Not Fire, Not a Rainbow
First, let's clear up the name. A 'fire rainbow' has nothing to do with fire, and it isn't a rainbow in the traditional sense, which is formed by raindrops. The captivating phenomenon is known to scientists as a circumhorizontal arc. It's a type of ice
halo, an optical marvel created by the interaction of light with ice crystals suspended high in the atmosphere. The term 'fire rainbow' likely comes from its vibrant, flame-like appearance, often seen in wispy, patchy clouds that look like they're ablaze with colour. Unlike a true rainbow, which forms an arc opposite the sun, a circumhorizontal arc is a broad, colourful band that runs parallel to the horizon.
The Sun's Crucial Position
One of the reasons these arcs are so rare is the first key ingredient: the sun's position. For a circumhorizontal arc to form, the sun must be extremely high in the sky—at an elevation of at least 58 degrees above the horizon. This condition is most often met during the summer months in the mid-latitudes. In locations farther north or south (above 55 degrees latitude), the sun never gets high enough for these arcs to be visible. For example, a city like Los Angeles has hundreds of hours per year where the sun is high enough, while London has far fewer, making sightings much less common there. This strict solar requirement means the time of day and time of year have to be just right.
A Symphony of Ice Crystals
The second essential ingredient is the presence of the right kind of clouds. Fire rainbows form in high-altitude cirrus clouds, which are thin, wispy clouds that float more than 6,000 meters above the ground where temperatures are freezing. These clouds are composed of tiny ice crystals. But not just any ice crystals will do. For a circumhorizontal arc, the crystals must be flat, hexagonal plates. Crucially, as these crystals drift down through the air, they must align themselves horizontally, like a collection of tiny floating dinner plates. If the crystals are randomly oriented, other optical phenomena like a 22-degree halo might form, but the distinct, broad spectrum of a fire rainbow will not appear.
How Light Performs the Trick
With the sun high in the sky and a layer of perfectly aligned, plate-shaped ice crystals, the stage is set. Sunlight strikes the crystals, entering through their vertical side faces. The light then refracts, or bends, as it passes through the ice and exits through the horizontal bottom face. This 90-degree turn acts like a prism, splitting the white sunlight into its component colours and projecting a vibrant spectrum. Red appears at the top of the arc, with violet at the bottom. Because the light is refracted through millions of tiny, similarly-oriented crystals across a wide cloud, the effect is a huge, colourful band painted across the sky.
NASA's Image Sharpens the Focus
The headline's 'new picture' refers to NASA's Astronomy Picture of the Day (APOD) for August 2, 2026, which featured a spectacular circumhorizontal arc photographed over West Virginia. While the physics of these arcs are well-understood, such clear images and the accompanying explanations from NASA play a crucial role in shaping public interpretation. They provide a perfect, real-world example of this delicate interplay of light and ice. For scientists and atmospheric optics enthusiasts, these high-quality captures serve as valuable data, confirming the precise conditions needed for such a pure display of colour. It’s not that the science has changed, but that each new, stunning photograph helps refine and reinforce our understanding of this beautiful phenomenon for a global audience.
