http://SaturnianCosmology.Org/ mirrored file
For complete access to all the files of this collection
	see http://SaturnianCosmology.org/search.php 
==========================================================
Recreational Aviation Australia home page <../index.html> 	

Atmospheric light phenomena

* Aviation Meteorology*
'''

Quick link page index <../remoteindex2.html> 	R6 ? page content was last
changed February 1, 2005

Module content

* 12.1 Light scatter <#light_scatter>
* 12.2 Atmospheric optical displays <#optical_displays>
* 12.3 Moon phases <#moonphase>

*Except for white-out conditions the atmospheric light phenomena
described below have no effect on aircraft navigation but certainly
provide some wondrous visual effects for the alert observer.*

12.1 Light scatter

12.1.1 Rayleigh and Mie scatter

Some of the visible light radiation from the sun, passing through the
atmosphere ( refer 1.7.1 )
<../meteorology/section1b.html#atmospheric_temperature>, bounces off
atoms, molecules and other particles and is scattered in all directions
without losing energy or altering frequency. Gas molecules being very
much smaller than the wavelength of visible light ( 0.4 to 0.8 microns,
refer 1.8 <../meteorology/section1b.html#electromagnetic_spectrum> )
scatter the shorter violet and blue wavelengths much more strongly than
the longer yellow and red wavelengths, but as the human eye is not very
sensitive to violet light the *skyglow* appears blue.

Human eye sensitivity chart

Atmospheric dust and smoke particles are considerably larger than the
gas molecules but may still be smaller than the wavelengths of visible
light and thus also selectively scatter the blue end of the spectrum,
but more strongly than the gas molecules. This phenomenon is termed
*selective scatter* or *Rayleigh scatter*.

However cloud droplets and small ice crystals are some 50 times larger
than the light wavelengths (refer 3.1)
<../meteorology/section3.html#cloud_formation> and scatter all equally.
Thus the light scattered from clouds retains the white light spectra,
*Mie scatter*, and even though the droplets are colourless and
transparent the clouds appear white. Thicker clouds have darker bases
because most of the light is scattered out the top and sides.

When the sun is directly overhead the direct parallel rays that reach
the eye from the sun's disc travel only a short distance through the
atmosphere and the sun?s disc appears white. As the sun lowers the
distance travelled through the atmosphere increases, as does the
scattering of the blue end, and the depleted unscattered light that
reaches the eye makes the disc appear yellow to orange to red, depending
on the number and size of non-gaseous particles in the air. If there is
a lot of dust or smoke haze in the path only the red end of the sun?s
rays will remain unscattered, and even the scattered light becomes reddish.

The amount of the visible light spectrum scattered is dependent on
line-of-sight distance through the atmosphere. The sky near the the
horizon appears less blue, or whiter, at mid-day than the sky overhead,
thus if the atmosphere were thicker, the sky would be whiter.

Similarly when looking horizontally at a series of mountain ranges they
appear bluer at a distance until a point where the far ranges start to
appear whiter than those in the middle distance. The trees on the ranges
emit terpenes or essential oils, hydrocarbon molecules about 0.2 micron
diameter, which combine with ozone infiltrating from the stratosphere.
These molecules selectively scatter blue light ? hence the *blue haze*.

Air molecules selectively scatter sunlight forward and backward equally,
and at about twice the intensity of the light scattered at right angles
to the beam. For particles larger than the wavelengths of light, back
scattered light is less intense than that for gas molecules but forward
scatter is much more intense. Thus, in an atmosphere containing many
large particles, the sky is less bright than blue sky when looking
downsun and much brighter when looking in the azimuth of the sun.

*White-out *conditions can occur when the surface has a complete snow or
ice cover, matched with an extensive cloud cover. The brightness of the
cloud cover is increased by light that is successively scattered many
times between surface and cloud with little absorption. The light is
travelling in all directions and at all angles. In such conditions there
can be no shadows, the horizon line disappears and the form of the
landscape is no longer discernible.

