2st PART



Optical phenomena of water Droplet

  • The Optical Phénomena of Ice Crystal

    Besides the optical phenomena due to droplets of clouds, luminous phenomena took place due to diffraction, refraction, reflection and dispersion of the light by flat hexagonal ice crystals or in hexagonal columns.

    We find them suspended in the cirrus, the cirrocumulus, cirrostratus or during ice fog. When temperatures are low in winter it is possible to see these optical phenomena, and when the sun is low on the horizon and the ground is snow-covered.

The various phenomena of light are caused by ice crystals in clouds.

The many optical phenomena rarely occur simultaneously. All this depend on the shape and position of the crystals, agitation of the atmosphere so many factors come into play for the number of optical phenomena visible in the sky.

  • Large and Small lunar and solar halos

Around the Sun or the Moon it is possible to see a luminous ring, called a halo. This circle of light appears of a pale white with sometimes a red inside and blue colour outside.

There are three types of halos :

  1. The 9° halo is white and as it is located near the Sun it is difficult to identify it.

  2. The 22° halo is the most common and the brightest. It can be white or coloured according to the number, type and position of the crystals. This optical phenomenon is visible around 100 times a year in Europe based on the 10 years of research.

  3. The halo of 46° is very large and very brilliant. Infrequent, it is possible to see the 3,90 times a year.

The halos are sometimes accompanied by parhelia on each side of the circle... It is in high latitude that these optical phenomena are more often observed.

The halos are formed by reflection or refraction of light of the ice crystals in clouds of the upper floor (cirrus, cirrostratus ...) or by aerosols in the atmosphere. These ice crystals are composed of six-sided prisms and cross sections at six angles

Click here to enlarge

A 22° solar halo I photography
on 10/01/2004 at 15:17
With a fujifilm S5000

At left, a 22° lunar halo. At right, a 22° and 46° halo around the
Sun with two parhelia, upper tangent arc, circumzenithal arc.
Photographed at the South Pole in 1999 by Marko Riikonen

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