Carneddau Halos

Solar halos over Carnedd Llewelyn and Yr Elen


28 January 2012

Carnedd Gwenllian, Carneddau

SH 68520 66535; 53.17961°N, 3.96883°W


This impressive display of atmospheric optical phenomena – caused by ice crystals in high cirriform clouds refracting and reflecting the light from the sun – was witnessed around 2pm on 28 January 2012. It was viewed from a spot on the slopes of Carnedd Gwenllian (formerly known as Garnedd Uchaf) looking south over Carnedd Llewelyn and Yr Elen in the Carneddau range of mountains in North Wales. The various components of the display are described below.

Individual components of the optical display

Circumzenithal Arc
The circumzenithal arc (CZA) is a vividly coloured arc centred on the zenith and extending for about 90°. Appearing like an ‘upside down rainbow’, this arc is caused by horizontal plate crystals acting as 90° prisms (light passes through a horizontal hexagonal face and one of the vertical side faces).

Supralateral Arc
This coloured arc is caused by horizontally oriented columnar crystals acting as 90° prisms (light passes through a vertical end hexagonal face and one of the long side faces of the column).

Upper Tangent Arc
The gull-wing shaped arc above the 22° halo is formed by refraction through side faces inclined at 60° to each other of horizontally oriented hexagonal column crystals.

22° Halo
A solar halo is a bright circle centred on the sun and is formed by refraction. The 22° halo is the most common and occurs when hexagonal crystals with a range of different orientations act as tiny 60° prisms (light enters through one side of the hexagonal cross section and exits through a next-to-adjacent side).

Also known as a ‘mock sun’ or ‘sundog’, a parhelion is a bright, coloured spot seen on either side of and at the same height as the sun. Parhelia are most commonly seen 22° away from the sun and are formed by refraction through the vertical side faces (inclined at 60° to each other) of horizontal plate crystals.

Parhelic Circle
Caused by reflection off vertical faces of ice crystals (both externally and internally), this faint white circle forms parallel to the horizon at the same height as the sun and can extend for as much as 360°.

This is a sundog appearing 120° away from the sun and occurs when light passing through horizontal plate crystals is reflected internally multiple times.

Warning: when looking for solar halos, never look directly at the sun.

(Featured in Atmospheric Optics Picture of the Day)

Circumzenithal Arc

Parhelion and a fragment of the parherlic circle

22° halo and parhelia seen above Carnedd Llewelyn and Yr Elen

Paranthelion (top right) seen 120° to the right of the sun

22° halo with the sun lurking to the side of Yr Elen (viewed at about 0930 from SH 65801 66765)

15 thoughts on “Carneddau Halos

  1. Good to see you back out in the mountains and what an impressive set of photos. I wouldn’t have had a clue what I was seeing…I have noticed things like this while climbing Munros in winter, so this is all very interesting…thanks for all the info. The sun looked strange from where we were yesterday but i was too preoccupied with the underworld to look properly- shame on me! 🙂


  2. It is great to have not only the top photo and then your detailed descriptions, but sewn up with individual photos of each phenomena. this is all very informative. thank you for doing this. And so what about the halo effect we see occasionally around the moon? It’s supposedly a pre-warning of wet weather.


    • Light from the moon can produce the same halos as those created by light from sun. The thing that they have in common is the clouds high in the sky bearing ice crystals. If you are interested in reading more about these phenomena, there is an excellent site here.

      With regards to being harbingers of wet weather, the above-mentioned site has this to say:

      Does it foretell of rain? Not necessarily. High cirrostratus cloud forms the halo and this same cloud can be on the advancing edge of a warm front with its associated heavy rain. If a wind stirs from the south west, the sky becomes hazy, a halo forms around the sun which is then dimmed and finally obscured by increasingly thick cloud then rain is likely within a few hours. But very many halos are not associated with frontal systems and to see one is not at all a reliable sign of wet weather.


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