MET 200 Lecture 14 Nature’s Light Show Atmospheric Optics Ahrens Chapter 15 1 Vog Bank 2 Atmospheric Optics • Scattering • Reflection • Refraction • Diffraction The amazing variety of optical phenomena observed in the atmosphere can be explained by four physical mechanisms. 3 Scattering Things to look for • Blue Sky • White Clouds • Blue Smoke • Red Sunsets • Crepuscular Rays 4
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Scattering - SOEST · Scattering by Air Molecules Rayleigh Scattering results in blue sky 9 Scattering by Air Molecules 10 Scattering Mie Scattering Larger objects such as
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The amazing variety of optical phenomena observed in the atmosphere can be explained by four physical mechanisms.
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Scattering
Things to look for• Blue Sky• White Clouds• Blue Smoke• Red Sunsets• Crepuscular Rays
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Why is this Sunset Red?
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Scattering
Light is scattered by the air molecules, cloud droplets, and aerosols.
The resulting optics depend on the size of the scatterer.
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Scattering
Rayleigh Scattering
Small objects such as air molecules and fine smoke particles most effectively scatter blue light.
sunlight
air molecule
Rayleigh Scattering is nearly equal in all directions.
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Rayleigh Scattering
Scattering of blue light by air molecules is more than 9 times greater than scattering of red light.
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Scattering by Air Molecules
Rayleigh Scattering results in blue sky
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Scattering by Air Molecules
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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
Mie scattering is greatest parallel to incident light. Rayleigh scatter is nearly equal in all directions.
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Scattering by Air Molecules and Clouds
Rayleigh and Mie Scattering
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Rayleigh and Mie Scattering
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Scattering by Air Molecules and Aerosols
Rayleigh and Mie Scattering
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Rayleigh and Mie Scattering
Mie scattering results in white clouds and the glare around the sun.
Blue sky is the result of Rayleigh scattering.
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Scattering by Smoke
Larger particles appear brown and smaller particles scatter blue.
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Scattering by Cloud Droplets
Mie scattering results in white clouds with black bottoms when the clouds are tall enough.
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When all visible light is scattered away by cloud drops the cloud turns black.
Scattering by Cloud Particles
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Avoid exposed places during thunderstorms
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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.
Scattering by Cloud Particles
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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.
Scattering by Cloud Particles
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Rayleigh and Mie Scattering
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Rayleigh and Mie Scattering
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Rayleigh and Mie Scattering
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Scattering by Aerosols
Crepuscular Rays
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Scattering by Fog Droplets
Crepuscular Rays
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Scattering by Fog Droplets
Crepuscular Rays
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Scattering by Aerosols
Crepuscular Rays
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Scattering by Fog Droplets
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
Mountain’s Shadow
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Anti-crepuscular Rays
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Summarizing: ScatteringScattering of light in the atmosphere causes many familiar effects: blue skies, white clouds, hazy days, colorful sunsets, crepuscular rays
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Impact of Vog on Sunset
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Reflection
The Law of Reflection The angle a) of incident light
equals the angle b) of reflected light.
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Reflection
Things to look for:• Sun Pillars• Circumhorizontal Arcs
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Reflection by Water
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Reflection by Water
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Reflection by Water
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Sunlight reflecting off of the ocean can produce a sun pillar.
Reflection by Water
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Reflection by Ice Crystals
Sun Pillars The Law of Reflection
The angle a) of incident light equals the angle b) of reflected light.
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Reflection by Ice Crystals
Sunlight reflecting off of plate-shaped ice crystals can produce a sun pillar.
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! Sun pillar
Reflection by Ice Crystals
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Sun pillars commonly occur beneath an altostratus cloud just after sunset.
Reflection by Ice Crystals
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Sun pillars commonly occur beneath an altostratus cloud just after sunset.
Reflection by Ice Crystals
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Reflection by Ice Crystals
Sun pillars commonly occur beneath an altostratus cloud just after sunset.
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Sun Pillar
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Sun Pillar
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Reflection and Scatteringby Dew Drops
Heiligenschein
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Reflection and Scatteringby Dew Drops
Heiligenschein
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Atmospheric OpticsThe amazing variety of optical phenomena observed in the atmosphere can be explained by four physical mechanisms.• Scattering• Reflection• Refraction • Diffraction
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Rayleigh and Mie Scattering
Things to look for• Blue Sky• White Clouds• Blue Smoke• Red Sunsets• Crepuscular Rays• Heiligenschein
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RefractionThings to look for:• Mirage• Green Flash• Halo• Tangent Arc• Rainbow
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RefractionLight 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.
Normal
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RefractionLight 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.
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Refraction
The amount of bending depends on the wavelength (color) of the light, leading to dispersion or separation of colors.
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Refraction in Air
Inferior Mirage
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Refraction in Air
Inferior Mirage
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Refraction in Air
Inferior Mirage
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Refraction in Air
Superior Mirage
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Refraction in Air
Superior Mirage
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Refraction in Air
Superior Mirage
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Refraction in Air
Superior Mirage
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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. Note: the atmosphere also refracts the sunlight, which causes red and green flashes.
Fog Bow: smaller drizzle droplets reduce the dispersion of colors.
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Fog Bow in Kilauea steam vent: cloud drops do not allow dispersion of colors.
Refraction and Reflection
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Refraction and Reflection
Fog Bow simulation: small cloud drops do not allow dispersion of colors.
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Questions?
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Diffraction
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DiffractionConstructive 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.
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Diffraction causes Interference
Diffraction: the apparent bending of waves around small obstacles and the spreading out of waves past small openings.