Atmospheric Optics
Nature’s Light Show
Lecture 17! !
Nature’s
Light Show
Atmospheric Optics
Scattering
Reflection
Ahrens Chapter 15
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Scattering
Atmospheric Optics
Things to look for
• Blue Sky
• White Clouds
• Blue Smoke
• Red Sunsets
• Crepuscular Rays
• Heiligenschein
The amazing variety of optical phenomena
observed in the atmosphere can be
explained by four physical mechanisms.
• Scattering
• Reflection
• Refraction
• Diffraction
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Scattering
Scattering
Rayleigh Scattering
Small objects such
as air molecules and
fine smoke particles
most effectively
scatter blue light.
Light is scattered by the
air molecules, cloud
droplets, and aerosols.
The resulting optics
depend on the size of
the scatterer.
sunlight
air molecule
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Rayleigh Scattering
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Scattering by Air Molecules
Scattering of blue light by air molecules is more
than 9 times greater than scattering of red light.
Rayleigh Scattering results in blue sky
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Scattering by Air Molecules
Scattering
Mie Scattering
Larger objects such
as cloud droplets
and ice crystals
scatter all visible
light equally well.
sunlight
Cloud drop
Mie scattering is greatest parallel to incident
light.
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Scattering
Scattering by Air Molecules and Clouds
Mie scattering is greatest parallel to incident light.
Rayleigh scatter is nearly equal in all direction.
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Rayleigh and Mie Scattering
Scattering by Air Molecules and Aerosols
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Rayleigh and Mie Scattering
Scattering by Smoke
Mie scattering results in white clouds and
the glare around the sun. Blue sky is the
result of Rayleigh scattering.
Larger particles appear brown and smaller
particles scatter blue.
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Scattering by Cloud Droplets
Scattering by Cloud Particles
When all visible light is
scattered away by
cloud drops the cloud
turns black.
Mie scattering results in white clouds, with
black bottoms if they are thick enough.
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Red Sunsets
Avoid exposed
places during
thunderstorms
Most of the blue light is scattered out, leaving red light,
which is scattered toward the observer by clouds.
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Scattering by Cloud Particles
Scattering by Cloud Particles
When the sun sets or rises, the sunlight passes through a
long path of air. Most of the blue light is scattered out,
leaving red light, which is scattered toward the observer by
clouds.
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Scattering by Aerosols
Scattering by Aerosols
Crepuscular Rays
Crepuscular Rays
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Scattering by Aerosols
Scattering by Cloud Droplets
Crepuscular Rays
Crepuscular Rays
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Crepuscular Rays
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Crepuscular Rays
Shadows cast by clouds or trees on hazy days result
in crepuscular rays, also known as Jacob’s ladders.
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Crepuscular Rays
Anti-crepuscular Rays
Mountain’s Shadow
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Reflection
Reflection
The Law of Reflection
The angle a) of incident
light equals the angle b)
of reflected light.
• Sun Pillars
• Circumhorizontal Arcs
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Reflection by Water
Reflection by Water
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Reflection by Water
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Reflection by Water
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Reflection by Water
Reflection by Ice Crystals
Sun Pillars
The Law of Reflection
The angle a) of
incident light equals
the angle b) of
reflected light.
Sunlight reflecting off of the ocean can
produce a sun pillar.
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Reflection by Ice Crystals
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Reflection by Ice Crystals
Sun pillar
Sunlight reflecting off of plate-shaped ice
crystals can produce a sun pillar.
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Reflection by Ice Crystals
Reflection by Ice Crystals
Sun pillars commonly
occur beneath an
altostratus cloud just
after sunset.
Sun pillars commonly occur beneath
an altostratus cloud just after sunset.
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Reflection by Ice Crystals
Reflection and Scattering
by Dew Drops
Sun pillars commonly occur beneath
an altostratus cloud just after sunset.
Heiligenschein
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Reflection and Scattering
by Dew Drops
Questions?
Heiligenschein
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Atmospheric Optics
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Rayleigh and Mie Scattering
The amazing variety of optical phenomena
observed in the atmosphere can be explained by
four physical mechanisms.
Things to look for
• Blue Sky
• White Clouds
• Blue Smoke
• Red Sunsets
• Crepuscular Rays
• Heiligenschein
• Scattering
• Reflection
• Refraction
• Diffraction
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Scattering by Cloud Particles
Scattering by Cloud Particles
When the sun sets or rises, the sunlight passes through a
long path of air. Most of the blue light is Rayleigh
scattered out, leaving red light, which is Mie scattered
toward the observer by clouds.
