Not Fire, Not a Rainbow
The captivating phenomenon that has social media buzzing has a misleading name. A 'fire rainbow' is not related to fire, and it isn't a true rainbow either. Its scientific name is a circumhorizontal arc. Unlike a traditional rainbow, which is formed by
the refraction and reflection of light in raindrops, a circumhorizontal arc is created by light passing through ice crystals in high-altitude clouds. It appears as a vibrant band of colours parallel to the horizon, often looking like a colourful smear in the sky rather than the distinct arch of a rainbow. The colours are pure and well-separated, with red on top, just like a prism's spectrum.
The Recipe for a Sky-High Illusion
For this rare spectacle to occur, a precise set of ingredients must come together perfectly. The first requirement is the presence of cirrus clouds—thin, wispy clouds that form at altitudes above 6,000 metres, where temperatures are low enough for them to be composed of millions of tiny, hexagonal ice crystals. Crucially, these plate-shaped ice crystals must be oriented horizontally, like tiny floating plates, for the effect to work. Finally, the light source—the Sun—must be at a very specific height in the sky. When all these conditions align, the ice crystals act as a vast, dispersed prism, bending sunlight and splitting it into its constituent colours.
The Magic Number: A Sun Angle Above 58 Degrees
The most critical factor, and the one highlighted by NASA's recent image, is the sun's angle. For a circumhorizontal arc to be visible, the sun must be very high in the sky—at an elevation of at least 58 degrees above the horizon. Sunlight enters the vertical side face of the hexagonal ice crystals and exits through the horizontal bottom face. This 90-degree refraction splits the light, creating the stunning colours. If the sun is lower than 58 degrees, the light cannot pass through the crystals in the required manner, and the arc will not form. The phenomenon is most intense when the sun reaches an elevation of about 68 degrees.
Can You See One In India?
The 58-degree requirement means that these 'fire rainbows' are a phenomenon of the mid-latitudes. They are impossible to see at locations north of 55°N or south of 55°S, because the sun never gets high enough in the sky. Fortunately, all of India lies well within the latitudes where circumhorizontal arcs can be seen. For observers in India, the best chance to spot one is during the summer months, around midday, when the sun is at its highest point in the sky. Your chances are better the further south you are, but the phenomenon is theoretically visible across the country, provided the right kind of clouds are present on a day with a high sun.
How to Spot This Sky-High Wonder
While considered rare, you can increase your chances of spotting a circumhorizontal arc. Look for thin, wispy cirrus clouds, which can sometimes appear patchy or fragmented. The best time to look is on clear summer days, in the hours surrounding noon, when the sun is highest. You'll need to look below the sun, as the arc forms parallel to the horizon. It’s important not to confuse them with another phenomenon called cloud iridescence, which can also create colourful clouds but appears much closer to the sun and is caused by a different process called diffraction. A circumhorizontal arc will always be located far below the sun, presenting a broad, band-like spectrum of colour.
