3st PART



Optical phenomena of water Droplet

  • Parhelia or (Sundog)

The parhelia also called sundogs and mock suns are optical phenomena as one can see on either side of the sun between 22° and 46°. Often they take place at the same time as the halos. More the Sun is high in altitude more parhelia are far away from 22° halo.

You can see parhelia caused by reflection and scattering of light from the moon witch they are called parselena or "mock moons". But they are more difficult and rare to see.

The color sequence is always that of the light spectrum as the red of the rainbow is always oriented towards the Sun and blue outward with sometimes followed by a tail of white light. This white light can be so brilliant sometimes, it looks like another sun.

They are among the optical phenomena caused by ice crystals most common. To see the sun must be low enough. In Europe, we can see about 73 per year. Their duration of occurrence can be long.

A sundog I photographed on
11/12/2004 to 5:45 p.m.
with a Fujifilm S5000
and zoomed X.9

Two parhelia photographed by Ivo Brezina in 2003

We can also see another type of parhelia 120° of the Sun a white color. Often they appear on the circle Parhelic. But these arcs are very rare. They are caused by flat ice crystals and horizontally oriented.

  • Upper or Lower tangent Arc

    At 22° above or below the Sun may be a tangent arc touching the lower halo. The shape of an upper tangent arc varies with the elevation of the sun. More the Sun is high more arcs are flat until the extremities of the upper and lower arc join when the Sun is 32° above the horizon, for forming a circumscribed halo around the 22° halo. Then this halo touches the 22° halo when the Sun amounts to 70°.

    The upper tangent arc is more often visible than the lower tangent arc. This is when the Sun is about 25° above the horizon that the lower arc tangent is visible under the little 22° halo.

    The circumscribed halo has an oval shape. As it approaches the 22° halo is becoming more brilliant until it becomes exactly circular and merging with the 22° halo.

    These are column horizontal oriented crystals that create the upper and lower tangent arc and the circumscribed halo.




    The evolution of the tangent arc and the circumscribed
    halo according to the height of the sun.
    Diagram Manual of Geophysics, Volume VIII

    A 22° halo around the sun at a height of 51,11° with
    its left tangent arc approaching it forms the circumscribed halo.
    Photographed the 04/06/2005 at 1:15 p.m. at St. Gaudens .

An upper tangent arc above the small halo.
Photographed with a digital camera.
  • Parry Arc

    Above the upper tangent arc and below the lower tangent arc can be seen Parry arc named William Edward Parry (1790 - 1855) who has described many of these optical phenomena.

    There are two types of Parry Arc :

    - The "suncave" Parry arc :

    1°) The upper "suncave" Parry arc is the highest and has the form of a caret. Higher the sun is relative the horizon nearer is the arc to the halo and more brilliant. It occurs mainly when the Sun is located between 15° and 40° above the horizon.

    2°)The lower "suncave" Parry arc is the opposite, under the lower tangent arc and it is rarer than his brother. In rare cases, when the Sun is about 50° of the horizon that arc may be visible below the 22° halo as a "\/", and when the sun is higher, 70° it touches the 22° halo.

    Upper tangent arc and above the Parry arc "sunvex"
    with his right to a portion of the "suncave" Parry arc
    shaped "^", made on 04.26.2004 by Pierre-Paul Feyte .
    Source : www.kitao.tk

    - "sunvex" Parry arc :

    1°) The upper "sunvex" Parry arc is between the upper "suncave" Parry arc and upper tangent arc and has as "V". In a color spectrum of light, red is always oriented towards the Sun. Lower is the sun at horizon more luminous and nearer is the tangent arc This is when the Sun is between 7 and 12° of the horizon we see the best of "sunvex" Parry arc and when the Sun reaches 15° to 20° is no longer seen. Higher the sun rises in the sky the flatter the arc is and the lesser visible.

    These arcs are rare and it is especially rare to see the "suncave" and "sunvex" Parry arc simultaneously.

    The evolutions of the Parry arc along the height of the Sun.

  • Parhelic Circle

    When parhelia are visible in the sky, you can see a parhelic circle. It is a white band from the Sun through the parhelia. Lower the sun is flatter the strip is and more mounts the sun and more amount the two bands forming a large circle always starting from the Sun.

    It consists of millions of ice crystals to form a vertical reflecting sunlight in the sky. The brightness of the parhelic circle will depend on the altitude of the sun and the crystal thickness.

    This optical phenomenon is rarely observed, we see that 4 every year.

A 22° halo and a Parhelic circle photographed
by Lawrence Laveder with a Zuiko 16 mm
open to 3.5 and a shutter speed of 1/500 sec on a film
Kodachrome 64 in Cap d'Antibes in September 1990 .
Source : http://www.photoastronomique.net

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