Parry Arcs

Last updated on September 16, 2023

Parry Arcs: A Phenomenon of Spectacular Atmospheric Optics

Parry arcs are a captivating and mesmerizing phenomenon that can be observed in the sky when it is filled with a dense concentration of ice crystals. These arcs are often part of a larger display of atmospheric optics, creating a truly spectacular scene for those lucky enough to witness it. While only a small proportion of ice crystals need to have the Parry orientation to produce an arc, their presence adds a touch of magic to the skies above.

What are Parry Arcs?

Parry arcs are a type of halo that forms around the sun or moon, appearing as a faint and relatively sharp arc above the much brighter upper tangent arc. They are named after William Edward Parry, a British Arctic explorer who first documented these arcs during his expeditions in the early 19th century. Parry arcs are caused by the interaction of sunlight or moonlight with horizontally oriented ice crystals in the atmosphere.

The Science Behind Parry Arcs

To understand how Parry arcs form, we need to delve into the intricate physics of light and ice crystals. When sunlight or moonlight passes through these ice crystals, it undergoes a process called refraction. The ice crystals act as tiny prisms, bending the light rays as they pass through them. The bending of light causes the rays to separate into their individual colors, similar to how light passes through a prism and creates a rainbow.

The orientation of the ice crystals plays a crucial role in determining the shape and position of Parry arcs. For a Parry arc to form, the ice crystals must have a specific orientation known as the Parry orientation. These crystals align horizontally in the atmosphere, with their flat faces parallel to the ground. When sunlight or moonlight enters these horizontally oriented crystals, it undergoes refraction and creates the distinct arc shape that characterizes Parry arcs.

Observing Parry Arcs

Parry arcs are not a common sight in the sky. They require specific atmospheric conditions, namely a high concentration of ice crystals, to form. Therefore, they are most likely to be observed in polar regions or during cold winter days when the air is saturated with moisture and ice crystals. When Parry arcs do appear, they often accompany other atmospheric optical phenomena such as sundogs, halos, and tangent arcs, enhancing the overall spectacle in the sky.

Capturing the Beauty of Parry Arcs

Photographing Parry arcs can be a challenging task due to their faint and elusive nature. To capture their beauty, photographers need to be patient and well-prepared. The best time to photograph Parry arcs is when the sun or moon is relatively low on the horizon, as this provides a greater contrast between the arc and the surrounding sky. Additionally, using a polarizing filter can help enhance the visibility of Parry arcs by reducing glare and increasing color saturation.

The Wonder of Nature's Optical Masterpieces

Parry arcs are just one example of the countless wonders that can be found in the natural world. Atmospheric optics offers a glimpse into the intricate interplay between light and the atmosphere, creating breathtaking displays that leave us in awe of nature's artistic abilities. From shimmering rainbows to glowing halos, these optical masterpieces remind us of the beauty and complexity that surrounds us every day.

In conclusion, Parry arcs are a captivating phenomenon of atmospheric optics that add a touch of magic to the skies above. Formed by the refraction of sunlight or moonlight through horizontally oriented ice crystals, these arcs create a distinct and faint arc shape above the upper tangent arc. While observing Parry arcs requires specific atmospheric conditions, their appearance is a rare treat for those fortunate enough to witness them. Photographing these elusive arcs requires patience and preparation, but the resulting images capture the beauty of nature's optical masterpieces. So, keep your eyes on the sky and be ready to be amazed by the wonders that unfold above us.

Display at Whistler - Blackcomb Mountain, BC, Canada on 16th January 2002. Parry arcs oft reveal themselves during spectacular displays when the skies are dense with ice crystals. Only a small proportion need have the Parry orientation to produce an arc. Here it is characteristically sharp and relatively faint above the much brighter upper tangent arc. The solar elevation was 20°. Lynn Fink captured these images while skiing. The left hand sundog is also shown here. Images ©Lynn Fink reproduced with permission.

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