Atmospheric optics never ceases to amaze with its stunning displays of light and color. One such captivating phenomenon is the Polish Wegener arcs, which add a touch of mystery to the sky. These arcs, named after the German meteorologist Alfred Wegener, are relatively rare and require specific atmospheric conditions to occur. In this article, we will delve into the intricacies of Polish Wegener arcs, exploring their formation, characteristics, and the factors that contribute to their occurrence.
Polish Wegener arcs manifest as faint, intersecting arcs that form a cross at a specific point in the sky called the anthelic point. The anthelic point is opposite to the sun and at the same altitude as the parhelic circle, a bright white arc that encircles the sun. These arcs are formed by column crystals present in cirrus clouds, with their long axes oriented horizontally. The column crystals responsible for Polish Wegener arcs have prism sides inclined at an angle of 60°, similar to those that form other atmospheric phenomena like the circumscribed halo and the upper tangent arc.
However, what sets Polish Wegener arcs apart is the unique path of light rays within the crystal. As the light enters the crystal, it undergoes a single reflection from one end face before exiting. This critical difference distinguishes Polish Wegener arcs from other halos in the sky.
To witness the ethereal beauty of Polish Wegener arcs, specific crystal characteristics are necessary. These arcs require relatively short crystals with nearly perfect end faces. The end faces of crystals play a crucial role in determining the visibility and clarity of these arcs. Imperfections or irregularities on the end faces can hinder the formation of Polish Wegener arcs or make them less pronounced.
In the realm of atmospheric optics, simulations are often employed to understand and visualize the behavior of light in different atmospheric conditions. HaloSim, a ray-tracing simulation, has been instrumental in studying various halos, including Polish Wegener arcs. However, it is essential to note that HaloSim employs virtual crystals that are perfect in nature. In reality, crystal imperfections are more likely to occur, making the formation of Polish Wegener arcs a delicate and rare occurrence.
The occurrence of Polish Wegener arcs relies on several atmospheric factors aligning perfectly. Here are some of the key elements that contribute to the appearance of these arcs:
Presence of cirrus clouds: Polish Wegener arcs form within cirrus clouds, which consist of thin, wispy ice crystals at high altitudes. The presence of these clouds is a prerequisite for the formation of these arcs.
Horizontal orientation of column crystals: The crystals responsible for Polish Wegener arcs have their long axes oriented horizontally. This orientation allows the light rays to interact with the crystal in a specific manner, leading to the formation of the arcs.
Crystal size and quality: The crystals involved in creating Polish Wegener arcs need to be relatively short in length and possess near-perfect end faces. These characteristics play a vital role in determining the visibility and clarity of the arcs.
Atmospheric conditions: The overall atmospheric conditions, such as temperature, humidity, and air stability, can influence the formation of cirrus clouds and, consequently, the occurrence of Polish Wegener arcs.
Witnessing the elusive beauty of Polish Wegener arcs can be a remarkable experience for sky enthusiasts and photographers alike. However, due to their rarity and specific atmospheric requirements, capturing these arcs can be quite challenging. To increase your chances of observing and photographing Polish Wegener arcs:
Monitor weather conditions: Keep an eye on weather forecasts and look for conditions that favor the formation of cirrus clouds, such as stable atmospheric conditions and high humidity levels.
Choose an optimal location: Find a location with minimal light pollution and a clear view of the sky to maximize your chances of observing these arcs.
Patience is key: Polish Wegener arcs are relatively rare, so be prepared to spend time patiently observing the sky. Sometimes, nature's wonders require persistence and a bit of luck.
Polish Wegener arcs are a captivating manifestation of atmospheric optics, adding a touch of intrigue and beauty to the sky. These intersecting arcs, formed by column crystals in cirrus clouds, exhibit unique characteristics that differentiate them from other atmospheric halos. Witnessing Polish Wegener arcs is a testament to the delicate interplay between crystal properties, atmospheric conditions, and the observer's patience. So, keep your eyes on the sky, embrace the wonder of atmospheric optics, and you may just catch a glimpse of the elusive Polish Wegener arcs.
Rare Wegener arcs imaged by Marcin Polkowski in Poland, June '08. ©Marcin Polkowski, shown with permission.
The bright white arc is part of the parhelic circle at the 'anthelic point' opposite and at the same altitude as the sun. Two faint arcs form a cross at the anthelic point - Wegener arcs. The arcs are formed by column crystals drifting in cirrus with their long axes horizontal. Wegener arc rays pass between prism sides inclined at 60° just like the rays that form the much more common circumscribed halo and (at lower sun) the upper tangent arc. The critical difference is that while inside the crystal, Wegener arc rays are reflected once from an end face.
All the halos in the all-sky HaloSim ray tracing (with the exception of the faint 22° halo) were made by horizontal column crystals. HaloSim's virtual crystals are perfect. In the real sky, Wegener arcs need relatively short crystals with near perfect end faces. End faces are most prone to imperfections.
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"Polish Wegener arcs". Atmospheric Optics. Accessed on March 1, 2024. https://atoptics.co.uk/blog/polish-wegener-arcs/.
"Polish Wegener arcs". Atmospheric Optics, https://atoptics.co.uk/blog/polish-wegener-arcs/. Accessed 1 March, 2024
Polish Wegener arcs. Atmospheric Optics. Retrieved from https://atoptics.co.uk/blog/polish-wegener-arcs/.