The Circumzenithal Arc

The circumzenithal arc is upside down rainbow which is also called circumsenith arc. This upside down rainbow and the Bravias arc is an optical phenomenon which is similar to the appearance of the rainbow but this circumzenith arc belongs to the family Halos which arise from the refraction of the sunlight through ice crystals, generally in citrus or cirrostratus clouds.

The circumzenithal arc is located above the noticed sun and forms a quarter of a circle which is an arc in the centered position on the zenith. The circumzenith is one of the brightest and colourful members of the family of the halos.  It is much purer than the rainbow as it has much less overlap in their formation proccess and it’s colours starts from violet at the top and red at the bottom. When the sun observed the CZA is the brightest as ever.

The circumzenithal arc’s intensity distribution requires several affects such as

  • Frensel’s reflection and transmission amplitude.
  • Atmospheric attention
  • Azimuthal angular dispersion
  • Geometrical constraints
  • Chromatic dispersion

Contrarily to the public awareness this Circumzenithal arc is not a rare phenomenon and it appears so far overhead so, the people are not aware of this phenomenon.

The light which forms the circumzenithal arc enters the ice crystal through its flat top face and it comes out from a side face of the prism.

The circle colour separation and the purity of colour is due to the refraction of the parallel sun through a 90° prism. The circumzenithal arc is brightest when the sun is at 22° above the horizon which causes the sunlight to enter in and out of the crystal at the minimum deviation angle this has 22° in radius and 3° in width.

The radius of the circumzenithal arc varies between 32.2° and 0° depending on the solar attitude. When the sun is observed above 32.2° the light comes through bottom face of the crystals which leads to almost colourless parhelic circle. The ice crystals also requires that it have a common orientation it occurs only when there is no significant up-or downraft and also in the absence of turbulence.

Like all the halos the circumzenithal arc occurs in both sun and moon and the one which occurs with the moon is called lunar circumzenithal arc. This lunar circumzenithal arc is a rarest circumzenithal which occurs only when the moon is brightest which refers to only the case around full moon.

We can make an artificial circumzenithal arc by a water glass experiment. Under the shallow angle illuminating the top air-water interface of a completely water-filled cylindrical glass which refracts the light into the water and the second refraction in the cylinders side face is called Skew- ray refraction.

Then the overall refraction turns out to be a equivalent to the refraction through an upright hexagonal plate crystal when the rotational averaging is taken into account. Then the colourful artificial circumzenithal arc will appear on the floor.