Parry Orientation & Parry Arcs

Last updated on September 16, 2023

Parry Orientation & Parry Arcs: Exploring the Intricacies of Atmospheric Optics

Atmospheric optics never ceases to amaze us with its intricate phenomena, and one such captivating phenomenon is Parry orientation and the resulting Parry arcs. Named after the explorer William Edward Parry, who first observed them, Parry arcs are a testament to the astonishing properties of ice crystals in the atmosphere. In this article, we will delve deeper into the world of Parry orientation and its various manifestations, shedding light on the rare and the more commonly observed Parry arcs.

The Improbable Orientation of Parry Crystals

If the near-perfect orientation of column crystals seems improbable, then the orientation of Parry crystals must appear wildly so. Parry crystals have a long crystal axis that is horizontal, along with two prism side faces that are also horizontal. This leaves them with only one degree of freedom: the ability to rotate about a vertical axis passing through the crystal's center and perpendicular to the two horizontal faces. It is this unique orientation that gives rise to the mesmerizing Parry arcs.

Unveiling the Parry Arcs

Parry arcs can be classified into different types based on the path of light rays passing through the ice crystals:

1. Sunvex and Suncave Parry Arcs: When light rays pass through two prism side faces with a wedge angle of 60°, they create sunvex and suncave Parry arcs. These arcs are the most common manifestations of Parry orientation and can be observed with careful observation a few times a year.

2. Parry Supralateral (Tape) and Infralateral Arcs: Rays traveling between a side face and a vertical end face, forming a wedge angle of 90°, produce the exceedingly rare Parry supralateral (Tape) and infralateral arcs. These arcs are a true testament to the extraordinary nature of Parry orientation.

3. Circumzenithal and Circumhorizon Arcs: Rays passing between the horizontal and vertical faces of the Parry crystals create the circumzenithal and circumhorizon arcs. These arcs are relatively more commonly produced by plate crystals, but their occurrence in Parry orientation adds to the diversity of atmospheric optics phenomena.

4. Helic Arcs: Reflection from a sloping prism face gives rise to the very rare helic (heliac) arc. Until recently, this arc was predominantly observed in the Antarctic, adding an element of mystique to its rarity.

Exploring the Fascinating Visuals

To better understand the beauty of Parry arcs, let's take a closer look at some of their visual manifestations:

• Suncave Parry and Upper Tangent Arcs: These arcs create a captivating interplay of light, forming a halo-like shape that can be observed when the sun is low in the sky.

• Sunvex and Suncave Parry Arcs: These arcs also form a halo-like shape but are observed when the sun is higher in the sky. They are often seen in conjunction with other atmospheric optical phenomena.

• Parry Supralateral (Tape) Arc: This arc is an exceptionally rare phenomenon that occurs when light rays pass through the vertical end face of a Parry crystal.

• Parry Infralateral Arc: Similar to the supralateral arc, this arc is also extremely rare and occurs when light rays pass through the side face of a Parry crystal.

• Helic Arc: The helic arc, characterized by its spiral shape, is an elusive phenomenon that has been primarily observed in the Antarctic. Its rarity adds to its allure.

Capturing the Beauty

To fully appreciate the intricate details of Parry arcs, several talented photographers have managed to capture these elusive phenomena. Their stunning images bring to life the ethereal beauty of atmospheric optics. Some notable photographs include:

• Suncave Parry by Alastair Adams
• Double Parry by Loren Hall
• Tape Arc by Max Emerson
• Parry Infralateral by Marko Riikonen
• Helic Arc by Alastair Adams

These captivating images showcase the diversity and enchantment of Parry arcs, further emphasizing the allure of atmospheric optics.

In conclusion, Parry orientation and the resulting Parry arcs are a testament to the astonishing properties of ice crystals in the atmosphere. The unique orientation of Parry crystals gives rise to a range of captivating arcs, from the more commonly observed sunvex and suncave Parry arcs to the exceedingly rare Parry supralateral (Tape) and infralateral arcs. The beauty of Parry arcs is best captured through the lens of talented photographers who have managed to freeze these ethereal moments in time. So, the next time you gaze up at the sky, keep an eye out for the mesmerizing Parry arcs that may grace the heavens above.

If the near perfect orientation of column crystals sounds improbable, that of Parry oriented crystals must seem wildly so. The long crystal c axis is horizontal AND so are two of the prism side faces. The only degree of freedom is the ability of the crystal to take all rotational positions about a vertical axis passing through the crystal centre and perpendicular to the two horizontal faces.The resulting halos are called Parry arcs. They are named after the explorer William Edward Parry who first observed one of them. Some Parry arcs are not particularly rare and with careful observation will be seen a few times a year .Rays passing through two prism side faces with a wedge of 60° produce sunvex and suncave Parry arcs (column orientations make tangent arcs). These are the most common Parry arcs.Rays travelling between a side face and a vertical end face - wedge of 90° - make the exceeding rare Parry supralateral (Tape) and infralateral arcs. Rays passing between the horizontal and vertical faces make the circumzenithal and circumhorizon arcs much more commonly produced by plate crystals.Reflection from a sloping prism face gives a very rare helic (heliac) arc until recently only seen in the Antarctic.

suncave Parry and upper tangent arcs sunvex and suncave Parry arcs Parry supralateral (Tape) arc Parry infralateral helic arc

Images: suncave Parry, Alastair Adams double Parry , Loren HallTape arc, Max Emerson Parry infralateral, Marko Riikonenhelic ,Alastair Adams Thumbnails lacking alarger version will be updated soon.

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