Wegeners

Last updated on September 16, 2023

Wegener Arcs: Rare and Captivating Atmospheric Phenomena

Wegener arcs, named after the German meteorologist Alfred Wegener, are a captivating and rare atmospheric optical phenomenon. These ice halos form a cross on the parhelic circle, which is located opposite the position of the sun. While all ice halos are considered rare occurrences, Wegener arcs are even more exceptional when they appear in such brightness.

These elusive arcs gracefully skirt the zenith and intersect tangentially with both the 22° halo and the circumscribed halo. If one is fortunate enough to witness this spectacle, it is possible to observe a faint glimpse of Wegener arcs in the enhanced view towards the sun.

The formation of Wegener arcs is closely linked to the presence of column ice crystals that are drifting with their long axes nearly horizontal. These specific crystal orientations are responsible for producing these mesmerizing halos. The ray path of Wegener arcs is similar to that of the upper tangent or circumscribed arcs, with the only difference being that the ray reflects internally from a crystal end face.

Due to their unique formation requirements, Wegener arcs are exceptionally rare. The geometry must align perfectly, and the column end faces of ice crystals must possess the necessary optical perfection. These factors contribute to the infrequency of sightings of this particular atmospheric optical phenomenon.

It is important to note that Wegener arcs are not commonly observed and are considered a rare treat for those who study atmospheric optics. Their occurrence is a testament to the intricate and complex interplay between light and ice crystals in our atmosphere.

In summary, Wegener arcs are a captivating and elusive atmospheric optical phenomenon that forms a cross on the parhelic circle opposite the sun. These rare halos gracefully skirt the zenith and tangentially intersect with the 22° and circumscribed halos. The presence of column ice crystals drifting with their long axes nearly horizontal plays a crucial role in their formation. However, due to the challenging geometry and the requirement of optical perfection in the crystal end faces, Wegener arcs are rarely observed. Witnessing these breathtaking halos serves as a reminder of the awe-inspiring wonders that can occur within our atmosphere.

Wegener Arcs � Ice halos imaged by Peter Upton, Yukon, Canada June 18. ©Peter Upton, shown with permission.

Wegener arcs form a cross on the parhelic circle opposite the sun. They are rare and when seen this bright even more so.

They skirt the zenith and meet the 22� and circumscribed halos tangentially. One of them can just be seen in the enhanced sunward view below.

Column ice crystals drifting with their long axes nearly horizontal produce them.

Their ray path is almost the same as that of the upper tangent or circumscribed arcs except that the ray reflects internally from a crystal end face.

Thus their rarity because the geometry is unfavourable and column end faces do not usually have the required optical perfection.

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Reference Atmospheric Optics

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