12.1.2 Twilight effects

The characteristic light, during the morning and evening twilight
periods, is due to atmospheric scattering. The duration of twilight is
geometrically dependent on latitude, season and the observer's
elevation. Evening *civil twilight* is the period from sunset until the
centre of the sun?s disc is 6° below the normal horizon i.e. ignoring
the topography. If the sky is clear it is usually practicable to carry
out normal outdoor activities without artificial light.

*Last light* is the end of evening civil twilight; and the official *end
of daylight* in air navigation regulations: *first light* is the
beginning of morning civil twilight. Evening *nautical twilight* ends
when the sun is 12° below the horizon. During this period the western
horizon is still clearly defined, weather permitting, and the brighter
stars are visible, thus providing good conditions for ocean navigators
to take star sights; hence nautical. Noctilucent clouds may be seen in
higher latitudes. Evening *astronomical twilight * ends when the sun is
18° below the horizon after which all scattered sunlight disappears from
the upper atmosphere and the stargazers have good viewing conditions.
The morning twilight periods are reversed, of course.

The *twilight wedge*, or curve, divides the earth?s shadow from that
part of the sky lit by direct sunlight. It appears on clear days as a
blue-grey arc next to the eastern horizon as the sun disappears, highest
at the antisolar point and curving down to the horizon. Initially there
is a fairly sharp boundary bordered by a reddish band, the *counterglow*
, then becoming diffuse as it rises. An airborne observer should see a
sharp boundary above the horizon. Similar shadowing occurs at sunrise on
the western horizon.

Usually after sunset the sky above that point is pale yellow with a
blue-white arch above, the *twilight arch*, with yellow above and orange
sky to either side. As twilight progresses the arch above the sunset
point becomes pink with yellow and orange below. These areas gradually
flatten as the sky above changes from blue-grey through to dark blue.
The final glimmers on the horizon are possibly greenish-yellow. Very
rarely, and mostly when viewed over water when the air is free from any
form of haze, a *green flash* is seen on the top of the sun?s disc, just
before it disappears.

*Zodiacal light* is a faint luminous glow in the night sky, easily seen
in low to mid latitudes at twilight in moonless conditions. Caused by
sunlight scattered by dust particles in interplanetary space. Zodiacal
light extends over the entire sky but is brightest in the zodiacal band,
and at about 30° angular distance from the sun, where the intensity is
about three times that of the brightest part of the Milky Way. It is
best seen when the ecliptic is close to vertical i.e autumn evenings and
spring mornings. Brightness decreases with angular distance from the
sun, being lowest at 120° then gradually increasing to the 180°
antisolar point. The enhanced brightness near the the solar point, and
covering an area 6° by 10°, is the /Gegenschein/ or *counter-glow* .

*Airglow* is visible IR and UV emissions from the atoms and molecules in
the ionisation layers, refer 1.1, caused by absorption of much of the
solar UV radiation and of cosmic radiation. Daytime airglow, *dayglow* ,
may be seen from the surface at twilight when the blue skyglow is
sufficiently weak. Dayglow is mainly caused by the dissociation of atoms
whereas *nightglow* emissions are due to recombination. The sum of all
visible nightglow emissions, together with zodiacal light and scattered
starlight can be seen as the faint light between stars.

*Crepuscular /(twilight)/ rays* are alternate light and dark bands which
appear to diverge fan-like from the sun?s position when it is hidden
behind a cloud bank or the topography, in a humid or hazy atmosphere.
The rays pass through gaps, like light beams shining through high
windows. The divergence is due to perspective, if the rays pass overhead
they then appear to converge on the antisolar point ? *anticrepuscular*
rays.

There are three types of crepuscular rays:
? rays of light passing through gaps in low clouds
? rays of light diverging from behind a cloud bank
? pinkish rays radiating from below the horizon.

