Kolkata: Kolkatans witnessed a red and blue ring around the sun, popularly known as ‘22 degree circular halo’was sighted on Wednesday.
The phenomenon popularly known as the 22 degree circular halo of the sun or occasionally the Moon (also called a moon ring or winter halo), occurs when the sun’s or moon’s rays get deflected/ refracted through the hexagonal ice crystals present in cirrus clouds, a senior researcher with the MP Birla Planetarium told.
“These kind of cirrus clouds are generally formed when water vapour freezes into ice crystals at altitudes five to ten kilometres above the earth’s surface,” the researcher said.
What was this ring of light around the sun?
When the overhead sun’s rays pass through hexagonal ice crystals suspended in air (or in high cirrus clouds during the summer in India), they get twice refracted within these crystals, and emerge at an angle of about 22 degree. The different colours of light are deviated slightly, so the ray emerges as a rainbow-coloured ring. Together, these rays appear as a coloured ring around the sun. It is called the solar halo or the 22 degree halo.
Does a halo forecast rain or storm?
Icy cirrus clouds never produce rain. But they often indicate moisture high up in the air which might trigger weather change within 48-72 hours.
How is it different from a rainbow?
There are two major differences. First, rainbows are caused by water droplets suspended in the air after a shower. The solar halo is caused by ice crystals. Second, the rainbow appears to us on the opposite side of the sky from the sun. This is because sunlight gets reflected within the rainwater droplets before reaching us, so the sun needs to be behind us when we see a rainbow.
The solar halo happens because of sunlight passing through ice crystals, so the sun needs to be high in the sky behind the cloud of ice crystals. The halo was seen in Calcutta around midday when the sun was overhead.
Like a halo, a rainbow is also circular, but because it happens when the sun is low in the sky, and the rainbow is consequently close to the horizon, we see it as semicircular.
A halo is more common than rainbows in cold places. Even in tropical regions, it is certainly not a rarity.
Similarly, another 22 degree halo was last observed in May 2016 and April 2013 in the suburbs of the city, the meteorologist said, adding, the phenomenon takes place before an impending storm.