Tangent arc - Alaska

Last updated on September 16, 2023

Tangent Arc - A Rare Atmospheric Phenomenon in Alaska

The atmospheric optics phenomenon known as the tangent arc is a captivating sight that occasionally graces the skies above Alaska. While this phenomenon may not be as well-known as other atmospheric events, such as rainbows or halos, it is a stunning display that has been captured by astute observers. In this article, we will delve into the details of the tangent arc, exploring its characteristics, formation, and rarity, with a particular focus on its occurrence in the Alaskan sky.

What is a Tangent Arc?

A tangent arc is an optical phenomenon that occurs when sunlight interacts with ice crystals suspended in the atmosphere. It manifests as a luminous arc that appears to touch or be tangent to another optical feature, such as a halo or sundog. The tangent arc is characterized by its position directly above the sun and its distinctive reddish hue on the side facing the sun. Although it may appear tilted in certain photographs due to perspective, the tangent arc is actually parallel to the horizon.

Formation of Tangent Arcs

Tangent arcs are formed by hexagonal ice crystals with specific orientations. These elongated crystals act as prisms, refracting and reflecting sunlight in a way that produces the tangent arc. When light enters the upper faces of these crystals, it undergoes refraction, bending at an angle determined by the crystal's shape. The refracted light then strikes the crystal's side face and undergoes internal reflection before exiting through another upper face. This intricate path results in the creation of the tangent arc.

The Role of Weather Conditions

The occurrence of tangent arcs is closely linked to specific weather conditions. To observe this phenomenon, one needs a combination of low temperatures and high-altitude ice clouds containing the necessary hexagonal ice crystals. Alaska's climate provides an ideal environment for tangent arcs due to its cold winters and frequent presence of ice clouds. The unique geography of Alaska, with its towering mountains and vast expanses of wilderness, further contributes to the occurrence of this rare optical display.

The Tangent Arc in Alaskan Skies

Alaska's remote and sparsely populated regions offer ample opportunities for keen observers to witness the elusive tangent arc. One notable location where these phenomena have been observed is Fairbanks. On the morning of March 6th, 2002, Brian Hartmann captured a mesmerizing display of atmospheric optics in Fairbanks. The photograph he took showcases a complex arrangement of halos, including the tangent arc, a sundog, a parhelic circle, and a supralateral arc. This captivating image provides a glimpse into the wonders that can be witnessed in the Alaskan sky.

Rarity and Significance

Tangent arcs are considered relatively rare compared to more commonly observed atmospheric phenomena. Their formation requires precise conditions and specific crystal orientations, making them a captivating sight to behold. The occurrence of a tangent arc adds to the allure of Alaska's already remarkable natural beauty. Witnessing such a phenomenon serves as a reminder of the intricate interactions between light, ice, and the atmosphere, highlighting the captivating wonders that can occur in our skies.

Capturing the Tangent Arc

To capture the beauty of a tangent arc, one must be prepared with the right equipment and timing. Wide-angle lenses are often employed to capture the full extent of these optical displays, as they can encompass a larger portion of the sky. Patience and persistence are key, as witnessing a tangent arc requires favorable weather conditions and a bit of luck. However, the reward of capturing such a rare atmospheric phenomenon is undoubtedly worth the effort.

In conclusion, the tangent arc is a remarkable atmospheric optics phenomenon that occasionally graces the skies of Alaska. Its formation relies on precise weather conditions and specific ice crystal orientations. While relatively rare, the occurrence of a tangent arc is a sight to behold, adding to the allure of Alaska's already stunning natural landscapes. As we continue to explore and appreciate the wonders of our atmosphere, let us remain in awe of the captivating displays that can unfold above us.

Halos at Fairbanks, Alaska. The sun was 11º high on the morning of 6th March '02 when Brian Hartmann captured this complex display. It is dominated by a luminous upper tangent arc characteristically edged red sunwards. The arc touches the 22º halo directly above the sun and only appears tilted here because of the wide angle lens perspective. A sundog flanks the sun and passing horizontally through it and the sun is a white parhelic circle. A broad sun pillar extends up from the sun. The coloured arc at top right? A rare supralateral arc created by the same long hexagonal prism crystals which produced the upper tangent arc. A rather more rare 46° halo was also faintly visible. Image ©2002 Brian Hartmann, reproduced with permission.

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