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Atmospheric Lecture 17! ! Nature’s Show Nature’s Light Show

Atmospheric Optics Scattering Reflection Ahrens Chapter 15

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Scattering Atmospheric Optics The amazing variety of Things to look for observed in the can be • Blue explained by four physical mechanisms. • White • Blue Smoke • Scattering • Red • Reflection • Crepuscular Rays •

3 4 Scattering Scattering Small objects such Light is scattered by the air molecules, as air molecules and droplets, and aerosols. fine smoke particles most effectively air molecule The resulting optics scatter blue light. depend on the size of the scatterer.

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Rayleigh Scattering 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

7 8 Scattering by Air Molecules Scattering

Mie Scattering sunlight Larger objects such as cloud droplets and ice

scatter all visible Cloud drop light equally well.

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.

11 12 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 . Blue sky is the Larger particles appear brown and smaller result of Rayleigh scattering. particles scatter blue.

15 16 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.

19 20 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

23 24 Scattering by Aerosols Scattering by Cloud Droplets

Crepuscular Rays Crepuscular Rays

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Crepuscular Rays Crepuscular Rays

Shadows cast by clouds or trees on hazy days result in crepuscular rays, also known as Jacob’s ladders.

27 28 Crepuscular Rays Anti-crepuscular Rays

Mountain’s Shadow

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Reflection Reflection

The Law of Reflection • Sun Pillars The angle a) of incident light equals the angle b) • Circumhorizontal Arcs of reflected light.

31 32 Reflection by Water Reflection by Water

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Reflection by Water Reflection by Water

35 36 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 Reflection by Ice Crystals

Sun pillar

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

39 40 Reflection by Ice Crystals Reflection by Ice Crystals

Sun pillars commonly occur beneath an altostratus cloud just after .

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 Heiligenschein

an altostratus cloud just after sunset.

43 44 Reflection and Scattering Questions? by Dew Drops

Heiligenschein

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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 • Scattering • White Clouds • Reflection • Blue Smoke • Refraction • Red Sunsets • Diffraction • Crepuscular Rays • Heiligenschein

47 48 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

• Sun Pillars • • Circumhorizontal Arcs • • Halo • Tangent Arc •

51 52 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 approaching a beach. The amount of bending depends on the wavelength (color) of the light, leading to or separation of colors.

Mirage

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Refraction in Air Refraction in Air

Mirage

Green Flash

55 56 Refraction in Air Refraction in Air

Green Flash Green Flash

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Refraction in Air Refraction in Air

Green Flash Red Flash

59 60 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

63 64 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 Comple x Halo Comple x

67 68 Refraction by Ice Crystals Refraction by Ice Crystals

46˚ Halo? Lower Tangent Arc

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Refraction by Ice Crystals Refraction and Scattering

Lower Tangent Arc Sun Pillar and ?

71 72 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 • Scattering • White Clouds • Reflection • Blue Smoke • Refraction • Red Sunsets • Diffraction • Crepuscular Rays • Heiligenschein

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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 Rayleigh scattering by air molecules results in a blue sky. scattered out, leaving red light, which is Mie scattered Mie scattering by droplets results in white/grey clouds. toward the observer by clouds .

75 76 Reflection Refraction

• Sun Pillars • Green Flash • Circumhorizontal Arcs • Mirage • Halo • Tangent Arc • Rainbow

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Refraction Refraction Causes Light slows down as it passes Mirage from a less dense to a more dense medium. Green Flash As light slows it bends toward the Halos denser medium. Similar to waves approaching a beach. Sundogs The amount of bending depends on the wavelength (color) of the light, leading to dispersion or separation of colors.

Halo Complex results from refraction by ice crystals

79 80 Refraction by Water Drops Refraction by Water Drops

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Refraction by Rain Refraction by Water Drops

Violet

Red

Primary Rainbow

83 84 Refraction by Rain Refraction by Rain

Double Rainbow Double Rainbow

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Refraction by Rain Refraction by Rain

Partial Rainbow Double Rainbow

87 88 Refraction by Rain Refraction by Rain

Rainbow seen from Airplane

High Sun: Low Rainbow

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Diffraction 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.

91 92 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 Diffraction by Cloud Droplets

Artificial Iridescence Corona

95 96 Diffraction by Pollen Diffraction and Reflection

Corona

Glory

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Diffraction and Reflection Diffraction and Reflection

Glory Glory

99 100 Diffraction and Reflection Diffraction and Reflection

Supernumerary Rays Bow: larger drops do not allow dispersion of colors.

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

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