Recall that about half of all incoming solar radiation that reaches the atmosphere is in the visible spectrum. All incoming radiation, especially that in the visible (400 - 700 nanometers), can be either absorbed, reflected, scattered, or transmitted through a molecule or particle. The interaction of solar radiation with elements in the atmosphere or surface depend on their general qualities (composition, density, temperature, color, etc.) A color that is perceived by the eye reflects the average total of all the radiation in the visible spectrum, and does not purport an emitter to have sent out a single wavelength of radiation. Case in point: The Sun; though it often appears yellow in the sky, it is emitting radiation all across the spectrum, from blue to green to red. Objects that aren't hot enough to produce radiation can still have a color, because they can absorb all radiation except a specific color (example: A yellow taxicab) and reflect that to our eyes. Some surfaces absorb all visible radiation wavelengths and reflect no light at all. That would appear as black. The opposite, reflecting all wavelengths, would be white. How does this process happen in the atmosphere?

Scattering and Reflection

    When sunlight bounces off a surface at the same angle from with it strikes, it is said to be Reflected. There are plenty of things in the atmosphere that tend to deflect incoming radiation in all directions, which is referred to as Scattering. During these processes, no energy is gained or lost between the incoming radiation or the particle that reflects or scatters it, so there is no net temperature change. In the atmosphere, scattering and reflection are caused by the small particles, even those that make up the huge clouds. Cloud droplets that are sufficiently large (about 20 µm in diameter) and spherical so they can scatter all wavelengths of visible radiation equally (this is known as Mie Scattering). Even thin clouds, because of the huge amount of molecules in them, are able to scatter a lot of sunlight, and at the same time they are poor absorbers. Clouds therefore often appear white because they're scattering all visible light wavelengths in all directions. As a cloud grows taller, the amount of light that can make it all the way to the base of the cloud without yet being scattered decreases. This makes the base of the cloud appear darker. The darkness is compounded when water droplets near the base of the cloud grow larger, effectively becoming better absorbers than scatterers. If there are lots of dust and particles and the like in a thin cloud, the amount of absorption is increased and scattering of all wavelengths isn't as effective. These two concepts are why darker clouds are often responsible for precipitation, and why some thin clouds are capable of being quite dark.

    Scattering also occurs in the cloudless sky as well. Any molecule or particle whose diameter is small when compared to the wavelength of incoming radiation can scatter that radiation. This scattering is unlike that of the clouds which can scatter all light about evenly in all directions. Nitrogen and Oxygen air molecules are what are known as Selective Scatterers. They each scatter shorter wavelengths of visible light much better than longer wavelengths. So on a normal day, sunlight enters the atmosphere and is immediately scattered by atmospheric gases. The shorter wavelengths (blue, violet, etc.) are scattered much more than the longer ones (orange, red, etc.). The weighted-by-color amount of scattering all hits the eye at once, and the result (much like mixing certain quantities of colors and coming out with a resultant color) is a blue. The concept of this type of scattering is referred to as Rayleigh Scatterin, named after Lord Rayleigh, an early influential mind in this field. Since the atmospheric molecules would treat all light in this fashion, a 'blueing' of light is not restricted to sunlight. Often distant mountains on the horizon will have a blue-ish tint to them as light reflected from them is scattered all the way out to the eye by the air molecules in between. The result is not only a gradual blue-ish color, but also a decrease in contrast between objects on the horizon and air above it.

