On the morning of Monday, 24th of May, the residents of the 'Garden City of India' witnessed an exceptional phenomenon - a rainbow-coloured ring around the sun. The 'rainbow-ring' was first spotted at about 10:50 AM, and this lasted for about an hour in the bright sky of the capital city. Bengalureans, who are pretty famous for flaunting their moderate and cool climate, eagerly await the onset of the summer monsoon. This spectacular sight brings the hope of changing weather. Pictures went viral on all social media platforms, and scores of people took to Twitter celebrating the beautiful phenomenon called the Sun Halo.
Photo Credits: Mr Selvakumar Deaver, Ms Meghna Dutta & Ms Prerana Sudarshan
A Sun Halo is an optical phenomenon that occurs when the sun's rays (white light) gets refracted and deflected to create an impression of a rainbow around the sun.
To understand how the halo was formed, consider the simple concept of refraction that we learnt in school. When white light is incident on a hexagonal prism, as shown in the figure, the light rays get refracted twice at the 2 surfaces, and split into the colours of the rainbow. The resultant angle of deflection is approximately 22° (average value).
When the moisture in the atmosphere crystalizes into hexagonal ice crystals, particularly on the upper surfaces of the cirrus or cirrostratus clouds, they deflect the incident light rays and act as perfect tools for the occurrence of the optical phenomenon that Bengaluru witnessed - an ideal 22-degrees halo or 22° halo.
The clouds that form these hexagonal ice crystals often arrive before the onset of a storm, and hence it is a popular belief that the appearance of this halo is a sign of changing weather. From our past experiences, we realize that the event in region-specific given that it necessary for the sun to be aligned at a particular angle with respect to the ice crystals. While many argue that this is a rare occurrence, the phenomenon is quite common. It occurs several times around the world in any given year.
Another similar yet rarer phenomenon known as Winter halo or Moon halo occurs when the moonlight is refracted and dispersed similar to the sunlight in this sun halo.
NOTE: The author of this article does not claim any ownership of the photographs attached. They have been posted with permission.