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Atmospheric OpticsGeronimo R. Rosario
a collective name for everything having to do with light and color in the atmosphere, which includes subjects on the ground.
Atmospheric Optics
It deals with how the unique optical properties of the earth’s atmosphere cause a wide range of spectacular optical phenomena.
Why the sky is blue?
What causes the color of the sunset and sunrise?
What is halo?
And many more optical wonders.. Or illusions?
All electromagnetic radiation is light, but we can only see a small portion of this radiation—the portion we call visible light.
Possible fates of light as it enters atmosphere (absorbed, reflected, scattered, transmitted)
Our eyes are sensitive to light which lies in a very small region of the electromagnetic spectrum labeled "visible light".
This "visible light" corresponds to a wavelength range of 400-700 nanometers (nm) and a color range of violet through red
Visible light
Reflection – occurs when sunlight bounces off a surface at same angle
Scattering – occurs when sunlight is deflected in all directions
Refraction- bending of light due to density changes.
Diffraction – bending of light around objects
Transmission- the process by which light energy is transmitted
Absorption- the process by which light is taken by the substance.
Visible light
The organ of adaptation is our eyes and, more specifically, specialized receptors located on a layer of cells called the retina at the back of the eye.
Visible Light
These special receptors consist of two types of cells:
1.Rods respond to all the wavelengths of light, and can only differentiate between light and dark.
2. Cones respond to specific wavelengths within the visible spectrum and our brain interprets the signals from cones activated by longer wavelengths as red and those by shorter wavelengths as blue with those in between as green.
Perception of color:◦Each color
corresponds to a particular wavelength.
◦White: all wavelengths are present with equal intensity;
◦Black: no light is emitted and/or reflected from the object.
Color
◦ The sun appears white (in the atmosphere).
◦ Colder stars look redder◦ Hotter stars look bluer
◦ Our Sun produces more yellow light than any other color because its surface temperature is 5,500°C.
◦ If the Sun's surface were cooler—say 3,000°C—it would look reddish, like the star Betelgeuse.
◦ If the Sun were hotter—say, 12,000°C—it would look blue, like the star Rigel.
Color and Temperature
Color is a function of the human visual system, and is not an intrinsic property.
Objects don't "have" color, they give off light that "appears" to be a color.
Spectral power distributions exist in the physical world, but color exists only in the mind of the beholder.
The color of the object is based upon the wavelength of visible light that is reflected by the object.
The red apple looks red because the all short wave lengths are absorbed except which is reflected back.
Color of the objects
Scattering of Light The scattering of light in the atmosphere depends on the size of
the scattering particles, R, and on the wavelength, l, of the scattered light.
Geometric scattering: R>> l◦ Rain drops (R~10-100 mm) ◦ All wavelengths equally scattered◦ Optical effects: white clouds
Mie scattering: R~ l◦ Aerosols (R~0.01-1 mm)◦ Red scattered better than blue◦ Blue moon, blue sun
Rayleigh scattering: R<< l◦ Air molecules (R~0.0001-0.001 mm)◦ Blues scattered better than red◦ Blue sky, blue mountains, red sunsets
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R
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Scattering of Light
Blue Sky◦ Involves gases, or other scattering
agents that are smaller than the energy wavelengths
◦ Scatter energy forward and backward◦ Partial to shorter wavelength energy,
such as those which inhabit the shorter portion of the visible spectrum
◦ A blue sky results Hazy Sky
◦ When larger particles such as dust and pollutants are present in the atmosphere, particularly when water vapor condenses on them to form tiny water droplets), they are large enough to scatter all wavelengths of light
◦ The sky then takes on a milky white appearance
Scattering of light
White Clouds◦ Water vapor (clouds) scatter all
wavelengths equally. The result is white.
◦ When clouds are thick (like thunderclouds) they absorb much of the light. Water drops also tend to absorb light. The result is a darker cloud.