Refraction and Dispersion

    There are some other key concepts in understanding the behavior of light and the interaction between atmospheric particles. Light that passes through a substance is said to be transmitted. This by-passing of light only works when the density is the same on both sides of the equation. If incoming light enters a denser substance, it slows down. It also, if it enters the substance at an angle, will bend it's path. The bending of light along its path due to the interaction between it and a denser medium is called refraction. The amount of refraction depends upon the density of the entered medium and the angle at which it is incident. The bending of light from a denser medium is always toward a perpendicular line running along the border of the two mediums (call this the "normal"), while that of a less dense medium will bend away from the "normal". When light encounters a denser medium that is comprised of several different lights (traditional "white light" and sunlight are examples of this), each portion of light of different wavelengths is bent differently. The red wavelengths bend the least and the violet wavelengths bend the most. This would cause sunlight, when attempting to transmit through a denser medium, to separate into it's visible spectrum components and be viewed instead as a rainbow of colors rather than a single resultant color. This phenomenon is known as Dispersion. 
    Radiant light not only acts like particles of energy, but they also have wave properties. When light encounters an object, it tries to bend around it, much like when moving water encounters a rock. This is called Diffraction. The little ripples that develop can either cancel each other out ("destructive interference") or combine and amplify ("constructive interference"). With light, we would perceive diffraction as alternating bands of light and dark, or perhaps even with color. Nearly all optical phenomena seen in or involving the atmosphere entails scattering, reflecting, refracting, or diffracting.

Observable Atmospheric Phenomena

Below is a list of most of the observable optical phenomena in the atmosphere. Click on any one on the list, and you will begin to learn about it.

 Blue Sky

    The reason the sky is blue (as aforementioned) is because of Rayleigh Scattering. When an air molecule's diameter is short when compared with the wavelengths of the incoming light, it can scatter that light in all directions.
    Towards the horizon, the contrast between the colors of the landscape and that of the air overhead diminishes, and distant mountains will have a blue-ish tint to them. This is also due to scattering of light molecules, and the lower likelihood of light scattered from far distances actually reaching the eye.

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Red Sunsets

    This is another example of scattering but with a twist. Sunsets become reddened when they are very low or just below the horizon and the incoming light must travel through a much greater thickness of atmosphere to reach the eye. In fact, the sunlight is refracted due to the increasingly dense atmospheric medium to the point that the actual sun's position when it's about to set to the viewer is actually almost a full diameter below the horizon. 

    Just like in viewing distant places where light coming from there is often scattered out, so it is with the sun far off toward the horizon. The difference is that the incoming sunlight is comprised of many different wavelengths and the reds aren't scattered nearly as often or well as the blues. The result then allows for the incoming light to have a red tint to it. The more aerosols and particles in the air, the deeper the red as more of the other colors are being scattered away. The best sunsets are often photographed during events like volcanic eruptions.

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The Green Flash

    There have been no less than a million ideas on how "The Green Flash" forms, and I suppose there are different ways for it to happen based on the different atmospheric conditions under which it formed. However, most often at sea-level, the green flash is visible through a complex rendering of several different optical processes. First, there's some reflection going on of the lower part of the sun that shows up at the horizon. It's not the reflection of the sun on the ocean, but rather a mirage rendition of part of the sun a little higher up. During the sunset process, this reflection eventually 'joins' up with the rest of the sun itself, looking like some odd-shaped vase. This condition is best observed in utterly clear days with a stable atmosphere and a bit of a temperature inversion. As the sun sinks lower, the sun-image begins to look like the capital Greek letter Omega. This is often a good sign to the trained observer that a green flash is just a few minutes away, and to begin looking right where the bottom of the sun-image becomes spherical because once the setting sun dips to that point, the green flash is often visible. When the sun sinks to the line where the reflected and visible images combine, there's where the green flash is most evident. The sun seems to finish its setting above the surface of the horizon, but again that's owing to the curvature of the earth, and the refraction of the sunlight through a thicker atmospheric medium. But why green?
    Light slows down ever so slightly as it travels through the denser air of the atmosphere at the horizon. Since refraction can depend on wavelength, light in the visible spectrum get slowed and bent more (or less) by color. Since blue is best scattered by atmospheric molecules, incident blue light from the sun (remember, sunlight is not just one particular color or wavelength, but a combination of all) is bent more than the red would be. The in between colors are also bent, and though they are minutely separated, you could think of the sun now existing in one sun-image for each color of the visible spectrum do to the ever so slight refractional differences. Green, however, is certainly in the middle, and in fact by color, the sun has more of the green wavelengths in it than either blue or red. Since all the colored sun images are so close to one another, they combine as normal in the visible to give a resultant view of the sun similar to what it would be normally, except on the fringes, where the red sun-image would be on the bottom and the blue sun image (usually scattered away and blended in with the rest of the sky) on the top. When the sun sets, in a way each colored sun-image will set in turn (though all happening very quickly compared with the observer, but when it gets to green it lasts just a bit longer. And when all the colors that have already dipped below the horizon are blocked by that horizon, and when most of the blue light is scattered out anyway, often the top rim of the green sun-image can be seen for it's brief or 'flash' journey below the horizon. When the sun is in a rough 'mirage' type of state in the horizon, this flash can be accentuated and much more easily visible by the naked eye.