Back to top <#top>

12.2 Atmospheric optical displays

12.2.1 Electromagnetic wave refraction, reflection and diffraction

When a light ray passes obliquely from one transparent medium to
another, or between layers of different density within the same medium,
part of the ray is returned back at the boundary and the remainder,
passing through, is deviated from its original course, i.e. its
direction changes. The deviation is dependent on angle of incidence, the
wave lengths of the light beam, or radio wave, and the refractive index
for that medium. The refractive index is the ratio of the speed of
electro-magnetic radiation in free space to the speed of radiation in
that medium, in air it is effectively 1.0, in water it is 1.33.

*Refraction* has two components, deviation and dispersion. As the
components of sunlight have different wavelengths, in the atmosphere the
deviated light ray is dispersed into its component colours but the red
light deviates less than the blue light when passing from air through
ice crystals or water droplets.

/*Radio waves* in the High Frequency [HF] bands are refracted by the
ionisation layers in the atmosphere. The downward bending of the wave is
sufficient to redirect the wave back to the earth's surface but at a
distance from the transmission point. If there is sufficient energy the
wave may then be reflected back to the ionosphere, thus a high energy HF
transmission is able to 'skip', between the surface and the ionosphere,
for a considerable distance around the world./

*Reflection* is the bounce back of all, or part, of a light ray when it
encounters the boundary of the two media and the angle of reflection
equals the angle of incidence. The amount of light reflected depends on
the ratio of the refractive indices for the two media.

*Diffraction* is the bending of a light beam (or radio wave) into the
region of the geometric shadow of an obstacle, or the spreading of light
waves around obstacles, producing a series of light and dark bands or
rings or coloured spectra, from the inter-ray interference, constructive
interference results in light bands, destructive interference in dark
bands. The degree of diffraction depends on wave length ? red light is
diffracted more than blue ? and particle size.

12.2.2 Ice crystal displays

*Halos* are a range of optical phenomena that result when the sun or
moon shines through thin cloud, particularly Cs, fog or haze composed of
ice crystals. The small ice crystals that grow in the troposphere tend
to be hexagonal flat plates or hexagonal columns. Light passing through
the sides of an hexagonal ice crystal is refracted in exactly the same
way as if it were passing through a 60° prism.

Hexagonal crystal

The magnitude of the deviation angle depends on the orientation of the
crystal but for a 60° ice prism the minimum deviation angle for all
orientations is 22° and for small rotations of the crystal, at the
minimum deviation angle, the variation from 22° is insignificant. Thus
in an atmosphere of randomly oriented crystals there will be a
concentration of rays deviated by 22°. The deviation of light from its
original path, through many hexagonal crystals, brings sunlight or
moonlight to the observer?s eye from different directions and in varying
intensities. However the concentration of refracted rays around 22°
produces a solar or lunar halo whose inner, red edge has an angular
radius of 22° from the observer?s eye. The red edge merges into a yellow
band then all the colours overlap in an outer white band. Halos are
minimum deviation effects, each colour has a concentration at its
minimum deviation angle, but also has a significant amount of light
refracted at greater angles, overlapping other colours. Only the red,
with the lowest deviation, cannot be overlapped.

Light passing through one side and an end of an hexagonal crystal is
refracted in the same way as in a 90° prism and, in this case, there
will be a concentration of rays at a 46° deviation angle. In suitable
conditions a very large solar or lunar halo with an angular radius of
46° may appear but it will be much less intense than the 22° halo and
will rarely be complete. The 22° halo is the most frequently observed of
all the ice crystal displays, the 46° halo is rather rare.

As cloud crystals grow during fall ( flat plates perhaps 50 microns
thick and several mm across, columns perhaps 100 microns across and
several mm long ) the drag creates lee eddies and the crystals tend to
orient with their longest dimension near horizontal. They oscillate
randomly as they fall in a spiral path, producing complicated optical
effects through reflection, refraction and diffraction.

