Sky Optics and Observation
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.
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.
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.
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".
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.
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).
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. |
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.
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.
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.
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.
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.
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.
Back.
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".
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.
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.
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.
Iridescent Clouds
|
|
|
|
Iridescence occurs all the time with the appearance of a corona, however,
because of the brightness of the sun, a corona is often hard to detect.
Some clouds, with just the right uniformity in droplet size and shape, can
produce a patch of corona out to about 20° of an angle to the sun.
Whenever a corona is not clearly visible but yet a cloud nearby is
illuminated, it is said to be iridescent.
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.
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.