Circumhorizon Arc, Summer's halo - OPOD

Last updated on September 16, 2023

Circumhorizon Arc: Exploring the Summer's Halo

As the summer season unfolds, the sun ascends to great heights, casting its radiant light upon the Earth's atmosphere. Amidst the high-altitude cirrus clouds, a breathtaking phenomenon known as the circumhorizon arc often graces the sky. This awe-inspiring display of colors has become a common sight during summer in various regions around the world. Let us embark on a journey to unravel the mysteries surrounding this mesmerizing optical marvel.

Captured by Stephen Gledhill in the picturesque Cascade Mountains of NW Washington State, USA, we are presented with a magnificent example of the circumhorizon arc. Stretching over 75° along the horizon, this immense and vibrant arc is a testament to the beauty that nature can bestow upon us.

Contrary to popular belief, the circumhorizon arc is not an exceedingly rare occurrence. Rather, it is a relatively common sight during summer in many parts of the world. Dispelling another misconception, this phenomenon does not solely require the sun to be positioned exactly 58° high. In fact, the sun must be at an elevation of 58° or higher for the circumhorizon arc to manifest its full splendor.

Often mistaken for a rainbow, the circumhorizon arc is actually an ice halo formed by sunlight refracting through plate-shaped ice crystals. These crystals, suspended within cirrus clouds or haze, disperse light and create a stunning display of pure spectral colors. When fully formed, the arc extends nearly 90° around the horizon, filling the fragmented clouds with vivid hues.

To witness this ethereal spectacle, one must direct their gaze beneath the high sun, where cirrus clouds or haze are present in the sky. As you observe, you may notice an inner circular halo surrounding the sun. However, if you look twice as far away from the sun, you will discover the circumhorizon arc, exhibiting its radiant colors with red hues predominantly positioned at the top.

While it may be tempting to rely on photographs to confirm the presence of the circumhorizon arc, it is best to initially assess its parallel alignment with the horizon through the naked eye. This arc, always running parallel to the horizon, distinguishes itself from a similar phenomenon known as the infralateral arc, which curves upwards at its end due to horizontal column crystals.

The inner circular halo, often mistaken for a 22° halo, can also lead to some confusion. In reality, it is more likely a circumscribed halo, exhibiting a near-circular shape when the sun is at its zenith. Occasionally, both the circumscribed and 22° halos can coexist, further enhancing the complexity of the atmospheric optics on display.

In our quest to comprehend the wonders of the natural world, let us remember that the circumhorizon arc serves as a reminder of the intricate interplay between sunlight and ice crystals suspended high above us. As summer graces us with its warmth and brilliance, take a moment to marvel at the breathtaking spectacle of the circumhorizon arc, an ephemeral masterpiece painted across the sky.

Summer's Halo ~ Circumhorizon Arc

Summer is here. The sun climbs high. Immense and colourful circumhorizon arcs are a common sight in high and icy cirrus cloud. This magnificent example was pictured by Stephen Gledhill (virtuallygrey.co.uk) in the Cascade Mountains of NW Washington State, USA. Scroll right for the full view - the arc stretched more than 75° along the horizon.

Image ©Stephen Gledhill, shown with permission

The are many myths attached to the circumhorizon arc. It is extremely rare - Untrue it is a common sight in summer in many parts of the world. It needs the sun exactly 58° high - Untrue, the sun must be 58° or higher. It is a form of rainbow - Untrue, it is an ice halo formed by sunlight refracted through plate shaped ice crystals. A journalist even coined a silly name for it that makes halo observers cringe.

Look for it beneath the sun when it is high and the sky has cirrus cloud or haze. The sun will often have an inner circular (or apparently circular) halo. Look twice as far away. It has pure spectral colours with red uppermost. When complete it extends almost 90° around the horizon. More often it is smaller, filling fragmentary clouds with vivid colour.

It is always parallel to the horizon. Check this by eye rather than later on photographs. A similar arc but one that curves up at its end is an infralateral arc from horizontal column crystals. The inner ‘circular’ halo can be misleading too. Often it is a circumscribed halo rather than a 22° halo - or sometimes both. The circumscribed halo is almost circular at high sun.

Note: this article has been automatically converted from the old site and may not appear as intended. You can find the original article here.

Reference Atmospheric Optics

If you use any of the definitions, information, or data presented on Atmospheric Optics, please copy the link or reference below to properly credit us as the reference source. Thank you!