Red/Orange Clouds◦ At sunset or sunrise the colour
of sunlight can be yellow to deep red due to the scattering of the blue component of sunlight as the light travels a longer path through the atmosphere.
Cloud’s color
Grey clouds As a cloud grows thicker, more
sunlight is reflected from it and less light can penetrate through it.
Since little sunlight reaches the underside of the cloud, less light is scattered, and the cloud base appears grey. Moreover, if the water droplets near the cloud base grow larger, they become less effective scatterers and more effective
absorbers. Blue clouds Common to noctilucent clouds
particularly in the Antarctic region. Caused by increased methane
production in the higher atmosphere. Sign of climate change
Cloud’s color
This works particularly well after major Volcanic Eruptions, when sunlight is scattered at sulfuric acid droplets in the stratosphere, as after the eruption of Mt. Pinatubo.
Color of the sky
The sun looks yellow, since a part of the blue light has been scattered away.
Near sunrise and sunset the path through the atmosphere (air mass factor) is very long, the major part of the blue light has been scattered away, the orange and red part of the spectrum remains.
Due to Mie-Scattering at dust particles in the atmosphere also the surrounding of the sun is red or orange.
Color of the Sun
Earth Shadows (dark segments) are names for the shadow that the Earth itself casts on its atmosphere.
Earth’s Shadows
Known as Venus girdle or anti-twilight arch, is a pink to brownish border separating the dark shadow band of the earth from the sky above it.
Belt of Venus
Shadow casts by cloud appearing darker. Cloud shadow occurring at sunset is formed when
light is cast from a low level cloud onto a higher level cloud.
Cloud shadow
Mountain shadow The triangular shadow cast by the mountain at the
top of the summit regardless of the mountain’s shapes.
The finite size of the sun causes the umbral (fully shaded) parts of the shadow to converge and eventually taper away.
Higher mountains are best to cast long triangular shadow.
Sun rays, also called crepuscular rays, streaming through gaps in clouds are parallel columns of sunlit air separated by darker cloud shadowed regions.
Crepuscular rays
Anticrepuscular rays or antisolar rays are similar to crepuscular rays, but seen opposite the sun in the sky.
Anticrepuscular rays
The color of the moon is also caused by the scattering. The orange color of the moon is because other colors are scattered except for the orange and red that remain. The whitish color is due to dust particles in the atmosphere.
Color of the Moon
Green flashes and green rays are optical phenomena that sometimes occur right after sunset or right before sunrise.
Green flash
When the conditions are right, a green spot is visible above the upper rim of the disk of the sun. The green appearance usually lasts for no more than a second or two.
Even more elusive than the “green flash” are its blue and violet variants. Blue and violet light is subject to larger refraction, but also to more intense scattering.
Blue and violet flash
Due to the refraction of light the objects on the sky appear higher than they actually are.◦ Star location and
scintillations;◦ Timing of the sunset and
the sunrise;◦ The sun on the horizon
looks flattened;◦ Twilight duration.
True and Apparent positions of objects
Scintillation-The apparent twinkling of star due to refraction.
The changing density will change the amount of refraction with cold, dense pockets refracting more and warm pockets less
As the air in the atmosphere flows and moves these pockets, the “apparent” position of the star will change with the star appearing to flicker or “twinkle”
Scintillation
The bending of sunlight by the atmosphere causes the sun to rise about two minutes earlier, and set about two minutes later, than it would otherwise.
Timing of sunrise and sunset
Light in the atmosphere travels along a curved path, due to continuous refraction. When the sun is at the horizon, light from the lower edge is significantly stronger refracted than from the upper edge – and appears to be higher – resulting in a flattened image of the sun.
Distorted Celestial bodies
Les Cowley
This is even more pronounced when observing a moonset from the International Space Station (ISS), since the path through the atmosphere is twice as long.
Distorted Celestial bodies
Atmospheric layers with different air density can cause bizarre distortions of the sun’s image .
Distorted Celestial bodies
Double Sun
An unusually warm layer of air over the ocean can produce an inferior mirage causing double sun.
