Effects of Pollution on Visibility, Ultraviolet Radiation, and Colors in the Sky - Air Pollution and Global Warming: History, Science, and Solutions - page 157

published in 1871 (Rayleigh, 1871), 23 years before
his discovery of argon. White

Noon Afternoon

7.1.2.1. Gas Scattering Extinction Coefficient
The gas scattering extinction coefficient (cm − 1 )of
total air (Rayleigh scattering extinction coefficient),
which is analogous to that of the gas absorption extinc-
tion coefficient, is defined as

Yellow

Red

Blue

Blue Green

Sunset /
Twilight σ s , g =

N a b s , g (7.3)

Green

Blue

Red

where N a is the number concentration of air molecules
(molec cm − 3 )atagiven altitude and b s,g is the scattering
cross section (cm 2 molec − 1 )ofatypical air molecule.
The scattering cross section of an air molecule is an
effective cross section that results in radiation reduction
by scattering and is proportional to the inverse of the
fourth power of the wavelength. This means that gas
molecules in the air scatter short (blue) wavelengths
preferentially over long (red) wavelengths.

Red

Earth Atmosphere

Space

Figure 7.6. Colors of the sun. At noon, the sun
appears white because red, green, and some blue
light transmit to a viewer's eye. In the afternoon,
sunlight traverses a longer path in the atmosphere,
removing more blue. At sunset, most green is
removed from the line of sight, leaving a red sun.
After sunset, the sky appears red due to refraction
between space and the atmosphere.

7.1.2.2. Colors of the Sky and Sun
The variation of the Rayleigh scattering cross section
with wavelength explains why the sun appears white at
noon, yellow in the afternoon, and red at sunset. It also
explains why the sky is blue. White sunlight that enters
the Earth's atmosphere travels a shorter distance before
reaching a viewer's eye at noon than at any other time
during the day, as illustrated in Figure 7.6. During white
light's travel through the atmosphere, blue wavelengths
are preferentially scattered out of the direct beam by air
molecules, but not enough blue light is scattered for a
person looking at the sun to notice that the incident beam
has changed its color from white. Thus, a person looking
at the sun at noon often sees a white sun .Theblue light
that scatters out of the direct beam is scattered by gas
molecules multiple times, and some of it eventually
enters the viewer's eye when the viewer looks away
from the sun. As such, a viewer looking away from the
sun sees a blue sky .
Although Lord Rayleigh formalized the theory of
scattering by gases and aerosol particles much smaller
than the wavelength of light, it may have been
Leonardo da Vinci (1452-1519) who first suggested
that blue colors in the sky were due to interactions of
sunlight with atmospheric constituents. He wrote in his
notebooks (c. 1500, Chapter VI, Section 300):

darkness of the fiery sphere which lies beyond and
includes it. ...Ifyouproduce a small quantity of smoke
from dry wood and the rays of the sun fall on this smoke
and if you place (behind it) a piece of black velvet on
which the sun does not fall, you will see that the black
stuffwill appear of abeautiful blue color. ...Water
violently ejected in a fine spray and in a dark chamber
where the sunbeams are admitted produces then blue
rays. . . . Thus it follows, as I say, that the atmosphere
assumes this azure hue by reason of the particles of
moisture which catch the rays of the sun.

What da Vinci saw, however, was not scattering by
gasmolecules but scattering by small aerosol parti-
cles, namely, dry wood smoke particles and small liquid
water drops. Because such particles are smaller than
the peak wavelength of visible light, they preferentially
scatter blue light, as do gas molecules. Small aerosol
particles are well known to scatter blue light prefer-
entially. Preferential scattering of blue light by small
aerosol particles in the air is responsible for the blue
appearance of the Blue Ridge Mountains in Virginia
and the Blue Mountains in Australia. Occasionally,
after a forest fire, a blue moon appears due to the scat-
tering of blue light by small organic aerosol particles.
Similarly, after a heavy rain, the relative humidity is low,
and aerosol particles lose much of their water, shrinking

Isaythatthe blueness we see in the atmosphere is
not intrinsic color, but is caused by warm vapor evapo-
rated in minute and insensible atoms on which the solar
rays fall, rendering them luminous against the infinite