*Sun pillars* are vertical columns of light that appear above or below
the sun, or both, when the sun is near the horizon. They are caused by
reflection of sunlight from the near horizontal surfaces of ice crystals
and are similar to the *glitter path* of sunlight reflected on water.
Light pillars are also associated with the moon. A *subsun* is a
particular form of sun pillar seen from an aircraft when the sun is
high, becoming a reflected, elongated image of the sun in nearly
horizontal ice crystals in lower clouds. The image appears as far below
the horizon as the sun is above. Sun pillars may be associated with Ac.

The *parhelic circle* is a reflection from the vertical surfaces of
horizontally oriented flat plate or columnar crystals when very small
ice crystals, *diamond dust* , fall through the air. The crystals
reflect the light in all directions of the azimuth but always downward
at the same elevation as the sun. Thus if the sun?s elevation is 25° an
observer would see the parhelic circle 360° around the horizon by
looking up 25°, but usually only part of the faint white circle is seen.
The parhelic circle and a sun pillar may form a cross in the sky,
centred on the sun.

If falling plate crystals maintain a horizontal position, with the sun
low in the sky, they have the possibility to refract light to the
observer from the sides of the 22° halo, but not from other positions in
the halo. The result is a spot of increased light intensity and colour
separation, red towards the sun, in the 22° halo each side of the sun,
where the halo would intersect the parhelic circle, and sometimes with a
white tail pointing away from the sun. As the sun elevation increases
the spots move further from the sun and outside the halo, disappearing
at sun elevations greater than 60°. These intensified light spots are
called *parhelia* , *sundogs* or *mock suns* and are the most common ice
crystal phenomenon after 22° halos, often associated with Ci, Cs and
possibly Ac.. Similar effects associated with the moon are *paraselena*
or *mock moons* .

Refraction through the edges of plate crystals with nearly horizontal
bases may produce a *circumzenithal arc* which is part of a circle,
possibly one third, centred directly above the observer?s head and above
the sun, just outside the 46° halo position. The halo may also be
visible. The circumzenithal arc cannot occur when the sun?s elevation
exceeds 32°. Colour separation occurs with red on the outer rim, blue on
the inner, as below. The arc may be associated with Ci and Cs.

An *anthelion* is a concentration of back reflected light at the
*anthelic point*, 180° from the sun and at the same elevation. The
anthelic point may be the centrepoint for various reflection /
refraction phenomena - the *anthelic arcs*.

Various other light intensifications are associated chiefly with
refraction and may appear in ice crystal displays in Antarctic
conditions, among them are:
? Parry arcs
? Circumhorizontal arcs
? Supralateral arcs
? Infralateral arcs
? Contact arcs
? Upper and lower tangents to the 22° halo

12.2.3 Cloud droplet effects

The moon or sun when viewed through Cc, Ac, thin As or Sc may be
surrounded by a diffraction disc, or aureole of light, of varying size
and intensity. The aureole is bluish near the sun or moon, whiter
further out with a red/brown periphery. The aureole may be enclosed by
rings with blue inner and red outer edges forming a *corona*. The size
of the rings depends on droplet size, smaller droplets produce larger
rings. If there is a wide mixture of droplets of varying size then the
diffraction rings will be of widely varying size, overlapping each other
and blurring into a uniform illumination, leaving only the aureole visible.

Cloud *irisation* or *iridescence* /( Iris = the Greek rainbow goddess
)/ appears when a cloud element or streak, usually Ac or Cc and
sometimes lenticularis, is evaporating around its edges so that the
droplet size changes quickly over a short angular distance. Also the
entire element or small cloud is contained in roughly the same angular
distance from the sun. The diffraction pattern traces blue light around
the edge of the cloud where the droplets are smallest and red light
where the drops are uniformly larger. The result is iridescent bands,
predominantly pinks and blues or greens with pastel shades, appearing
along the thinner edges of individual cloud elements, below left. Cloud
iridescence is common but the cloud must be within 20° of the sun and
thus not readily noticeable, it can occur in thin Sc or As but also in
nacreous clouds.

The corona is the diffraction pattern seen in cloud droplets when
looking towards the sun, the *glory* is the diffraction pattern seen in
cloud or fog droplets when looking toward the antisolar point. /(A glory
is the circle of light or aureole around the depiction of the head of a
saint etc.)/ When flying in sunlight over a cloud layer the coloured
rings of glory may be seen around the the antisolar point, i.e. around
the aircraft shadow if it is not diffused. The antisolar point is that
of the observer so the luminous coloured halos are centred on the
position of the observer?s head shadow. As in other diffraction rings
the blue halo is on the inside and the red on the outside.

The ?*silver lining*? which may be seen around the outer edges of
heavier clouds, containing larger droplets, is a diffraction effect.

12.2.4 Rainbows

As a light ray from the sun strikes a small spherical raindrop /(drops
less than 150 microns diameter are held as a sphere by surface tension,
larger raindrops are distorted by drag into a flattened sphere)/ some
light is reflected by the outer surface and some passes through and
reaches the opposite inner surface, where a fraction of the light is
reflected internally and the rest passes out of the drop. A ray may be
reflected only once inside a drop, or many times, but each reflection is
accompanied by light leaving the drop, so each internal reflection
diminishes the reflected ray.

Each spherical raindrop reflects and refracts, in all directions, the
light rays that are striking it. However due to the spherical surface
there is a concentration of first reflection rays reflected back,
towards the sun, around a maximum angle of about 42° to the axis line
joining the raindrop and the sun. The red light is refracted less than
the other colours and has a concentration at about 42° while the blue
light is concentrated at 40° with the other colours in between.

The observer will see this concentration of reflected light rays as an
intensified coloured light band. This band consists of the first
reflection rays from all the raindrops which lie on the surface of a
cone, subtended at the observers eye, with an angular radius of 42° from
an axis line drawn from the sun (directly behind the observer) through
the observer?s head and extended down-sun to the *antisolar point* i.e.
below the horizon where the shadow of the observer?s head might be. This
*primary rainbow * will have the red band on the outer edge. An observer
on the earth?s surface sees only an arc of the rainbow circle. When the
sun is 40° above the horizon just the top of the bow can be seen above
the opposite horizon. The rainbow will rise as the sun lowers until a
maximum half circle is reached when the sun is on the horizon. An
airborne observer could possibly see the full circle

Light that is reflected twice within the raindrops has a deviation angle
of 51° and produces the weaker *secondary rainbow*, concentric with and
outside the primary but with the red band on the inner edge. Thus the
observer is seeing the concentration of twice reflected rays from all
the raindrops that lie on the surface of a 51° cone, at the same time he
is seeing the first reflections from the raindrops on the 42° cone.
Third and fourth reflection rays would also form rainbows with angular
radii of 40° and 46° respectively but are so weak, and would also form
up-sun, so that they are most unlikely to be seen, except against a dark
cloud.

As the first reflection rays from spherical raindrops have a maximum
deviation angle of 42° it follows that all the low angle reflections
coming back to the observer?s eye, from all the raindrops enclosed
within the 42° cone, will increase the brightness of the sky within the
primary bow. Similarly the sky is also brighter outside the secondary bow.

The rainbow ends are frequently brighter than the rest of the bow,
particularly when the sun is low. This comes from the approximate
straight back reflection / refraction in the larger, flatter raindrops
added to the reflection / refraction of the smaller, spherical drops.

Diffraction interference of light rays /( the waves are out of
synchronisation )/ produces changes in light intensity which may appear
as a series of light / dark bands within, and close to, the primary
rainbow.

