Scattering Atmospheric Optics - SOEST · 2006. 10. 23. · Sun pillars commonly occur beneath an...

Lecture 17! !

Nature’s

Light Show

Atmospheric OpticsScatteringReflection

Ahrens Chapter 15

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Atmospheric Optics

Nature’s Light Show

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Atmospheric Optics

• Scattering• Reflection• Refraction • Diffraction

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• Heiligenschein

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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

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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 direction.

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Scattering by Air Molecules and Clouds

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Scattering by Air Molecules and Aerosols

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Rayleigh and Mie Scattering

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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 if they are thick 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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Most of the blue light is scattered out, leaving red light, which is scattered toward the observer by clouds.

Red Sunsets

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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 scattered out, leaving red light, which is scattered toward the observer by clouds.


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Scattering by Aerosols

Crepuscular Rays23

Crepuscular Rays


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Crepuscular Rays


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Scattering by Cloud 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

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Crepuscular Rays

Mountain’s Shadow

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Anti-crepuscular Rays

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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

• 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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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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Sunlight reflecting off of plate-shaped ice crystals can produce a sun pillar.


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Sun pillar


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Sun pillars commonly occur beneath an altostratus cloud just after sunset.


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Reflection and Scattering

by Dew Drops

Heiligenschein

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Reflection and Scattering

by Dew Drops

Heiligenschein

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Questions?

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Atmospheric Optics



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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.


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Reflection


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Refraction

• Green Flash• Mirage• Halo• Tangent Arc• Rainbow

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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.

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Refraction in Air

Mirage

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Refraction in Air

Mirage

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Refraction in Air

Green Flash

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Refraction in Air

Green Flash

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Refraction in Air

Green Flash

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Refraction in Air

Green Flash

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Refraction in Air

Red Flash

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Refraction by Ice Crystals

22 1/2˚ Halo

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Sun Dogs

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Sun Dog

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22 1/2˚ Halo

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22 1/2˚ Halo and Upper Tangent Arc

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22 1/2˚ Halo

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Halo Complex

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Halo Complex

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46˚ Halo?

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Lower Tangent Arc

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Lower Tangent Arc

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Refraction and Scattering

Sun Pillar and Sun Dog?

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Atmospheric Optics



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Rayleigh scattering by air molecules results in a blue sky. Mie scattering by droplets results in white/grey clouds.


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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.


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Reflection


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Refraction

• Green Flash• Mirage• Halo• Tangent Arc• Rainbow

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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.

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Refraction Causes

Halo Complex results from refraction by ice crystals

MirageGreen FlashHalosSundogsRainbows

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Refraction by Water Drops

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Refraction by Rain

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Primary Rainbow

Violet

Red

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Refraction by Rain

Double Rainbow

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Refraction by Rain

Double Rainbow

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Partial Rainbow

Refraction by Rain

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Double Rainbow

Refraction by Rain

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High Sun: Low Rainbow

Refraction by Rain

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Refraction by Rain

Rainbow seen from Airplane

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Diffraction

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.

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Diffraction causes Interference

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 Results in• Iridescence• Corona• Glory• Supernumerary bows

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Diffraction by Cloud Drops

Iridescence

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Diffraction by Airplane Window

Artificial Iridescence

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Diffraction by Cloud Droplets

Corona

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Diffraction by Pollen

Corona

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Diffraction and Reflection

Glory

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Glory

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Glory

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Supernumerary Rays

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Fog Bow: larger drops do not allow dispersion of colors.

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Fog Bow: larger drops do not allow dispersion of colors.

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Summary: Atmospheric Optics



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Scattering Atmospheric Optics - SOEST · 2006. 10. 23. · Sun pillars commonly occur beneath an...

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