Summer Arc

Last updated on September 16, 2023

The Summer Arc: A Spectacular Atmospheric Phenomenon

The summer arc, also known as the circumhorizon arc, is a mesmerizing atmospheric optical phenomenon that occurs during the summer months. It is a halo of light that forms parallel to the horizon beneath the sun, creating a stunning display in the sky. This remarkable sight is caused by sunlight passing through ice crystals in the atmosphere, resulting in a vibrant and colorful arc.

The Conditions for the Summer Arc

To witness the summer arc, the sun must be at least 58° high in the sky. This requirement is due to the specific angle at which the sunlight interacts with the ice crystals. When the sun reaches this elevation, its rays pass into the side faces of plate-like ice crystals present in the atmosphere. These crystals act as prisms, refracting and dispersing the sunlight into its constituent colors. As the light exits through the larger lower face of the crystals, it forms the magnificent summer arc.

The Appearance of the Summer Arc

The summer arc is a massive halo that stretches across the sky beneath the sun. Its shape resembles an upside-down rainbow, with vibrant colors ranging from red on the top to violet on the bottom. The arc appears parallel to the horizon, creating a breathtaking visual effect. It is a rare and elusive phenomenon, only occurring under specific conditions during the summer months.

How Ice Crystals Create the Summer Arc

Ice crystals in the atmosphere play a crucial role in the formation of the summer arc. These crystals are typically plate-like in shape, allowing sunlight to enter through their side faces. As the light passes through the crystal, it undergoes refraction and dispersion, separating into its constituent colors. This dispersion creates a beautiful band of colors that make up the summer arc. The size and orientation of the ice crystals determine the intensity and appearance of the phenomenon.

Spectacular Displays and Photography Opportunities

When conditions are favorable, the summer arc can create breathtaking displays in the sky. Its vibrant colors and massive size make it a sight to behold. The arc often appears as a vivid streak of colors against the blue sky, adding a touch of magic to the summer atmosphere. Photographers eagerly capture this phenomenon, aiming to preserve its beauty for others to admire. The interplay of light, ice crystals, and the atmosphere makes each summer arc unique and awe-inspiring.

The Rarity of the Summer Arc

While the summer arc is a remarkable sight, it is relatively rare to witness. Its occurrence depends on specific conditions, including the sun's elevation and the presence of plate-like ice crystals in the atmosphere. These factors must align perfectly for the summer arc to manifest. Therefore, those lucky enough to witness this phenomenon should consider themselves fortunate to have experienced such a rare and captivating display.

Similar Atmospheric Optics Phenomena

The summer arc belongs to a family of atmospheric optical phenomena known as halos. These halos occur when sunlight interacts with ice crystals or water droplets in the atmosphere. Some other notable halos include:

• Sundogs: Bright spots of light that appear on either side of the sun, often accompanied by a halo.
• Sun pillars: Vertical columns of light that extend above or below the sun, caused by the reflection of sunlight off ice crystals.
• Circumzenithal arc: An upside-down rainbow that appears high in the sky, characterized by its vibrant colors.

Each of these phenomena adds to the wonder and beauty of our natural world, captivating observers with their ethereal appearances.

The Science Behind Atmospheric Optics

Understanding atmospheric optics involves studying the interaction of light with particles in the atmosphere, such as ice crystals or water droplets. The shape, size, and orientation of these particles determine how light is refracted, dispersed, and reflected, leading to the formation of various optical phenomena. Scientists and researchers use advanced instruments and models to study these interactions and gain insights into the complex behavior of light in the atmosphere.

Appreciating Nature's Wonders

The summer arc serves as a reminder of the incredible beauty that can be found in our natural world. This phenomenon, with its vibrant colors and majestic presence, invites us to pause and appreciate the wonders that surround us. It is a testament to the harmonious interplay between light, ice crystals, and the atmosphere. So, the next time you find yourself gazing at the summer sky, keep an eye out for this awe-inspiring display, as you may be fortunate enough to witness the elusive and captivating summer arc.

Circumhorizon Arc

Imaged by Sue Wild-Kim during the great Spokane, Washington display June '06. ©Sue Wild-Kim, shown with permission.

This ice halo is a herald of summer because the sun has to be at least 58° high. It is huge and lies beneath the sun parallel to the horizon. Sunlight passing into the side faces of plate crystals and leaving through the large lower face forms it.

Circumhorizon arc ice crystals

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