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Reflection
Refraction
• Green Flash
• Mirage
• Halo
• Tangent Arc
• Rainbow
• Sun Pillars
• Circumhorizontal Arcs
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Refraction
Refraction in Air
Light slows down as it passes
from a less dense to a more
dense medium.
As light slows it bends toward the
denser medium. Similar to waves
approaching a beach.
The amount of bending depends
on the wavelength (color) of the
light, leading to dispersion or
separation of colors.
Mirage
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Refraction in Air
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Refraction in Air
Mirage
Green Flash
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Refraction in Air
Refraction in Air
Green Flash
Green Flash
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Refraction in Air
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Refraction in Air
Green Flash
Red Flash
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Refraction by Ice Crystals
Refraction by Ice Crystals
22 1/2˚ Halo
Sun Dogs
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Refraction by Ice Crystals
Refraction by Ice Crystals
Sun Dog
22 1/2˚ Halo
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Refraction by Ice Crystals
Refraction by Ice Crystals
22 1/2˚ Halo and Upper Tangent Arc
22 1/2˚ Halo
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Refraction by Ice Crystals
Refraction by Ice Crystals
Halo Complex
Halo Complex
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Refraction by Ice Crystals
Refraction by Ice Crystals
46˚ Halo?
Lower Tangent Arc
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Refraction by Ice Crystals
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Refraction and Scattering
Lower Tangent Arc
Sun Pillar and Sun Dog?
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Atmospheric Optics
Rayleigh and Mie Scattering
The amazing variety of optical phenomena
observed in the atmosphere can be explained by
four physical mechanisms.
Things to look for
• Blue Sky
• White Clouds
• Blue Smoke
• Red Sunsets
• Crepuscular Rays
• Heiligenschein
• Scattering
• Reflection
• Refraction
• Diffraction
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Scattering by Cloud Particles
Scattering by Cloud Particles
When the sun sets or rises, the sunlight passes through a
long path of air. Most of the blue light is Rayleigh
scattered out, leaving red light, which is Mie scattered
toward the observer by clouds.
Rayleigh scattering by air molecules results in a blue sky.
Mie scattering by droplets results in white/grey clouds.
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Reflection
Refraction
• Green Flash
• Mirage
• Halo
• Tangent Arc
• Rainbow
• Sun Pillars
• Circumhorizontal Arcs
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Refraction Causes
Refraction
Light slows down as it passes
from a less dense to a more
dense medium.
As light slows it bends toward the
denser medium. Similar to waves
approaching a beach.
The amount of bending depends
on the wavelength (color) of the
light, leading to dispersion or
separation of colors.
Mirage
Green Flash
Halos
Sundogs
Rainbows
Halo Complex results from refraction by ice crystals
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Refraction by Water Drops
Refraction by Water Drops
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Refraction by Rain
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Refraction by Water Drops
Violet
Red
Primary Rainbow
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Refraction by Rain
Refraction by Rain
Double Rainbow
Double Rainbow
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Refraction by Rain
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Refraction by Rain
Partial Rainbow
Double Rainbow
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Refraction by Rain
Refraction by Rain
Rainbow seen from
Airplane
High Sun: Low Rainbow
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Diffraction
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Diffraction causes Interference
Constructive interference of
light waves can produce color
separation.
The physical mechanism in
this case is called diffraction.
Produces colors on soap
films, oil slicks, and music
CDs.
Diffraction is the tendency of light waves to
bend as they pass the edges of objects.
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Diffraction by Soap Film
Diffraction by Cloud Drops
Diffraction Results in
• Iridescence
• Corona
• Glory
• Supernumerary bows
Iridescence
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Diffraction by Airplane Window
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Diffraction by Cloud Droplets
Artificial Iridescence
Corona
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Diffraction by Pollen
Diffraction and Reflection
Corona
Glory
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Diffraction and Reflection
Diffraction and Reflection
Glory
Glory
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Diffraction and Reflection
Diffraction and Reflection
Fog Bow: larger drops do not allow
dispersion of colors.
Supernumerary Rays
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Diffraction and Reflection
Summary: Atmospheric Optics
The amazing variety of optical phenomena
observed in the atmosphere can be
explained by four physical mechanisms.
• Scattering
• Reflection
• Refraction
• Diffraction
Fog Bow: larger drops do not
allow dispersion of colors.
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