Tangent arc images

Last updated on September 16, 2023

Tangent Arc Images: A Phenomenon of Atmospheric Optics

Atmospheric optics never ceases to amaze us with its captivating displays of light and color. One such intriguing phenomenon is the appearance of tangent arc images. These stunning arcs form a part of the larger family of halo phenomena that occur when sunlight interacts with ice crystals suspended in the atmosphere. In this article, we will delve into the fascinating world of tangent arc images, exploring their formation, characteristics, and the conditions required for their occurrence.

Understanding Tangent Arc Images

Tangent arc images are ethereal arcs of light that appear tangentially to a halo, such as a 22-degree halo or a circumzenithal arc. They typically manifest as gentle curves above or below the main halo, creating a mesmerizing celestial spectacle. These arcs are caused by the refraction and reflection of sunlight within hexagonal ice crystals present in cirrus clouds.

Formation of Tangent Arc Images

To comprehend how tangent arc images form, we must first understand the behavior of light within ice crystals. As sunlight passes through these hexagonal crystals, it undergoes refraction, bending at different angles depending on its wavelength. The rays then strike the internal faces of the crystal and undergo further refraction and reflection before emerging from the crystal.

In the case of tangent arcs, the sunlight enters a side face of a horizontal hexagonal prism and undergoes internal reflection, reflecting off the lower face before exiting through another side face. This process causes the formation of tangent arcs, which are tangent to the circumzenithal or 22-degree halo.

Characteristics of Tangent Arc Images

Tangent arc images possess distinct characteristics that set them apart from other atmospheric optical phenomena. Here are some key features:

1. Shape: Tangent arcs are typically smooth and gently curved, resembling segments of a circle.
2. Location: They appear tangentially to the main halo, either above or below it.
3. Size: The size of tangent arcs can vary, ranging from a few degrees to several degrees in angular extent.
4. Color: Like other halos, tangent arcs can exhibit a range of colors, including red, orange, yellow, and blue.

Conditions for Tangent Arc Image Formation

For tangent arc images to form, specific atmospheric conditions must be met. These include:

1. Presence of ice crystals: Tangent arcs require the presence of hexagonal ice crystals in the atmosphere, typically found in cirrus clouds.
2. Orientation of crystals: The ice crystals must have a specific orientation with their long axes roughly horizontal to the ground.
3. Sun's position: The sun must be at a relatively low elevation angle (around 10 to 30 degrees) for the tangent arcs to be visible.

Other Related Phenomena

Tangent arc images are just one member of the vast family of atmospheric optical phenomena. Here are some other related phenomena you might encounter:

1. Circumzenithal arc: A vibrant arc that appears at a higher angle than the sun, forming an upside-down rainbow.
2. 22-degree halo: A circular halo that appears around the sun or moon, caused by the refraction and reflection of light within ice crystals.
3. Parhelic circle: A circular band of light that encircles the sun, formed by sunlight reflecting off plate-shaped ice crystals.

Capturing and Observing Tangent Arc Images

To witness the beauty of tangent arc images, it is best to look towards the sky on days when cirrus clouds are present. These wispy, high-altitude clouds often contain the necessary ice crystals for these phenomena to occur. Keep an eye out for any halos or arcs around the sun or moon, as this could indicate the presence of tangent arcs.

If you are fortunate enough to spot tangent arc images, consider capturing their splendor through photography. Using a polarizing filter can enhance the visibility and colors of the arcs. Remember to adjust your camera settings to capture the delicate details and colors accurately.

The Magic of Tangent Arc Images

Tangent arc images remind us of the sheer beauty and complexity of the natural world. They serve as a reminder that even in the vast expanse of the atmosphere, hidden wonders await our discovery. So, the next time you gaze at the sky and witness a halo or arc, take a moment to appreciate the intricate dance of light and ice crystals that creates these mesmerizing phenomena.

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