Hong Kong Observatory explains Circumzenithal Arc and sundogs


16th May 2024 – (Hong Kong) Hong Kong Observatory took to Instagram today to address a captivating optical illusion that has been circulating online. Contrary to popular belief, the mesmerising sight captured in photos is not a reversed rainbow but a phenomenon known as a “Circumzenithal Arc.” This optical marvel forms an arched ring around the zenith, positioned directly above the sun, with a vibrant red hue at the bottom and a striking purple shade at the top. Additionally, one of the images also showcased a vivid coloured spot called sundogs appearing on either side of the sun at approximately the same altitude.

The Circumzenithal Arc, often abbreviated as CZA, occurs when sunlight interacts with ice crystals present in thin, high-level clouds. These ice crystals act as prisms, refracting and dispersing sunlight into a beautiful display of colours. The arc’s formation is a result of the specific angle at which sunlight enters and exits the ice crystals, causing the light to bend and create a distinctive arc shape. The intense red and vibrant purple colours observed in the CZA are due to the refraction of sunlight at different angles, emphasising these specific hues.