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The Twilight Wedge

    The twilight wedge is the visible shadow band of the earth cast upon the sky by the setting sun as it dips below the horizon. It also occurs at sunrise in the opposite direction. When the sun is just about to set, in the east direction this shadow band will start to edge on and rise. It "rises" quicker than the sun sets because of the increasing radial axis of the Earth's shadow with reference to the moving sun. Eventually, the colors of the dusk sky and night colors blend in this shadow. Often, a reddish or brownish haze can be seen along the bottom border of the shadow. For reasons unrelated to astronomy, this is referred to as "The Belt of Venus".

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Crepuscular Rays

    Crepuscular rays are seen often when light from the sun intercepts a very rough, edgy cloud like a cumulus or stratus cloud. Some of the sunlight is allowed to poke through and that which hits the actual cloud does not reach the eye. The effect is for this light to stand out, when it's being scattered by the affected air molecules below and it stands out when compared to the unilluminated air just beside it. It also has the appearance of fanning out from the sun, or perhaps connecting at some point just behind the cloud, however this is not true. Just as railroad tracks appear to converge way down the line, so too crepuscular rays have a similar appearance. The truth is these rays ultimately trace back to the sun, so for all intents and purposes they are nearly parallel at the Earth's surface.

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Mirages

    Fishermen often get down real low to the water, and others actually fish in the water because they know light refracts when water is in between, making them look larger than they are to the fish.  By the same token, a coveted item lying at the bottom of the swimming pool appears closer than it actually is. In the atmosphere, when refraction causes an object to appear displaced from its true position it is said to be a mirage. A mirage is not an invention of the mind, rather, of the atmosphere.
    The easiest cause of a mirage is through sharp differences in temperature through shallow layers of the atmosphere near the surface. This creates layers of different densities near the surface. This causes incident light to be bent as it travels into or out of the denser layers into or out of the less dense layers. For example, a black pavement in the hot sun will warm the air right above it to a scorching temperature, but because air is a poor conductor the air above that shallow layer will stay much cooler. The different densities refract incoming light and result in the blue sky being refracted on its way to our eyes as if it were coming off the ground. Since this occurs often when it's hot, many a desert traveler has mistook it for water.
    When the air near the ground is much warmer than the air slightly above it, sometimes objects not only appear to be lower than they really are, but also inverted. These are called Inferior Mirages. Light that hits objects near the surface move out in all directions. Those that hit the warmer, less dense air are refracted upward, entering the eye from below, and appearing in reverse order from the image as viewed straight on. The same object can be high enough such that light from parts of it overshoot the warm layer and enter the eye without modification, and the result to the eye is an object and it's inverted reflection beneath it.
    This can happen in cold weather too, where air at an icy surface is quite cold compared to a layer slightly above it. The light here gets bent less than normal as it heads to the eye and therefore makes the object viewed appear larger (this is called a Superior Mirage).

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Looming (and Stooping)

Looming is another way of defining a superior mirage. An object is said to be looming if the mirage makes an object appear larger or more elevated than they really are. The way this would occur would be in a situation where there's a shallow layer of air at the surface that is much colder than the air just above it. Here, an object AB is viewed from point O, but the light waves are refracted through the atmosphere and the result is the object appears to be A'B'.

