Rare for some, UK circumhorizon arc

Last updated on September 16, 2023

Rare for Some, UK Circumhorizon Arc: A Spectacular Atmospheric Phenomenon

Have you ever witnessed a breathtaking display of colors in the sky that wasn't a rainbow? If you're lucky, you may have caught a glimpse of a circumhorizon arc, an elusive and rare atmospheric phenomenon. Recently, in Hampshire, Southern England, Gary Bates captured a stunning image of a circumhorizon arc on July 17th. This captivating event is exceptionally rare at the latitudes of Northern Europe, making it a truly remarkable sight to behold.

A circumhorizon arc occurs when sunlight passes through hexagonal plate crystals suspended in high and cold cirrus clouds. These crystals act as prisms, bending and refracting the sunlight to create a vivid display of colors. However, for a circumhorizon arc to form, specific conditions must align:

1. The sun must be higher than 58° in the sky.
2. The cirrus clouds must contain hexagonal plate crystals.
3. The crystals must be oriented in a specific way to refract the sunlight effectively.

In the UK, these conditions are typically met for only a few weeks around the summer solstice. During this time, the sun's position allows for the necessary angle of sunlight to create the circumhorizon arc. However, even with favorable solar elevation, the presence of cumulus clouds often obscures the view of the southern horizon, making sightings even more uncommon.

In Gary Bates' photograph, the sun was positioned just 59° above the horizon, providing an ideal scenario for the formation of the circumhorizon arc. The image was captured using a 21.1 Megapixel Canon 5D Mk II camera, with minimal post-processing applied. By adjusting the contrast slightly, Bates aimed to enhance the dynamic range of the image without altering its natural beauty.

The rarity of this atmospheric phenomenon adds to its allure. While many people are familiar with rainbows and sun dogs, circumhorizon arcs remain a lesser-known spectacle. However, educating ourselves and others about these natural wonders can foster a deeper appreciation for the intricacies of our atmosphere.

To fully comprehend the rarity and beauty of circumhorizon arcs, it's crucial to understand the science behind their formation. The hexagonal plate crystals in the cirrus clouds act as prisms, refracting sunlight and separating it into its constituent colors. This dispersion creates a stunning display of vibrant hues stretching horizontally along the horizon.

Some key points to consider about circumhorizon arcs:

• Circumhorizon arcs are only visible when the sun is at an elevation higher than 58°.
• These arcs are formed by sunlight passing through hexagonal plate crystals in high and cold cirrus clouds.
• The orientation of the crystals determines the effectiveness of refracting sunlight, resulting in the arc's distinct shape.
• Due to their reliance on specific atmospheric conditions, circumhorizon arcs are rare in Northern Europe.
• The summer solstice offers a brief window of opportunity for these arcs to occur in the UK.
• Cumulus clouds often obstruct the southern horizon, further limiting sightings of circumhorizon arcs.

In conclusion, witnessing a circumhorizon arc is a remarkable experience that few are fortunate enough to encounter. The vibrant colors and ethereal nature of this atmospheric phenomenon leave observers in awe of the natural world's wonders. Understanding the science behind circumhorizon arcs can deepen our appreciation for these fleeting displays and inspire us to keep our eyes to the sky in search of their elusive beauty.

Very rare for some - A circumhorizon arc over Hampshire, Southern England July 17th imaged by Gary Bates. ©Gary Bates, shown with permission.

"I was watching a hawk flying display at Weyhill Hawk Conservancy when my daughter excitedly came running up to me and told me there was a sun dog in the sky! (although she's only 9 she knows that colours in the sky are not always rainbows and I've told her about sun dogs before!) - Further investigation on the internet revealed it to be a circumhorizon arc, which Richard at dewbow.co.uk confirmed. Richard suggested I send you a copy also as apparently they are quite rare at our latitude.

The image was recorded in RAW format on a 21.1 Megapixel, Canon 5D Mk II. Because the dynamic range was quite compressed and the picture was somewhat more washed out, I have applied "Auto Contrast" to the image to expand the dynamic range. No other processing has occurred on the picture."

Rare indeed at the latitudes of Northern Europe. The sun must be higher than 58° to form this colourful halo from hexagonal plate crystals drifting in high and cold cirrus. That only happens for a few weeks around the summer solstice in the UK and during that time the southern horizon is often masked by too many cumulus clouds.

Here the sun was just 59° high!

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