Circumhorizontal arc (fire rainbow): how it forms, where and when to see it

A circumhorizontal arc is a rare optical phenomenon that appears as a bright, horizontal band of colour in the sky. Often called a fire rainbow, it forms when sunlight refracts through flat, hexagonal plate crystals in high cirrus clouds. The sun must sit high in the sky for it to appear.

Key takeaways

• A circumhorizontal arc is formed by light refraction through ice crystals, not by fire or rain.

• The sun needs to be high enough for the arc to appear, which is why solar altitude matters.

• These displays are most likely in parts of northern Australia, especially near the Tropic of Capricorn and farther north.

• Cirrus clouds are the key cloud type to watch for.

• It helps to tell a circumhorizontal arc apart from cloud iridescence and other halo phenomena.

Circumhorizontal arc over the Nepalese Himalaya - By Фтещт Нфтлщмнш - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=44083669

How circumhorizontal arcs form in cirrus clouds

A circumhorizontal arc forms when sunlight passes through plate-shaped ice crystals in cirrus clouds and is split into colours by refraction. The crystal shape matters because it gives the light a set path through the ice.

A circumhorizontal arc appears as a broad, horizontal band of colour running parallel to the horizon, formed by sunlight refracting through plate-shaped ice crystals in high cirrus clouds.

The Bureau of Meteorology describes cirrus clouds as high, wispy clouds made of ice crystals. That is the right setting for a circumhorizontal arc. When the crystals are flat and aligned well enough, the arc can appear as a clean band of colour running parallel to the horizon.

Two rules matter here. First, the crystals need to be plate-shaped. Second, the sunlight needs to enter at the right angle. This is where Snell's Law and Bravais' Law come in. In plain terms, the light bends as it moves through the crystal, and the crystal shape helps keep the colours ordered rather than scattered.

A true circumhorizontal arc usually shows red at the bottom of the band and blue or violet higher up. If the colours look patchy or broken up, you may be seeing another optical phenomenon instead.

The 58-degree threshold: why latitude matters

The sun has to be high in the sky for a circumhorizontal arc to form. The key cut-off is the 58-degree threshold. If the sun is lower than that, the geometry does not work.

What does the 58-degree threshold mean?

It means the sun must sit at a solar altitude high enough for light to enter the side of the ice crystal and leave in a way that spreads the colours. If the sun is too low, the effect does not appear.

That is why latitude matters so much. The higher the sun gets over your location, the better your chances. In Australia, places closer to the Tropic of Capricorn and farther north have more opportunities than southern cities.

How a circumhorizontal arc forms: sunlight enters the vertical side of a horizontal plate ice crystal and exits the flat bottom face, effectively passing through a 90° prism that splits the light into its colours. The sun must be higher than about 58° above the horizon for the geometry to work.

Determining visibility in Northern vs Southern Australia

If you live in Northern Australia, you have a better shot at seeing a circumhorizontal arc than someone in the south. The reason is simple: the sun climbs higher there for more of the year.

Circumhorizontal Arc over peak 12225 in the Sierra Mountains of California - By Jxharley - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=152622718

When are circumhorizontal arcs most likely in Australia?

They are most likely in mid-latitude summers, when the sun runs highest across the sky. Warm weather itself does not cause the arc, but summer often gives the right solar altitude and cloud setup at the same time.

In northern parts of Queensland and the Northern Territory, the required sun angle is reached more often. In southern cities such as Melbourne or Hobart, the window is much narrower and usually limited to a short period around the middle of summer.

If you are watching the sky, look for thin cirrus clouds near midday. That is the best clue. A clear patch of sky below the cloud deck also helps, because the arc stands out more cleanly against a brighter backdrop.

The article types of clouds in Australia can help you pick the right cloud type before the light display starts.

Visibility varies sharply by latitude. Cairns and other locations north of the Tropic of Capricorn easily clear the 58° sun-altitude threshold for much of the year, while Melbourne and Hobart only get a short window around the summer solstice.

How to tell a fire rainbow from other halo phenomena

People often call many colourful sky sights a fire rainbow, but the label is not always correct. The main jobs are to check the shape, the cloud type, and the way the colour sits in the sky.

What is the difference between a fire rainbow and cloud iridescence?

A circumhorizontal arc is a structured band caused by light refraction through plate-shaped ice crystals. Cloud iridescence is different. It usually shows as small, shifting patches of colour near cloud edges, often from smaller droplets or very fine ice crystals.

In short, the arc is broad and banded. Iridescence is patchy and broken. That is the quickest field test.

If the colour sits along a long, level strip high in the sky, think circumhorizontal arc. If it shimmers in small blotches near a cloud edge, think cloud iridescence. Another similar effect to differentiate is the 22-degree solar halo, which forms a ring rather than a horizontal band.

A closer look at the spectral banding inside cirrostratus. Notice how the colour sits as one clean, level strip within the cloud, rather than in small shifting patches like cloud iridescence.

How to spot one in Australia

You do not need special gear. You need the right sun angle, the right clouds, and a bit of patience.

1. Look during late morning to early afternoon.

2. Scan for thin cirrus clouds, especially high and wispy ones.

3. Check that the sun is high enough in the sky.

4. Look for a long band of colour below or near the cloud.

5. Do not stare at the sun.

If you are near the coast, the display may stand out more when a broad sheet of cirrus moves through after a settled spell. If the cloud cover thickens or the sun drops lower, the arc usually fades quickly.

For cloud identification, the cloud optical phenomena guide can help separate a circumhorizontal arc from other sky effects.