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The colourful spectacle of the evening sky and the setting Sun is one
that no artist can truly duplicate. But few people appreciate that the colour
of most of the sunsets we see reveals pollution in the atmosphere. City
sunsets differ from those in clean country air. Eruptions of dust and ash
from volcanoes can change the palette of colours yet again, making the Sun
appear blue or even green, on certain rare occasions.

In an exceptionally clear, unpolluted atmosphere, the colours of sunset
are especially vivid. The Sun is bright yellow and the adjacent sky is shades
of orange and yellow. As the Sun disappears below the horizon, the colours
change gradually from orange to blue. Low-lying clouds continue to reflect
the light of the Sun, even after it has vanished. Higher, thin cloud shows
a tint of mauve, as the blue of the sky mingles with the red of the sunlight
it reflects. Several minutes later the sky is filled with exquisite shades
of blue that deepen upward.

But in a highly industrialised region, where pollutants in the form
of aerosols cloud the atmosphere, the colours in the sky are quite different.
The disc of the Sun appears orange-red and the sky dull red, different shades
of red showing the layers of pollutants. Sometime after sunset, the western
sky has two broad bands of colour, dull red at the horizon and murky blue
above. On a day when the pollution is particularly heavy, the Sun appears
as a dull red disc that may fade away even before it reaches the horizon.

But why does the Sun look yellow and the sky blue in the first place?
The explanation was first given by Lord Rayleigh at the end of the 19th
century. Someone on the Earth’s surface sees the sky through sunlight scattered
by the atmosphere. In clear, unpolluted air, most of the scattering is effected
by air molecules – oxygen and nitrogen in the main – which are very much
smaller than the wavelength of visible light. For such a case, Rayleigh’s
theory predicts that the scattering predominantly affects the shorter wavelengths
of light, at the blue end of the spectrum, so the sky itself appears blue.
Sunlight, which traverses the atmosphere directly, loses much of its blue
light through scattering, so the Sun itself appears bright yellow, white
light minus blue.

According to Rayleigh’s theory, the amount of scattering increases sharply
as the wavelength of the light decreases. So the shortest wavelength light,
violet, would be scattered most strongly, then indigo, blue and green light
much less. Why, then, do we see a blue sky and not a sky rich in violet
and indigo? As the scattered light travels through the intervening atmosphere,
it loses energy mainly by scattering. This energy loss is greatest for the
short wavelengths. Violet and indigo light, though strongly scattered, are
also strongly absorbed. The sky we see is a mixture of colours in the blue
region, giving the characteristic ‘sky blue’ colour.

Apart from scattering, sunlight is also absorbed in the atmosphere,
mainly by molecules of ozone and water vapour. The sunlight that eventually
reaches the ground has lost energy as a result of both scattering and absorption
in the intervening layers of the atmosphere. Such a loss of energy makes
it possible for us to enjoy looking at the Sun as it rises and sets; looking
directly at the Sun in broad daylight is dangerous.

At dawn or dusk, an observer views the Sun by sunlight that approaches
the Earth obliquely, traversing many layers of the atmosphere, particularly
the denser parts closer to the ground. This light loses much of its intensity
by scattering and absorption before reaching the observer, so the Sun looks
much less bright. At noon, light that reaches an observer approaches the
surface of the Earth at an angle close to 90 degrees, travelling through
a much thinner layer of atmosphere. As a result, the sunlight is hardly
reduced in intensity and is dazzling to the eye, as it is for most of the
day.

Just before the Sun sets, you may see a bright red glow around the disc
of the Sun. This glow, or aureole, is a result of the diffraction of sunlight
by particles much larger than air molecules – usually fine dust, floating
in the air close to the Earth’s surface. The aureole appears to extend outwards
from the centre of the Sun’s disc by about three diameters. Because the
angular extent of the glow depends on the wavelength of the light and the
size of the particles, I estimate that the diffracting particles are about
25 micrometres across, a reasonable size for dust. You do not usually see
this glow at sunset if a shower of rain has cleared the air beforehand.

Lord Rayleigh did not deal explicitly with the problem of a polluted
atmosphere – as we know it today. His theory predicts that the scattering
of light should increase sharply as the scattering particles get bigger.
But Rayleigh’s theory only applies to particles that are very much smaller
than the wavelength of light, such as air molecules; it fails for particles
more than about 0.025 micro-metres across. In today’s industrial world,
pollutants are often aerosols, made up of droplets that vary in size from
about 0.01 to 10 micrometres. Rayleigh’s scattering theory cannot be used
to explain their effects.

In 1908, Gustav Mie put forward a more general theory that covers a
wider range of particle sizes. Essentially, theory predicts that larger
particles dominate the scattering if there are enough of them in the atmosphere.
Mie’s scattering theory can predict the effects we see in urban skies and
is demonstrated in a simple experiment I set up to show the effect of larger
particles (described in the Box). It showed that larger particles scatter
more of the light, and that the effect depends on wavelength. The scattering
is stronger in the green-yellow parts of the spectrum as well as in the
blue regions.

