Kolkata: The action on Saturday was supposed to be on the ground, at the poll booths. But it was the sky that grabbed all the attention around noon when a halo wreathed the sun, mesmerizing voters and party workers alike. The halo kept people riveted for several minutes and injected a doze of excitement into the otherwise sedate day when heavy security quelled fear of violence.
News about the large ring around the sun spread like wildfire on social media, prompting everyone to gaze skyward. Even polling personnel took a break and stepped out of the booths to look upwards. The chatter on social media revolved around the appearance of the phenomenon on the very day star candidates, led by chief minister Mamata Banerjee, went to poll.
In reality, it was an atmospheric condition that led to the solar halo. As National Council of Science Museums physics curator Kanchan Choudhuri explained, the optical phenomenon is called the 22° halo and forms around the sun, and occasionally the moon, due to refraction of sunlight as it passes through hexagonal ice crystals in the atmosphere.
"Around this time of the year, when temperature and humidity are high, thin cirrus clouds are formed. When they drift up to an altitude of around 20,000 feet above the earth's surface, the water vapour in the clouds freezes to form of tiny crystals of 20.5 micro-meter diameter. These crystals act as prisms when sunlight passes through them, refracting the light twice - once at the point of entry and again at the exit. The deviation is 22° and hence, the name. At times, the inner edge of the circle takes a reddish tinge and the outer a bluish hue," he said, adding that usually, the 22° halo is followed by rain.
GC Debnath, the deputy director general of the Regional Meteorological Centre, Kolkata, said rain was indeed expected in the next couple of days.
"The heatwave will abate. From Tuesday, half of south Bengal will receive rainfall. We expect the conditions to favour Nor'westers," he said.
The city and rest of the state has witnessed only two Nor'westers in March, but none in April, mirroring what had happened in 2014. "Usually, there are three to four Nor'westers in April, but there have been none this year. We expect that situation to change. There should be about six to seven of them in May," Debnath added.