MET 102 Pacific
Climates and Cultures
Lecture 23: Atmospheric Optics Introduction
Discussion Questions –
WW2010
• Why is the sky blue?
• Selective (Rayleigh) Scattering by Oxygen and Nitrogen, which interact with shorter (blue) wavelengths.
• Why are sunsets red or different colored?
• As the sun sinks toward the horizon, sunlight enters the atmosphere at a much lower angle and consequently must pass through much more atmosphere before being seen by an observer. • Air molecules scatter away the shorter wavelengths of light (violet and blue) and the only light which penetrates through the atmosphere are the longer wavelengths of light (yellow, orange and red) which produce colorful sunsets. Red and other Colored Sunsets
• Most spectacular Sunsets occur when lots of fine particles in the atmosphere…
• Dust
• Volcanic Ash
• Vog
Volcanic sunsets depicted in
Munch's The Scream
1883 eruption of Krakatoa
Spectacular sunsets from the Sarychev
volcanic eruption June 2009 from the
International Space Station
Purple Sunset off Big Island
Date unknown
Discussion Questions – WW2010
• What optical phenomena are the result of light interacting with ice crystals?
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46 degree halos
22 degree halos
Sundogs (Mock Suns or Parhelia)
Sun Pillars
• What optical phenomena are the result of light interacting with water droplets?
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Corona
Silver Lining
Cloud Iridescence
Rainbows (Primary & Secondary)
White Light & The Nature
of Light
• The Law of Reflection
• Light rays always bounce off the reflecting surface at the same angle at which the meet at that surface.
• White light is composed of many wavelengths
• Atmospheric Gases, Clouds and particles interact differently with these wavelenghts
• Sky Color
• Optical Phenomena
On a SMOOTH
surface you can
easily see that the
angles are the same.
On a ROUGH
surface light will
strike (and reflect)
at different angles.
White Skies – Hazy, Fog
and SMOG
Fog
• Fog, haze or smog scatter light more equally in all wavelengths
Haze
Smog
Nature of Light – Internal Reflection
• Occurs when light that is traveling through a transparent material (like water) reaches the opposite surface and is reflected back into the transparent material.
Nature of Light – Refraction
• Some rays are not reflected
• Refraction is the bending of light as it passes obliquely (not at 90 deg) from on transparent medium to another. • The velocity of light varies depending on the medium.
• It is slower in water or ice than in a vacuum • What do you think of when you think of the word “Mirage?
Mirages
Mirages
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Usually associated with desert regions
Can happen anywhere
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Related to air of different densities
Different types
1. Inferior Mirage
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The image is BELOW the true location of the object
Usually warm air below, cold above
2. Superior Mirage
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The image is ABOVE the true location of the object
Usually warm air above, cold below
How inferior mirages work….
COOL AIR
• Change in air density = gradual bending of light rays
• Near the earth’s surface light traveling through less dense air develops a curvature in a direction OPPOSITE to the Earth’s Curvature.
HOT AIR
• When light rays pass from hotter to colder, they bend toward the direction of the cooler
air. • Makes objects appear below (or above) where they are, often inverted (upside down).
• Human brain perceived light as traveling in a straight line.
Superior Mirages
• The air close to the surface must be much colder than the air above it. • This condition is common over snow, ice and cold water surfaces. • When very cold air lies below warm air, light rays are bent downward toward the surface, thus tricking our eyes into thinking an object is located higher or is taller in appearance than it actually is. Superior Mirages
• Towering: changes in the
apparent size of an object
• Looming: Makes it appear
object is suspended over
the horizon.
Fata Morgana
• Named after the sister of King Arthur
• a.k.a Morgan La Fey –An evil witch, rival of Merlyn.
• Credited with the ability to create towering castles out of thin air.
Fata Morgana
• A complex mirage in which distant objects are distorted as well as elongated vertically. • For example, a relatively flat shoreline may appear to have tall cliffs, columns, and pedestals. • The phenomenon occurs under much the same meteorological conditions as the superior mirage with inversion, and contains features of both towering and inversion.
Glory
• Colored rings surrounding the shadow of the airplane
• Named after the “halo” it produces around a person’s shadow
• Can be see at the surface if the observer is located such that:
• Above a layer of fog
• Sun at his or her back
• The shadow on the ground the glory will enshroud the observer’s head.
• Not usually seen when the observer is on the ground … Easier to see when in an airplane
Corona
• Appears as a bright whitish disk centered on the Moon or Sun. But mostly the MOON!
• Sometimes colors are seen, but are rare
• Red outer band, blue in the middle
• Produced when a thin layer of water‐laden clouds (usually altostratus) cover the Moon (or the Sun)
• The Corona is due to the BENDING of light around the EDGES of the drops causing them to collide and interfere with each other.
• Diffraction – The bending of light around sharp edges.
• Interference – Occurs when light rays of different frequencies (colors) meet.
Rainbows
Cloud drops scatter white light
Rainbows
• Clarity of color varies
• Outermost band is always RED
• Outermost band is always VIOLET
• Usually see SIX Colors
• Red, Orange, Yellow, Green, Blue, Violet
• Usually seen when the observer has the Sun on one side and a rain shower on the opposite side.
• i.e. Sun at your back, facing the rain
• Fine mists from waterfalls and sprinklers can generate mini‐rainbows.
Rainbow Formation
• Need three things:
• 1) Sunlight
• 2) Water droplets
• 3) An observer in between the rain and the sun
• Refraction
• As light travels through water it is bent
• Different colors travel at different speed in water
• Each color is then bent at a different angle
• Violet is refracted and bent the most
• Red is refracted and bent the least
Rainbow Formation
• The angle between sunlight and the dispersed color is always:
• 42° for red
• 40° for violet
• The curved shape results from the fact that the light always travels at 42° from the path of sunlight
• An observer will only see one color from each raindrop
• Each observer sees their OWN rainbow!
• Dimmer
Double Rainbows
• Visible above the primary bow
• Makes a larger arch (by 8 deg)
• Narrow band
• COLORS are REVERSED!
• Outermost = Violet
• Innermost = Red
Secondary Rainbow
PRIMARY RAINBOW
• Forms the same way EXCEPT the dispersed light is refracted twice
• Reverses the colors!
• Results in a 50° angle for the color red…. 8° above the primary rainbow’s red.
• Extra refraction also makes it dimmer
SECONDARY RAINBOW
Halos
• Narrow whitish ring around the sun.
• Look for halos on days when the sky is covered with a thin layer of cirrus clouds.
• Whey are Halos White?
• Raindrops are almost always spherical.
• Ice crystals vary in shape and size
• Thus, they are “imperfect”
• The colors overlap and “wash” each other out
• If you do see a color, it’s usually reddish on the inside of the halo
Halos
• 22° halo
• Most Common type
• Subtends an angle of 22° from the observer
• 46° halo
• Less frequently observed
• Larger
Halo Formation
Column
• Similar to a rainbow
• Formed by dispersion of light
• ICE CRYSTALS instead of water drops
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Plate
Column
Capped Column
Bullet
Plate
Paths taken by light to produce 22° halo
Differences between 22° and 46° halos
• The path the light takes differs
• For 22° halo light strikes one side of the crystal and passes through the other side.
• For 46° halo light strikes the side of a crystal and then passes out either the top or bottom
Sun Dogs or Parhelia
• Two bright regions on either side of the sun
• Usually associated with a 22° halo
Sun Pillars
• Usually seen at sunrise or sunset
• Sunlight is reflected from the lower sides of falling plate crystals and capped columns
• Usually reddish in color
• Direct sunlight at sunrise and sunset is usually reddish in color