Sunset rays pass long and deep through the lower atmosphere.
They are bent and twisted around the very rim of the Earth.
The atmosphere acts as a giant lens which refracts low sunset rays
into long curved paths passing through up to 40 times as much air
than the rays from a high midday sun.
Air, dust, aerosols and water drops scatter and absorb the rays throughout
their long passage.
Reds, yellows and golds arise because the air itself, small dust and
aerosol particles smaller than the wavelengths of visible light, Rayleigh
scatterers*, scatter short wavelength blue and green rays
much more strongly than longer wavelength yellow and red. The remaining
direct unscattered light is dimmed but relatively enriched in reds
and yellows. Absorption of specific green and blue wavelengths
by ozone and water vapour molecules redden the light further. The
sunset rays are sometimes reflected back and forth between clouds
and the ground. All this goes to makes a spectacle seemingly painted
with every colour and shade of the palette.
When the upper atmosphere contains extra fine dust from a volcanic
eruption skies are reddened further.
Large dust particles and suspended water
droplets scatter light differently, they are Mie
scatterers** and do not produce vivid red sunsets, they merely
dim the sun.
Particles much smaller than wavelengths of light scatter light
in all directions. Their scattering is inversely proportional
to the fourth power of the wavelength. Blue (~450 nanometer
wavelength) is scattered over four times more strongly than
red (~650 nm). Very small dust particles are Rayleigh
scatterers. Some smoke particles are small enough
also, watch smoke from a fire, it looks red or brown when viewed against
a bright light but blue/white otherwise.
Particles larger than visible wavelengths scatter light predominantly
forwards in the direction of the original beam. Some,
like water droplets, also scatter strongly in other quite specific
directions to form rainbows, fogbows,
glories and coronae.
With the exception of these specific directions light of different
wavelengths is scattered much more equally than by Rayleigh scatterers.