Stooping mirages happen under the opposite conditions, where there is a warm layer at the ground, with drastically cooler air just above it. Objects viewed through this air appear smaller than they really are, and often inverted. This is another way of depicting an inferior mirage. Here the light is refracted such that object AB, when viewed from point O, is rendered to appear as A'B'. The sky will often appear on the ground after point P, as light waves from the sky are bent towards the ground on their way to the eye.

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Twinkling

    The twinkling of stars has had a bit of a debate surrounding it, however one thing is agreeable on its occurrence: It is caused by the light from the distant stars encountering mediums of different densities on it's way towards the eye and thus some of that light is refracted out. Some of this could be interstellar density differences (small roving areas of differing densities somewhere along the many light-year path from the star to the eye), and some of it could be owing to the different turbulent cells in the atmosphere that refract the light only when it has to travel through one. Either conclusion is viable, as both could clearly cause the twinkling effects. Planets, however, don't appear to twinkle. This is said to be because planets are close enough that they appear as disks of light that tend to average out any type of refractive elements and appear more steady.

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Fata Morgana

   
Fata Morgana is Italian for "Fairy Morgan", and as legend goes was the half-sister of King Arthur. She was said to live beneath the water and had magical powers capable of building huge cities out of thin air and the make them disappear. There's a certain place in Italy, when in Reggio looking across the Straits of Messina, where this legend has staying power in the eyes of numerous witnesses. It is actually a specific type of superior mirage, where the air temperature increases rapidly with height from a cooler beginning. It does so slowly, then rapidly, then more slowly again. Instead of two distinct density layers from which air can refract towards the eye, it's more of a gradual mesh that can create a looming mirage with some inverted properties at a varying height level, depending upon just how the temperature profile sets up. This type of mirage is often only seen for a few minutes, as mixing of the different temperature layers would even out all the variations in short order. These are best viewed when warm air sits over cold water, often in polar regions, and of course the Straits of Messina.

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Contrails

    Contrails have no refractive properties, but rather it's a type of cloud that only appears under certain conditions. There are many clouds that appear under singular circumstances and tell a particular thing about the atmosphere at the level at which it occurs. The easiest way to figure out what one is, is to break down the name into it's parts: Condensation Trails. They are left behind by passing jet aircraft. The jet exhaust is hot and humid and upon expulsion it mixes with the environmental air. Often this does not result in anything appearing in the sky. But if the air is conducive to cloud formation at that level, you will see a cloud forming in the wake of the aircraft exhaust. The more humidity there is at that level, the longer the contrail will last. Often you can 'eyeball' the amount of humidity in the upper levels of the atmosphere by examining the durability of contrails. If they last long enough, contrails can often be blown around a bit by the winds until they expand, covering a much larger portion of the sky. If a plane travels through slices of air of differing properties, then often you can see a contrail stop and then start back up again in the favorable sections of its flight path. If an area is a highly traveled spot, on days when contrails are favorable the sky can be nearly overcast in contrail cirrus clouds whereas it otherwise would be clear given the absence of air travel.

