As Kolkata residents ventured out to vote on a hot and humid Saturday morning, a long wait in the serpentine queues was enlivened by a rather curious sight. For about half an hour around noon, the sun appeared to be encircled by a rainbow, setting off murmurs in the voting lines and speculation on WhatsApp groups about whether the city had a UFO in its midst or divine intervention was at play, among other suggestions.

As it turned out, the city was witnessing a rare optical phenomenon called the 22 degree halo, which occurs when the sun’s – or moon’s – rays get deflected or refracted through the hexagonal ice crystals present in cirrus clouds.

“These kind of cirrus clouds are generally formed when water vapour freezes into ice crystals at altitudes 5-10km above the earth’s surface,” a researcher at the Birla Planetarium in Kolkata told Press Trust of India.

“It’s a very common phenomenon in the cold countries. But in our countries its occurrence is rare and cannot be predicted. The red and blue ring around the sun was seen for around 30 minutes,” he said, adding that the halo is traditionally a harbinger for a storm.

Amid warnings not to look directly at it, excited Kolkatans put up pictures of the halo on Twitter: