In
the sky it is possible to see luminous phenomena of all
kinds. The latter result from diffraction, refraction, reflection
and dispersion of the light by various water droplets of
clouds or of crystals of ice.
Optical
phenomena of water Droplet
When the clouds or the sky are wet, it
is possible that the spherical water drops involve the dispersion
of the sunlight which causes an astonishing variety of complicated
optical effects.
The
Corona
The
coronas are optical phenomena produced by the diffraction
of the light by water droplets of clouds. The
stratus, the thin clouds or mists and especially the
cirrocumulus, the altocumulus
and altostratus, clouds
of average height.
In
the clouds the droplets must be of uniform size, of mean
size to deviate the sunlight and also to reach the observer.
Around
the Sun or the Moon when there is a corona we see a ring
all around it composed of several subtly coloured rings
encircling the central aureole. It is around the Moon that
they are easiest to see.
A Lunar corona taken by Jerry Xiaojin Zhu the 12/2/2003
at 21:43
with digital Nikon coolpix 995, 4 seconds pause time, F4.2,
ISO 400
A solar corona taken
by Michael Ellestad the 08/03/2003
The
Iridescent Clouds
When the thin clouds and wet steps in the sky pass over
us and far from the colours of clouds are slightly diffracted.
We often observe them during the
cirrocumulus.
The colours are diffracted in the bands at the edges
of the clouds and are only organised out of coronal rings
when the size of the droplets is exactly uniform through
the clouds.
These optical phenomena are among most current, we can
see them approximately 100 times per year.
The Brocken specter is observed especially in the mountains
during the scattering of light by the droplets that is composed
fog. In order to observe it, you require an unobstructed
Sun shining behind you and a blanket of fog or cloud in
front of you. You then need to look downwards away from
the Sun. Then we observe a luminous circle and coloured
exactly opposite the sun with a dark central zone, which
is the shade of the observer. The circle of light is called
Glory. Its radius depending on the size of the drops. Its
ray depends on the dimension of the drops because the smaller
they are, the larger the Glory. And opposite.
It is also possible to see the Brocken specter in the
plane. That depends on the distance between the plane and
the clouds. To see Glory, it is necessary that the plane
is close to the clouds.
The Brocken specter owes its name to The Brocken, Blocksberg
or Bocksberg, is the highest peak of the Harz mountain in
North Germany, where it is frequently seen.
The
Brochen specter
The
Fogbows
On the hills, the mountains and the cold fogs of the
sea, it is possible to see white Arcs called fogbow in english.
This optical phenomena is known as the Brocken specter.
It is even possible to see both at the same time.
The White Arcs are almost as large as the rainbow and
wider except that they are white. The Sun must be no more
than 30 to 40° high above the horizon unless you are
on a hill so that the mist and the arc can be seen from
above. If the fog is at a distance of more 50 m of you it
is possible to see the fogbow more prolonged. They are located
at about 145° east or west of the Sun on the horizon.
A
fogbow taken by Michel Tournay on 01/25/2004 at 04:34
The
rainbows
The rainbows are optical phenomena caused by a curtain
of rain full of very small water drops, which reflect the
sunlight by changing the direction of the light rays which
creates an arc composed of all the colours of the visible
spectrum (red, orange, yellow, green, blue, indigo and purple).
The bigger the droplets, the more brilliant the colours.
We see the rainbows in the West in the morning, and
in the East in the afternoon. The Sun must be in our back.
It is possible to see it only if the sun is at an angle
less than 42°. On average, 10 rainbows are visible a
year and it is especially in the spring when we see them.
A video explaining the formation of rainbows
A rainbow taken in the Valley of Colorado.
It is possible also to see a secondary
arc which is less luminous. The order of the colours of
the secondary arc are reversed relative to the primary rainbow.
The primary arc has a ray of approximately 42° and the
secondary arc has a ray of approximately 51°.
When we are at an altitude and we stay
close to the curtain of rain, it is possible to see the
bottom of the circle rainbow ("reversed" rainbow).
But a rainbow is visible only if the angle
between the line sun-observer and the enlightened curtain
of rain is less than 42° (angle related to the phenomenon
of multi-reflection occurring in the drops of rain). So
for an observer on the ground, it is impossible to see a
rainbow if the sun is more than 42° above the horizon
(it is for that we see more rainbows during the morning
or the evening and/outside of the summer season).
WARNING Patricia
Régnier helped me correct mistakes, please you to
visit her
blog
I’m not english speaker, some improprieties can appear
to english masters.
Could you help me reporting by mail any fault you read.
Thank you for all.
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