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Rainbows

    Rainbows appear when the sun is shining in one part of the sky and it's raining in the other. Though it seems that observing a rainbow is rare, under certain conditions it will ALWAYS occur. When you think of Christmas, you don't think of it as a rare event because you always know its coming, but most people see rainbows several times a year and still think they're rare. In order to see a rainbow it first has to be sunny at one side of the horizon and raining at the other. Stand with the sun at your back and face the raining area. Rainbows will form in an arc that's 42° in angle when measured from the observer up from am imaginary point that would represent the opposite of the sun (or the 'anti-solar' point). If you wanted, you could create this phenomenon any sunny day you like with a garden hose, just make sure you're positioned correctly. Since none of us are walking protractors, the best way to get a generalized idea of where to look when you think you may be able to see a rainbow, is to put your back towards the sun, and look about half-way up where it's raining. Often that will be the place the rainbow is forming.
    So why is it that it's always a 42° arc? Well... actually, it's not that way always, but for a primary rainbow it is. Many of us have seen two rainbows at once, and that second rainbow has a specific angle it forms at two. When we look at a rainbow, we're looking at sunlight that has entered falling raindrops and has been not only refracted but also reflected back to our eyes. Since a raindrop is a denser medium than the air light was just passing through, it will slow down and bend. The shorter wavelengths (violets, blues) bend the most, while the reds bend the least. Most of the sunlight actually passes right on through the raindrops, but a fraction of it, because of the roughly spherical properties of raindrops, is actually reflected within the raindrop itself. The angle at which this happens is called a "Critical Angle", which for water happens to be 48°. For all light that hits the back of the raindrop at an angle greater than the 48°, it is reflected internally and sent towards our eyes. Since light is also being refracted at the same time, the light emerges in different colors, each bent at slightly different angles (red at 42° and violet at 40°). It would then seem that violet would be the highest color in the rainbow, but we know this not to be true. What happens is each drop only registers one color to the eye of the observer. When a raindrop is reflecting violet to the eye, the color red would be incident near the waist of the viewer. Therefore, the red that is reaching the eye is coming from the higher drops, and the violet from the lower drops.
    Sometimes, a second rainbow will appear in the sky, with the colors on it reversed from the primary rainbow. This rainbow is fainter than the primary rainbow. In this case, light has entered raindrops at such an angle such that it is reflected twice within the same raindrop. Each reflection causes the light to be a bit dimmer (like a poor mirror), and thus the second rainbow would appear dimmer. The way the light emerges the from the raindrop after a second internal reflection would cause violet this time to be on top, and red to be on the bottom. Remember, each raindrop exhibits one color in reference to the eye of the observer. If you move, the rainbow appears to move with you, because you are looking at the light reflected from different rainbows. In fact, a rainbow is going to be different for each observer. Between the two rainbows that form, there is a dark band of sky. This is because all the light reaching the observer from the falling raindrops are refracted into the two rainbow bands. The space inbetween has no internally reflected light that actually reaches the eye, so it appears darker because there is less light reaching the eye from there. Moreover, the sky under the primary rainbow is brighter than otherwise because of the combination of all the other colors sent out by the raindrops that the eye does not perceive directly blend together but also intensify the light. You can think of it as light hitting the raindrops, and then being refocused into a certain area of perspective, lessened in another, with the colors in between.
    Occasionally a rainbow will show more colors than the usual red, orange, yellow, green, blue, indigo, and violet. This is caused by the diffractional interference patterns light has when interacting with light from many other droplets if they are all of similar size. Sometimes this will show up as more colors as certain light waves combine, but usually it ends up being a few waves of light bands or dark bands, as the interference between the light waves stack or cancel out.

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Halos

    Halos is a representation of refracted light much like a rainbow, except it is achieved through ice crystals. Therefore, the presence of a halo means the clouds are cold, and thus usually cirriform. In the sky there are different types of ice crystals, but the two main forms are hexagonal shaped and either long ("columns") or wide ("plates"). If they are column-shaped ice crystals and are laying long-ways in your field of vision, sunlight entering them will be refracted towards the eye at a 22° arc. This could also happen if there are ice crystals of any dimension, yet with a diameter less than 20µm along the light path. If the same  ice crystals are lying with their narrow side, light will be refracted towards the eye at a 46° arc. The more ice crystals there are with uniform properties, the more sunlight will be concentrated from that direction as viewed from the eye, and thus a halo appearance will develop because of the concentrated light refraction. A good guess of a common 22° arc from an observational standpoint is to extend your thumb and pinky finger with your thumb on the sun. Your pinky should be about where the arc would form given a sufficient number of properly oriented ice crystals. Sometimes these halos will have a bit of color to them, which is the result of some color dispersion where, like rainbows, can occur with refraction.

