BENGALURU: Bengaluru residents were treated to a colourful sky at dusk on Monday and pre-dawn on Tuesday with shades of pink, green and yellow lighting up the horizon. While some thought it was the rare sight of Comet C/2023 A3 passing by the city, others compared it to a mini Aurora Borealis, or Northern Lights. But scientists dismissed these.
Dr Anand, Senior Scientific Officer at Jawaharlal Nehru Planetarium (JNP), told TNIE that the colours were caused by an environmental phenomenon, specifically a “local cloud phenomenon” called iridescent cloud. This happens when ice crystals in the cloud scatter light, creating a rainbow-like effect, similar to how light splits into colours when it passes through a prism.
Iridescent clouds happen when the sun is behind or close to thin clouds that have tiny water droplets or ice crystals in them. These clouds bend the sunlight, causing colourful patterns, like a rainbow, Dr Anand explained, adding that one usually sees this around sunrise or sunset when the sun is lower in the sky because the light passes through the clouds at just the precise angle.
Many people took to social media mentioning that Hyderabad might witness the same phenomenon after Bengaluru on October 2. However, Dr Anand clarified that this cannot be predicted as the iridescent cloud phenomenon is local and depends on specific atmospheric conditions like temperature and pressure. He explained that it varies depending on these factors, and even within Bengaluru, people across the city may not have witnessed this.
Although sometimes mistaken for a mini Aurora Borealis (or the Northern Lights), iridescent clouds tend to appear ‘more vibrant,’ especially around sunrise or sunset. In contrast, a mini Aurora Borealis, which occurs due to particles from the sun interacting with the Earth’s atmosphere, is generally ‘dimmer’ and often seen in darker skies, he said.
Iridescent clouds usually form in thin, high-altitude clouds like cirrus or altocumulus, which are found at altitudes between 3,000 to 7,500 metres (10,000 to 25,000 feet) above the ground. Cirrus clouds, in particular, are thin and wispy, appearing like delicate strands across the sky.
These clouds can cover a small patch of the sky, often surrounding the sun or the moon, and their colourful effect is limited to the specific area where sunlight or moonlight scatters through the ice crystals or water droplets at the right angle. While cirrus clouds generally occupy only a small portion of the sky, their brightness and vibrant colours can create appealing visual displays.
Moreover, iridescent clouds may not be visible across the entire city as they require thin, high-altitude clouds with ice crystals, which may only be present in certain parts of the sky at a given time.