An optical illusion described by Italian psychologist Mario Ponzo known as “ Ponzo illusion”, suggested that the human mind judges an object's size based on its background.
Bigger Sun and Moon
Supermoon is the coincidence of a full moon or a new moon with the closest approach the moon makes to the Earth on its elliptical orbit, resulting in the largest apparent size of the lunar disk as seen from Earth.
It is also known as Perigee Full Moon.
The technical name is the perigee-syzygy of the Earth–Moon–Sun system.
Supermoon
March 19, 2011, May 6, 2012, June 23, 2013August 10, 2014, September 28, 2015November 14, 2016, January 2, 2018
The full moon on November 14, 2016 was the closest supermoon of the year (356,509 kilometers or 221,524 miles). What’s more, this November 14, 2016 full moon will showcase the moon at its closest point to Earth thus far in the 21st century (2001 to 2100), and the moon won’t come this close again until the full moon of November 25, 2034.
Twilight is the name given to the time after sunset (and immediately before sunrise) when the sky remains illuminated and allows outdoor activities to continue without artificial lighting.
Twilight
Civil twilight lasts from sunset until the sun is 6° below the horizon.
Astronomical twilight lasts until the sky is completely dark and th astronomical observation of the faintest stars is possible.
The length of twilight depends on season and latitude.
During the summer in middle latitudes, twilight adds about 30 minutes of light to each morning and evening for outdoor activities.
The duration of twilight increases with increasing latitude, especially in summer.
At high latitudes during the summer, morning and evening twilight may converge, producing a white night — a nightlong twilight.
In general, without the atmosphere, there would be no refraction or scattering, and the sun would rise later and set earlier than it now does.
Instead of twilight, darkness would arrive immediately when the sun disappears below the horizon.
Twilight
Mirage – an image that appears displaced from its true position
Caused by light moving through different air densities
Inferior mirage – images that appear lower and inverted from original
Superior mirage – images that appear higher than original
Mirage
Inferior mirage Warm surface, decreasing temperatures aloft
• The proper conditions for an inferior mirage are commonly found over a hot road surface during summer, or over an unfrozen lake on a very cold day.
Superior mirage Cold surface, warmer temperature aloft• The proper conditions for a superior mirage are commonly
found over water and over snow-covered surfaces.
is an unusual and complex form of superior mirage that is seen in a narrow band right above the horizon.
Fata morgana
Fata Morgana mirages significantly distort the object or objects on which they are based, often such that the object is completely unrecognizable.
A Fata Morgana can be seen on land or at sea, in polar regions or in deserts. This kind of mirage can involve almost any kind of distant object, including boats, islands and the coastline.
an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky.
Halo
The preferred angular distance for halos are 22 and 46 degrees from the ice crystals which create them
Dispersion – breaking up of light into different wavelengths (prism)
Halo
Tangent arc - an arc of light tangent to a halo. It forms by refraction of light through ice crystals
Tangent arc
Refraction of sunlight through the ice crystals produces the bright arc of light.
When the sun is on the horizon, the arc that forms at the top of the halo is called an upper tangent arc.
When the sun is above the horizon, a lower tangent arc may form on the lower part of the halo beneath the sun.
atmospheric phenomenon that consists of a pair of bright spots on either side on the Sun, often co-occurring with a luminous ring known as a 22° halo.
Sundogs
Created by hexagonal ice crystals oriented horizontally
Ice crystals act like a prism
Sundogs are commonly seen when cirrostratus clouds are in the sky.
Moondog (paraselenae)- A relatively rare bright circular spot on a lunar halo caused by the refraction of moonlight by hexagonal-plate-shaped ice crystals in cirrus and cirrostratus clouds.
Moondogs
They are exactly analogous to sun dogs, but are rarer because to be produced the moon must be bright, about quarter moon or more.
Moondogs show little color to the unaided eye because their light is not bright enough to activate the cone cells of the human eye.