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Sundogs

    Sundogs are an extension of the halo development process, whereby instead of the column-shaped ice crystals are focused on, the plate-like ice crystals are refracting sunlight toward the viewer. Keep in mind in a given atmospheric picture, sunlight can be refracted simultaneously from all sorts of ice crystal formations, and can sometimes create quite a picture of light in the sky. When the plate-like ice crystals are oriented horizontally from the viewer, they can work to prevent the viewing of a halo, and rather refract incoming sunlight and act like small prisms. When the sun is nearing a horizon, and the ice crystals are suspended horizontally between it and the viewer, a pair of brightly colored spots, one on either side of the sun. These are the sun dogs, also known as 'parahelia', mock suns, etc. The light bent from these ice crystals will hit the eye from red on the inside towards the sun, to blue on the outside. Some days, where there aren't any ice crystals in the sky to the left of the sun (or right) sufficient to shine through, only one sun dog will obviously be visible.

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Auroras

    Auroras are quite different from the other optical phenomena visible in the sky in that it is not produced by sunlight necessarily reflecting, refracting or being scattered by any cloud or atmospheric molecule. Rather it's due to the solar wind disturbing the magnetosphere. The magnetosphere is the section of upper atmosphere that contains many charged particles and contains the flow of the Earth's electromagnetic field. The solar wind is a crazy thing, and will often launch waves of electromagnetic impulses that travel outward towards Earth at high speeds. Upon reaching the magnetosphere, the bits of incoming radiation will collide with the air molecules and transfer some of its energy to the air molecule. This places the air molecule in an excited state, and it becomes unstable. It will eventually release this energy, sometimes in the form of a re-emitted energy particle (called a "photon"). Like other aspects of radiation, it has a wavelength, where when in the visible range of the spectrum, can be seen by the eye. Since the poles have the highest concentration of electromagnetism (and consequently the Earth's magnetic field is higher at the poles), auroras are most commonly seen there. At the North Pole, it is called the "Aurora Borealis", in the South Pole, it's the "Aurora Australis", each referring to northern or southern "lights". Sometimes, when the solar wind will launch a quite large impulse of electomagnetism, called a "Coronal Mass Ejection", it can disturb the magnetosphere much farther away from the poles and bring the "lights" to an audience that rarely sees it locally.
    Just as air of different densities refract sunlight to different angles, so too does atoms and molecules of different properties emit charged particles of different wavelengths when excited by incoming radiation. The excitation of atomic oxygen at high altitudes will result in the emission of green light photons. Above 250km, oxygen will produce red light. Nitrogen can produce red and violet lights (note here that it depends on the charge structure of the atom and there's not a clean transition per molecule between red and violet in the visible like refraction and dispersion in water). These shades can flicker and soften, with light from it refracting as it travels through air of different densities (similar to the twinkling effect), and often appear in a wave-like line. Right along the arctic circle, where lines demarking the Earth's magnetic field emerge from the Earth's surface, one can see auroras nearly 80 times a year given a clear sky, giving rise to what is called "The Aurora Belt".

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Sun Pillars

    While sundogs and halos are examples of refraction through ice crystals, a sun pillar is the result of sunlight reflecting off ice crystals. Sun pillars appear usually during sunrises or sunsets resembling a vertical shaft of light extending either upward or downward from the sun. Pillars can form as the hexagonal plate-like type ice crystals fall very slowly at their terminal velocity with their flat bases oriented horizontally with reference to the viewer. As these crystals fall, they do so like tiny light leaves, tilting from side to side. The tilted surfaces can reflect the sunlight like little mirrors, focusing it above and below them and thus creating the pillar look as that light is then scattered to the eye. Sun pillars can also form with the column-shaped ice crystals, provided their long bases are oriented horizontally toward the viewer as they fall.

