layer just above the horizon in
the last view.
Figure 2
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, (see, for example, Figure 3), 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?
Figure 3
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 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 solid
decks of middle or high clouds that covers the entire sky except for
a narrow strip near the horizon. A five-minute sequence of such an event
over Baltimore, Maryland is shown in Figure 4. 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 the one in Figure 4 are perhaps most notable for the "bathed in red" effect which they produce. The entire landscape
takes on a surreal saffron hue as the clouds reflect 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. This particular example also illustrates how large particles ---
in this case rain falling from the upper level clouds in the left-most view ---
tend to mute sunset colors. The overall coloration at this point is a dusky
brownish-orange. Minutes later, once the rain has cleared the area, vibrant
shades of red and orange overspread the scene (right).
Figure 4
Twilight hues from volcanoes
Tropospheric clouds aren't the only ones which can enhance the
beauty of the twilight sky. As was already mentioned, particles in
the stratosphere can also produce colorful sunrises and sunsets.
Stratospheric particles are derived mainly from volcanic eruptions
and exist as thin veils of dust or sulfuric acid droplets at
altitudes of 12 to 18 miles. Like the stars and planets, these
clouds are usually invisible during the day because of scattering
in the troposphere. About 15 minutes after sunset, however, when
the stratosphere is being illuminated by sunlight passing through
the lower atmosphere to the west, these high-level clouds come into
view. Since their colors achieve greatest intensity after the sun
has set at the surface, volcanic twilights are known as
"afterglows."
Three different twilight afterglows are shown in Figure 5. All
three were observed over the eastern United States in September
1991 following the massive eruption of the Philippines volcano
Mount Pinatubo in June. As the photographs show, afterglows vary
markedly in appearance depending upon the depth and height of the
stratospheric clouds in the observer's vicinity. Color and
intensity are also affected by the amount of haze and tropospheric
cloudiness along the path of light reaching the stratosphere.
Figure 5
The first picture (left) shows a lilac afterglow high above the fading
light of a brilliant early fall sunset. The cirrus streaks in the
foreground have long since become shaded, but in the center of the
view, a distant tropospheric cloud tower below the horizon is
casting a dark shadow across the afterglow. Blue light scattered
downward through the thin cloud producing the afterglow, mixed with
the red light which illuminates it, is responsible for the lilac
hue.
The middle example shows a red-orange afterglow produced by a
thicker aerosol cloud. The nearer parts are being illuminated by
light which has passed through the troposphere and is therefore
strongly reddened. More direct sunlight illuminates the brighter
region close to the horizon. A similar cloud, viewed through a
hazier lower atmosphere, appears in the photo at the right. Because
of the haze, there is increased attenuation (especially along the
horizon), and the intense colors of the previous example have been
replaced by paler shades of pink and white.
Mount Pinatubo's sunset afterglows persisted to varying degrees for about
18 months after the initial explosion. In more recent years (e.g., 1998 and
2003), sunset colors in many areas have been subdued by the introduction of large
smoke particles into the lower atmosphere by forest fires across the western United
States and Canada.
The preceding pages have provided only a brief introduction to the physics
and meteorology of the twilight sky. Further understanding of the nature
of sunrise and sunset colors can only increase our appreciation of them.