A parhelic circle is a white band circling the sky and always at the same height above the horizon as the sun
Parhelic circle
If complete, it stretches all around the sky, but more commonly it only appears in sections.
Even fractions of parhelic circles are less common than sun dogs and 22° halos
A sun pillar is a vertical shaft of light extending upward or downward from the sun.
Sun Pillar
Caused by reflection of light off ice crystals
Typically seen during sunrise or sunset, sun pillars form when sunlight reflects off the surfaces of falling ice crystals associated with thin, high-level clouds (like cirrostratus clouds).
Sun Pillar
Sun pillars are created by reflection on tilted ice crystals
Rainbow-phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky.
Rainbow
Cloud drop act as prism, dispersing light Rainbows are caused by (double) refraction at the surface
and reflection. On the backside of spherical raindrops. This can happen
under different angles, but there is a maximum angle of 42°. Violet light emerges from the “front” side of the raindrop at a
40° angle and red at 42°
Rainbow
Rainbow
a phenomenon in which two rainbows appear
Double Rainbow
They are about 127° (violet) to 130° (red) wide.Since this is more than 90°, they are seen on the same side of the sky as the primary rainbow, about 10° above it at apparent angles of 50–53°. As a result of the "inside" of the secondary bow being "up" to the observer, the colours appear reversed compared to the primary bow. The secondary rainbow is fainter than the primary because more light escapes from two reflections compared to one and because the rainbow itself is spread over a greater area of the sky.
A second rainbow appears (at an angle of ~ 51°), when the light is reflected twice inside the raindrop. Note the reversed order of the colors and Alexander’s dark band in between.
Double Rainbow
Alexander's band or Alexander's dark band is an optical phenomenon associated with rainbows which was named after Alexander of Aphrodisias who first described it in 200 AD.
Alexander’s band
It occurs due to the deviation angles of the primary and secondary rainbows.
Both bows exist due to an optical effect called the angle of minimum deviation.
The refractive index of water prevents light from being deviated at smaller angles.
Twinned rainbow- appears as two rainbow arcs that split from a single base.
The colours in the second bow, rather than reversing as in a double rainbow, appear in the same order as the primary rainbow.
Twinned rainbow
Twinned rainbows are (most likely) caused by non-spherical, large raindrops
Due to air resistance, raindrops flatten as they fall, and flattening is more prominent in larger water drops.
When two rain showers with different-sized raindrops combine, they each produce slightly different rainbows which may combine and form a twinned rainbow
A complete rainbow, which is visible in the higher altitude and a clear sky.
Full circle rainbow
In theory every rainbow is a circle, but from the ground only its upper half can be seen.
Since the rainbow's centre is diametrically opposed to the sun's position in the sky, more of the circle comes into view as the sun approaches the horizon, meaning that the largest section of the circle normally seen is about 50% during sunset or sunrise.
A supernumerary rainbow—also known as a stacker rainbow—is an infrequent phenomenon, consisting of several faint rainbows on the inner side of the primary rainbow, and very rarely also outside the secondary rainbow.
Supernumerary rainbow
They are caused by interference – they provided a first hint that light can act like a wave.
Reflected bows are made by rays that are reflected by the water surface after they have passed through raindrops.
Reflected Rainbow
The lower bow, although it is called a "reflected bow", is not a reflection of the upper one.
Rainbows are not real objects and therefore cannot produce reflections.
The reflection inverts the rainbow and the bow centre is then above the horizon at the anthelic point. The reflection bow and ordinary bow meet in a cusp at the horizon.
A reflection rainbow may be produced where sunlight reflects off a body of water before reaching the raindrops, if the water body is large, quiet over its entire surface, and close to the rain curtain.
Reflection rainbow
The reflection rainbow appears above the horizon.It intersects the normal rainbow at the horizon, and its arc reaches higher in the sky, with its centre as high above the horizon as the normal rainbow's centre is below it. Due to the combination of requirements, a reflection rainbow is rarely visible.