So sunlight that has passed through many layers of polluted air – such
as in a city sunset – would be much less intense and would appear more red,
having lost its blue, yellow and green components. In addition to the scattering,
there is extra absorption from pollutants such as ozone and water vapour.
As a result, the disc of the Sun appears dulled, with an orange-red hue.

But what about the colour of the sky itself in a polluted atmosphere?
The pollutant aerosols in the atmosphere tend to settle with time into layers,
with larger particles forming the denser layers closest to the ground. Sunlight
fades gradually and takes on an orange-red hue as it traverses these layers.
The scattered light loses more of the shorter-wavelength colours, so that
mainly red gets through. The sky takes on a dull red glow; the shades of
red become darker nearer the Earth’s surface as the scattered light comes
from the increasingly dense layers of the lower atmosphere.

The type of sunset you see depends to a large extent on where you are.
On the ground, the brightness and the colour of the sunset would depend
on the season and the local conditions of the atmosphere each day. Someone
at a high altitude has quite a different view of a sunset or a sunrise.
Sometime after a sunset, an observer in a plane, say, would receive sunlight
scattered from a small part of the atmosphere close to the western horizon.
There is little or no atmosphere directly above to scatter the light towards
the observer.

During a sunrise, this scattered light is visible before the Sun rises.
As with sunsets, the colour of this patch of sky depends on the atmospheric
conditions. Vivid colours such as orange-yellow, purple and deep blue in
the sky before sunrise are an indication that the atmosphere to the east
is comparatively free of pollution. Once the Sun rises, most of the sky
becomes blue, with only a narrow band coloured orange and yellow close to
the horizon.

To an informed eye, the evening sky can unveil a polluted atmosphere.
Natural ‘pollution’ can have an effect too, particularly when volcanoes
erupt large quantities of ash, hot gases and vapours into the atmosphere.
The finer particles of ash and aerosols eventually form a layer at between
15 and 20 kilometres up. This layer scatters sunlight with exceptionally
vivid and colourful effects, especially at twilight, even for a few years
after an eruption.

After the Krakatoa eruption of 1883, people saw blue and green Suns
and they have been reported on several occasions since. This effect arises
from scattering according to Mie’s theory: a cloud of large particles, each
bigger than the wavelength of light, lies between an observer and the Sun
and scattering obscures most of the light. Mie’s theory says that least
light is lost in the blue region of the spectrum for particles of 0.85 micrometres
across, and least in the green region for particles 1.1 micrometres across.
Volcanic ash, dust or industrial smoke contain particles of just these sizes,
so if conditions are just right, on rare occasions the Sun can look blue
or green.

These spectacular effects do not make up for the other disadvantages
of pollution, whether natural or manufactured. But at least the pollutant
particles reveal their presence in colourful and subtle effects. We have
a great responsibility to restrict the pollution released into the atmosphere,
signalled by the dull red hue of city sunsets. It is only then that we can
ensure that future generations will continue to enjoy the colourful spectacle
of a sunset amid clean evening skies.

George Dissanaike is professor of physics at the University of Peradeniya,
Sri Lanka, presently on sabbatical at the University of Cambridge, Department
of Education and Hughes Hall.

Further reading: George Dissanaike has written Scattering of light,
Sunsets and Air Pollution, Science Education Series no. 19, Natural Resources,
Energy and Science Authority of Sri Lanka, 1984. Aden and Marjorie Meinel
discuss the effects of natural pollution in a fascinating book entitled
Sunsets, Twilights, and Evening Skies, Cambridge University Press, 1983.

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SUNSET IN THE COMFORT OF YOUR OWN LABORATORY

It is relatively easy to simulate the scattering of light that colours
sunsets under different atmospheric conditions. In my experiments I represented
the atmosphere by a solution in which colloidal particles of sulphur are
formed in suspension and allowed to grow. As they increase in size, the
optical properties of the solution change in a way that matches the effects
of increasing atmospheric pollution.

A narrow beam of white light shining through the solution was focused
on a screen to give a disc-shaped image that simulates the setting Sun.
The light scattered off the particles in the solution simulates the colour
of the sky.

When the colloidal particles are first formed, they are very much smaller
than the wavelength of light. The light the solution transmits is bright
yellow and the scattered light blue, simulating colours of the setting Sun
and the blue sky in a clean atmosphere containing mostly air molecules.
The blue is at first deeper and richer than ‘sky blue’. In the atmosphere,
the violet and indigo light that is strongly scattered is also strongly
absorbed. But the solution hardly absorbs violet and indigo because the
tube containing it is too narrow to absorb significant amounts of the scattered
light. These colours are transmitted with the blue, to give a richer hue
to the scattered light.

As the experiment progressed, the particles became larger and more and
more of the colours in the green-yellow part of the spectrum were scattered.
The scattered light became brighter and more white, with only a tint of
blue showing. At the same time, the light transmitted through the solution
gradually reduced in intensity and became more red. After an hour, the colours
matched those seen at sunset in a highly polluted atmosphere.