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Coronas

    Whenever the moon, or even car headlights are seen through a thin mist of water droplets, all roughly the same size and spherical shape, a ring of light will often appear around the light source. This also happens with the sun, but is often difficult to observe because of the sun's brightness. This phenomenon is called a corona, and it is due to the diffraction of light. The ring around the sun is actually comprised of several layers of diffraction, with the innermost ring being the brightest, to fainter rings father removed from the center. During spots of constructive interference (light waves combining after passing around tiny water droplets), the light bands are brighter, and where there's destructive interference, the light waves cancel out and the result is a dark band. Sometimes there are several bands of light and dark color. Other times there is just a light band, with gradual fading to black farther out. The most spectacular are the coronas with color. Whenever the clouds droplets or particles are of uniform size (and it's best to be tiny), coronas may be full of color. When the light bends around an object and diffracts, there are very slight variations in the amount of bending dependent upon wavelength (like many other sky optics). The red would appear on the outside of a ring and the blue on the inside. When droplet sizes are non-uniform, or perhaps uniform only in patches, the result would be a patchwork of corona. One side may be longer than the other, or perhaps there's patches of color in part of it.

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Glories

    The glory, like the corona, is also caused by diffraction. It is sometimes known by another term that helps explain its existence: The "Anti-corona". Usually flying in an aircraft is one of the main ways to see a glory, although other ways of observing one is by standing on a mountain ledge; someplace where you can see the sun direct light through you to water droplets on the other side. When spherical cloud droplets are sufficiently uniform and small (around 50 µm in diameter). The sunlight would diffract around you and then the interference pattern would be revealed in the clouds below or the fog beyond. The plane's shadow or a person's shadow would be in the center of this image, with the rings of light and/or color surrounding it. The angular distance of the glory depends on the size of the uniform water droplets, not the size of the shadow, so sometimes a plane can cast a tiny shadow on a cloud, but the glory will still show the same angular size.

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Iridescent Clouds

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Heiligenschein

    The Heiligenchein (German for "halo" or "holy aura" or whatever else that means the same thing) is often seen in the morning when there is a lot of dew on the grass. Stand facing the dew with your back to the sun and observe that there is a bright area around your head. This halo forms when sunlight shines on nearby dew droplets that are of uniform size and spherical character (getting pretty common, isn't it) is focused and reflected back along roughly the same path it took originally. This is a process called retroreflection. Each dew-drop or particle or what-have-you is acting like a lens and focusing the light behind the drop, once it strikes the blade of grass, part of it comes back to our eyes, but not exactly at 180°. There is just enough difference and it is spread out just enough to appear as a diffuse white light around the shadow of your head. This light is confined to about a 10° arc from the dewdrops themselves, so the halo would appear right around your head. If you were to have someone standing next to you and both look at the grass, each would have a halo, but each would only see his own because the retroreflection from the other does not reach the eyes. If you were to hold a camera out and photograph the image, the camera would have the halo, it being the new eyepiece, and you yourself would not have the halo. It's a neat thing to view between holes at the golf course during an early morning tee time.

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The Brocken Bow

    This is also known as the "Spectre of the Brocken", and it is named after the Brocken Mountains in Germany where it can be seen often. This optical phenomenon resembles the glory. For it to occur the sun must be to your back so that sunlight can be returned to your eyes from water droplets on the opposite side of the observer. Sunlight that enters the small spherical fog droplets right along the edge of the droplet is refracted, and then reflected off the back side of the droplet, it refracts again on its way out of the droplet and is returned towards the eye. In order for the light to be returned to the eye, it must slide along the edge of the droplet, becoming something of a surface wave for the short distance it's along the edge of the droplet. These surface waves can diffract and will cause little bands of light when viewed by the eye. Sometimes it may have color in it as well, owing to the refractive properties of the water droplet by wavelength. Like the heiligenschein, there is a small window of observability (about 10°), so companions standing next to an observer would not notice others having a Brocken Bow around them, just the observer personally.

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