Red rainbow or monochrome rainbow- a rare type of rainbow where it happen during sunrise or sunset with occasional shower, causing the shorter wavelengths like blue and green scattered of which the longer wavelength, red , remain.
Red Rainbow
Formation of rainbows of higher orders even up to sixth order.
Higher order rainbows
The order of a rainbow is determined by the number of light reflections inside the water droplets that create it: One reflection results in the first-order or primary rainbow; two reflections create the second- order or secondary rainbow. An additional challenge in observing the third-order (or tertiary) and fourth-order (quaternary) rainbows is their location in the direction of the sun.Brightness decreases as order increases.
is a rainbow produced by light reflected off the surface of the moon (as opposed to direct sunlight) refracting off of moisture laden clouds in the atmosphere.
Moonbow
Moonbows are relatively faint, due to the smaller amount of light reflected from the surface of the moon.They are always in the opposite part of the sky from the moon
A bow like the rainbow, formed in cloud droplets.
Cloud bow
As cloud droplets are much smaller than raindrops, the spectral colors of the white light have a much larger range of exit angles from a droplet, and as a result, the colors overlap so a white bow is produced.
Fogbows form in the same way as rainbows, but they are formed by much smaller cloud and fog droplets that diffract light extensively.
Fogbow
They are almost white with faint reds on the outside and blues inside.
The colours are dim because the bow in each colour is very broad and the colours overlap.
Brocken bow or mountain spectre, is the apparently enormous and magnified shadow of an observer, cast upon the upper surfaces of clouds opposite the sun.
Brocken bow/Brocken Spectre
From above a cloud you may see your own shadow (at the anti-solar point) as “Brocken Spectre“, surrounded by a glory.
The phenomenon is named after Brocken, the highest elevation in the Harz mountain range, where it can be frequently observed
An arc formed directly overhead, the zenith, about 45o above the sun as platelike ice crystals fall with their flat surfaces parallel to the ground.
Circumzenithal Arc
This is caused by refraction of light into ice crystals (cirrus clouds).
Circumzenithal arc can only form when the sun is lower than 32° above the horizon.
When the sun is higher than this angle, the refracted light cannot be seen by the observer.
While rainbow is formed when it rain, circumzenithal arc is not.
A glory is an optical phenomenon that resembles an iconic saint's halo about the shadow of the observer's head
Glory
Each observer will see a glory around its own head – and no glories around those of others. Sun must be to your back, cloud in front of youThe formation of glories is still not entirely understood.
Corona – Bright ring of light around the moon or sun
Corona
A corona is caused through diffraction by small particles – usually by cloud droplets Blue appears on inside of ring, red on outside.Need a cloud that has droplets of similar sizeNew clouds are the best corona producers
Heiligenschein – (German for halo) a faint white ring surrounding the shadow of an observer’s head on a dew-covered lawn.
Heiligenschein
The Heiligenschein forms when sunlight, which falls on nearly spherical dew drops, is focused and reflected back toward the sun along nearly the same path that it took originally. (Light reflected in this manner is said to be retroreflected.)
The light, however, does not travel along the exact path; it actually spreads out just enough to be seen as bright white light around the shadow of your head on a dew-covered lawn.
Cloud iridescence is the occurrence of colors in a cloud similar to those seen in oil films on puddles. Associated with thin clouds.
Cloud iridescence
It is a fairly uncommon phenomenon, most often observed in altocumulus, cirrocumulus, lenticular clouds and cirrus clouds.The colors are usually pastel, but can be very vivid. Iridescence is generally produced near the sun, with the sun's glare masking it, so it is more easily seen by hiding the sun behind a tree or building.
An aurora is a natural light display in the sky, predominantly seen in the high latitude (Arctic and Antarctica) regions.
Aurora
Aurora borealis- northern lightsAurora australis- southern lights
The aurora forms when charged particles emitted from the sun during a solar flare penetrate the earth's magnetic shield and collide with atoms and molecules in our atmosphere. These collisions result in countless little bursts of light, called photons, which make up the aurora.
