When a patch of cloud high in the atmosphere bursts into a brilliant, multicolored band, it often earns the evocative name of a “fire rainbow.” This stunning display is an optical illusion that occurs under a very specific set of atmospheric conditions, making it an infrequent sight. The vibrant colors and seemingly fiery appearance often lead to confusion about its actual origin. Understanding the true nature of this spectacle requires looking past the common name to the underlying science of light and ice in the upper atmosphere.
Defining the Circumhorizontal Arc
The proper scientific term for this atmospheric phenomenon is a circumhorizontal arc (CHA). It is not a true rainbow; unlike a standard rainbow formed by sunlight refracting through water droplets, the CHA is a type of ice halo. It forms when light passes through ice crystals. The arc appears as a broad, bright band of parallel colors that runs horizontally, often looking like a segment of a rainbow painted onto a wispy cloud. The common name is misleading because it has no connection to fire and differs fundamentally from a rainbow. The color separation is typically purer than a rainbow’s, appearing with red at the top and violet at the bottom.
Necessary Ingredients for Formation
The formation of a circumhorizontal arc requires a precise alignment of three physical factors in the atmosphere. High-altitude cirrus or cirrostratus clouds must be present to contain the necessary ice crystals. These ice crystals must be shaped like flat, hexagonal plates, which form in the cold temperatures of the upper troposphere. This specific geometry is foundational to the arc’s vibrant color separation.
Second, the sun must be high in the sky, positioned at an elevation of at least 58 degrees above the horizon. This high angle is necessary because light must enter the crystal through a vertical side face and exit through the near-horizontal bottom face. This specific 90-degree inclination within the crystal causes the light to refract and split into its constituent colors, acting much like a glass prism.
Third, the plate-shaped ice crystals must be oriented with their large, flat faces lying almost perfectly parallel to the ground. This horizontal alignment allows the light rays to follow the required path through the crystal structure. If the crystals were tumbling randomly, the light would be scattered in many directions, resulting in a general haze rather than a brilliant, focused arc.
Optimal Viewing Conditions and Rarity
The strict requirement for the sun to be at least 58 degrees above the horizon makes the circumhorizontal arc a geographically and seasonally limited event. This high solar elevation is impossible to achieve at latitudes greater than 55 degrees North or South, making the phenomenon invisible in regions such as northern Europe. The arc is most commonly observed near midday during the summer months in mid-latitude regions. For example, the sun is high enough for observation for only a brief period in London, while locations closer to the equator have visibility for many more hours across the year.
Successful viewing also depends on the presence of the correct type of cloud, specifically thin, high cirrus clouds containing the necessary plate-shaped ice crystals. Since the arc is often massive and runs parallel to the horizon, it is frequently seen in fragments rather than as a complete band. Although the arc is common in some parts of the United States during the summer, its occurrence remains a rare and transient spectacle globally.