On Friday afternoon, residents in the city were treated to a surprise from the sky. A rainbow-like halo encircled the sun in a perfect circle. This triggered excitement across the city and drew people in large numbers out of homes and offices.
“It’s an atmospheric phenomenon which just happens, something like aalangatti mazhai (hailstorm),” says N. Sanjay, a keen sky watcher. The solar halo happens when sunrays refract off tiny ice crystals, like prisms, suspended in the atmosphere. “When the temperature drops in the stratosphere, the upper most layer of the atmosphere, the condensed air mass, forms ice crystals shaped like flat plates or pencils. When the sun is aligned with these crystals at a particular angle, a halo appears. It disappears as the refraction reduces.”
Also called a kaleidoscopic effect, the phenomenon depends on the tilt of the ice crystals in the air and the altitude of the sun. “When the crystals are large enough, a halo appears. The only significance is that it is visually brilliant.”
If observed closely, one could see that the ring was slightly redder in the inside and bluish on the outer edges. The area within the ring was less bright compared to other parts of the sky. “The sun’s brightness obscures the ‘Vibgyor’,” explains Prof. K. Smiles Mascarenhas who writes regularly for the ‘Science Reporter’. A halo is very area-specific and regional. This time, besides Coimbatore, people in Palani and Mettupalayam could also see it.
The halo can appear around the moon, or around any light source too, says K. Sakthivel of Coimbatore Astronomy Club.
Coimbatore witnessed a similar halo in 2005. Terming it a normal phenomenon, D. Mangalaraj, Head, Department of Nano-Science and Nano-Technology, Bharathiar University, had a different reason to attribute to the halo.
He said that given the weather condition of this region, the halo was formed when water droplets that accumulated around the sun started evaporating. “When there is differential moisture content in the atmosphere surrounding the sun, evaporation takes place and because of the proximity of the sun, the white of the water is scattered into seven colours and hence the rainbow effect,” he said.
Curiously, a parched Coimbatore had this question to ask: Does it foretell rain? “Not necessarily,” says S. Anand of the Amateur Astronomy Club. “It’s just a beautiful phenomenon to watch in the sky. Anyway, this is the rainy season and any rain could be the result of the regular monsoon or the rain clouds.”
(With inputs from Amutha Kannan)
