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

MET 200Lecture 14Nature’sLight Show

Atmospheric Optics

Ahrens Chapter 15

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Vog Bank

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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

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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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Scattering by Air Molecules and Aerosols


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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 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.


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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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Scattering by Aerosols

Crepuscular Rays

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Scattering by Fog Droplets

Crepuscular Rays

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Crepuscular Rays

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Scattering by Aerosols

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

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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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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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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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.


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Refraction in Air

Green Flash is a form of superior mirage

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Refraction in Air

Green Flash

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Refraction in Air

Green Flash Animation

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Refraction in Air

Green Flash Animation

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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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22 1/2˚ Halo

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22 1/2˚ Halo

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22 1/2˚ Halo

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22 1/2˚ Halo around the moon

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22 1/2˚ Halo and Upper Tangent Arch

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Halo Complex77


Sun Dogs

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Sun Dogs

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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 Reflection

Sun Pillar and Sun Dog?

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Refraction by Water Drops

Rainbow in Hawaiian is ānuenue

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Descarte’s Schematic

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Primary Rainbowhttp://punchandbrodie.com/ncyclo/rainbows/

Sunlight Raindrops

Red

VioletObserver

40° 42°

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Refraction by Rain

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Refraction by Rain

Sunlight

Falling raindrops

Viol

et

Red

Observer

Double Rainbow91

Refraction by Rain

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Alexander’s Dark Band

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Refraction by Rain

Double Rainbow

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Refraction by Rain

Double Rainbow

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High Sun: Low Rainbow

Refraction by Rain

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Refraction by Rain

Rainbow seen from Airplane

Low Sun: High Rainbow

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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.


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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.

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Diffraction by Soap FilmDiffraction 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 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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Glory

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Glory

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Glory and Fog Bow

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Supernumerary Bows

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Supernumerary Bows

The faint bows in the inside of the primary rainbow are caused by interference or diffraction in reflected rays.

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Summary: 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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Observation Assignment 12/5

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Documents

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