39,000ft circumhorizon arc - OPOD

Last updated on September 16, 2023

Exploring the 39,000ft Circumhorizon Arc

Have you ever looked up at the sky and witnessed a stunning display of colors stretching across the horizon? One such phenomenon is the 39,000ft circumhorizon arc, also known as the circumhorizontal arc or the Fire Rainbow. This captivating optical phenomenon occurs when ice crystals in cirrus clouds high above us create a halo that appears parallel to the horizon. In this article, we will delve into the intricacies of this awe-inspiring sight and uncover the science behind its formation.

The Beauty of the Circumhorizon Arc

Captured by Grant Goodge over the Eastern Mediterranean Sea, the 35,000ft circumhorizon arc serves as a testament to the breathtaking beauty of this atmospheric phenomenon. Goodge describes the experience of flying at 35,000 feet and witnessing this incredible sight that lasted for about 20 minutes. The circumhorizon arc appeared intermittently, varying in brightness but always there, adding an ethereal touch to the sky.

Unveiling the Science Behind the Phenomenon

To understand how the circumhorizon arc forms, we must first delve into the role of ice crystals in cirrus clouds. These clouds, often found at high altitudes, consist of tiny ice crystals that act as prisms when sunlight passes through them. The sunlight enters the side faces of hexagonal horizontal plate crystals and exits through their lower faces. As a result, the refraction through the resulting 90° prism produces a wide dispersion of colors, creating the vibrant display of hues that we see in the circumhorizon arc.

Optimal Conditions for Circumhorizon Arc Formation

While witnessing a circumhorizon arc is undoubtedly a rare treat, certain conditions must align for this phenomenon to occur. Here are some factors that contribute to the formation of a circumhorizon arc:

• Latitude: The circumhorizon arc is more commonly observed in locations closer to the equator, where the sun is higher in the sky.
• Time of Year: The arc is most likely to be visible during the summer months when the sun is at its highest point in the sky.
• Cloud Type: Cirrus clouds, particularly those with ice crystals, are essential for the formation of the circumhorizon arc.

Distinguishing the Circumhorizon Arc from Other Optical Phenomena

The circumhorizon arc may bear some resemblance to other atmospheric optical phenomena, but there are distinct features that set it apart. Here are a few notable differences:

• Circumzenithal Arc: While the circumhorizon arc appears parallel to the horizon, the circumzenithal arc forms a similar display, but it is located closer to the zenith (the point directly overhead).
• Halos: Halos, such as the 22° halo, are circular rings around the sun or moon caused by the refraction and reflection of light in ice crystals. Unlike halos, which encircle the celestial body, the circumhorizon arc extends horizontally across the sky.

Exploring Similar Atmospheric Optical Phenomena

The world of atmospheric optics is filled with mesmerizing displays of light and color. Here are a few other phenomena that you may encounter:

• Sun Dogs: Also known as parhelia, sun dogs are bright spots that appear on either side of the sun. They are caused by the refraction of sunlight through ice crystals in cirrus clouds.
• Rainbows: Rainbows form when sunlight refracts, reflects, and disperses through raindrops, creating a spectrum of colors. They often appear after rain showers when the sun emerges from behind the clouds.
• Glories: Glories are circular rings of light that surround an observer's shadow when it falls on a cloud or mist. They occur due to the diffraction of light around tiny water droplets.

The Importance of Atmospheric Optics

Studying atmospheric optics not only allows us to appreciate the stunning beauty of natural phenomena but also provides valuable insights into the behavior of light in our atmosphere. By understanding how light interacts with various particles and atmospheric conditions, scientists can gain a deeper understanding of our planet's climate, weather patterns, and even the composition of distant celestial bodies.

Embrace the Wonder of the Skies

The 39,000ft circumhorizon arc is a testament to the incredible wonders that await us in the skies above. From the vibrant hues of its colors to the delicate interplay of light and ice crystals, this optical phenomenon offers a glimpse into the mesmerizing complexity of our atmosphere. So, next time you find yourself gazing up at the heavens, keep an eye out for this elusive spectacle and let it inspire you to explore the fascinating world of atmospheric optics.

35,000ft Circumhorizon arc ~ Imaged by Grant Goodge over the Eastern Mediterranean Sea. "It was incredible to be flying at 550 mph at 35,000' and see this feature that lasted about 20 minutes, sometimes brighter than other times, but always there."

©Grant Goodge, shown with permission

Ice crystals in cirrus cloud even higher than the aircraft formed the halo. Solar rays entered the side faces of hexagonal horizontal plate crystals to exit through their lower faces. The refraction through the resulting 90° prism produced the wide dispersion of colours.

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