Over the weekend, residents in parts of Victoria witnessed a rare atmospheric display known as iridescent clouds, producing shimmering, rainbow-colored effects across the sky. Photographs of the phenomenon were captured in Melbourne suburbs including Mount Waverley and Caroline Springs.

Iridescent clouds typically form within cirrus clouds, which are composed of ice crystals and located in the upper levels of the troposphere, where temperatures are extremely cold. According to Sarah Scully, a senior meteorologist at the Bureau of Meteorology, this optical phenomenon arises when sunlight is diffracted—bent as it passes around ice crystals of similar size—resulting in a rainbow-like spread of colors appearing as rings or waves.

While iridescent clouds are not uncommon in Australia, they often go unnoticed due to their transient nature. “People don’t often notice them, as they’re really fleeting,” Scully explained. The phenomenon is more visible when smaller clouds allow multiple rays of sunlight to interact with individual ice crystals, enhancing the diffraction effect.

Iridescent clouds differ from traditional rainbows in both formation and appearance. Rainbows result from the combined processes of refraction and reflection within water droplets, producing a consistent order of colorful bands. In contrast, iridescent clouds owe their hues solely to diffraction, which yields less regular patterns.

The phenomenon is also sometimes confused with sun dogs, another atmospheric optical effect associated with ice crystals. However, sun dogs are distinct in both formation and location, appearing as bright spots or pillars of light adjacent to or on either side of the sun, typically at lower altitudes near the horizon. Scully noted that sun dogs require water crystals at much lower atmospheric levels than those involved in iridescent clouds.

This recent display offered observers a brief but vivid glimpse of a complex meteorological process, showcasing the delicate interplay of light and atmospheric conditions over Victoria.