When rainbow rays pass through very small water droplets e.g. cloud or
fog droplets, they are spread by diffraction and each colour band is
broadened and overlaps adjoining bands. Where all the colours overlap
the result is a *white rainbow*, *cloud bow* or *fog bow*, often seen
from an aircraft flying over a smooth, extensive cloud layer. Near
sunset a white rainbow may appear as a *red rainbow* in a low cloud
bank. A full moon can produce a rainbow which appears to be white in the
low light conditions but, when photographed, is revealed as a normal
rainbow.

12.2.5 Atmospheric density layer effects

When a light ray passes through the atmosphere, where the density
changes gradually, the light ray changes direction in a curved path
rather than abruptly as when passing through an ice crystal. With
changes in atmospheric density the deviation path curves toward the
denser air, thus when a star is low in the sky the change in atmospheric
density with height, particularly with a cold surface layer under an
inversion, causes refraction to bend the light rays so that the star?s
apparent position is higher than actual and the dispersion may produce a
multi colour image ? upper part blue, middle white and lower part red;
giving the impression of an aircraft?s lights, and often reported as
strange moving lights in the sky, as the atmospheric effects make the
object appear to jiggle. At sunset or sunrise refraction can cause the
sun?s image to appear above the horizon when it is actually below.

Small scale atmospheric temperature and density variations in the line
of sight between the observer and a star, or other light emitting
object, produce the twinkling effect, *scintillation*, and the
shimmering of distant landscape. Parcels of cooler or warmer air can act
as lenses, reducing or increasing the apparent brightness or size of the
object.

*Mirages* are optical phenoma produced by refraction of light rays
through air layers with large temperature gradients. An *inferior
mirage* [i.e. it appears below its actual position] occurs when the
temperature initially decreases rapidly with height. For example, the
heat flux from a hot surface, such as tarmac or sand, greatly increases
the temperature of the adjacent shallow air layer and consequently the
density of that layer decreases, refer 1.2, equation of state. The
result is a layer of less dense air underlying denser air, the reverse
of the normal lapse rate. Light rays from the sky moving through the
layers will be refracted <#refraction> upward in the less dense air
[i.e. bent toward the denser air] giving the appearance of a layer of
water.

When seen from the ground or water a *superior mirage* [i.e. it appears
above its actual position] occurs when there is a pronounced inversion
near the surface, and normally over the sea or a large body of water. A
distant object within the inversion layer, even something below the
horizon, will appear in the sky above its actual position ? possibly
totally upside down or the upper portion upside down, but certainly
distorted and wavering. For more information enter the phrase "superior
mirage" into a search engine.

An inversion layer of cooler air with warmer air above and below acts as
a wave guide for light rays introduced into the layer at a small angle
to the horizontal. Unless there is a discontinuity in the layer the
trapped rays cannot escape and may be confined within the wave guide for
very long distances, following the curvature of the earth. In such
circumstances a spectacular superior mirage might be seen from an
aircraft flying over land within that wave guide. Whit Landvater is a
Nevada balloonist who experienced such a display on November 27, 2003
and said "It was like "living inside a Photoshop document while someone
was going crazy with the clone tool and filters!" See Whit's report
<http://www.area-fiftyone.com/balloon/inversion>.

Back to top <#top>

12.3 Moon phases

The geometry of the sun-earth-moon orbits gives rise to the eight
commonly recognised moon phases and the associated moonrise/moonset
periods.. The elapsed time from one full moon to the next is about 29.5
days.
Moon phases and moonrise / moonset periods Phase	Rises	Sets
*New moon*	dawn	dusk
Waxing crescent	 	 
*First quarter*	noon	midnight
Waxing gibbous	 	 
*Full moon*	dusk	dawn
Waning gibbous	 	 
*Last quarter*	midnight	noon
Waning crescent	 	 

|  Aviation meteorology contents page <../meteorology/index.html>  |  
back to top <../meteorology/section12.html#top>

Next - aviation weather reports <../meteorology/section13.html> The
next, and last, section of the Aviation Meteorology Guide covers
aviation weather reports and forecasts

Copyright © 2000 John Brandon ? mail to: web at auf.asn.au