1st PART



Optical phenomena of water Droplet


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.

iridescences which I photographed on 08/16/2004
at about 11:52 with a fujifilm S5000
Click here to see more photos of iridescence and larger

  • The Broken specter

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°.

A double rainbow


Inverted - upside down rainbow seen by plane
Source : Mountain Light, © Galen & Barbara Rowell

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).

Patricia Régnier helped me correct mistakes, please you to visit her blog
I’m not english speaker, some improprieties can appear to english masters.
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