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Sun-less city gets the shivers

5.30am: 15.4.

8.30am: 14.4.

11.30am: 15.4.

2.30pm: 18.4.

5.30pm: 16.8.

Calcutta went brrrrrr on Wednesday as pre-noon felt as cold as pre-dawn and the peak afternoon Celsius slid to eight degrees below normal.

With the maximum temperature failing to touch the 20 mark — the highest being 19.8 degrees Celsius, six notches below normal — the city shivered through a fog cover and a chilly breeze.

The weather oscillated between foggy-cloudy and mildly sunny throughout Wednesday with a persistent Northwesterly wind, sometimes blowing at 15kmph.

The city experienced what meteorologists described as “moderate to shallow fog” that extended up to 3km in height and did not allow the sun to make even a guest appearance before 1pm.

This kind of fog is called advection, which means horizontal movement. It is different from the normal fog in that it is taller and spread over a larger area and keeps the sunlight out for a longer period after daybreak, said a weather scientist.

“The cold that was felt throughout the day was because sunlight could not reach the ground. So the city could not receive solar radiation as the day progressed. Coupled with the strong winds, the feeling of cold was accentuated,” said G.K. Das, a meteorologist at the Regional Met Office at Calcutta.

The Met office said the weather would be “similar” on Thursday but start clearing from Friday.

Those who left home in the morning were caught off guard by how cold it felt. “When I started for office at the usual time of 8.30am, it was foggy and windy. I wished I had carried a muffler,” said Aparajita Basu, 26, a techie at Sector V.

It was no different for those stepping out at noon. Mandira Chatterjee, a private sector employee, heaved a sigh of relief in the “warmth and comfort” of a bus after a five-minute teeth-chattering wait at the Ruby roundabout on the EM Bypass.

The temperatures recorded throughout the day defied the standard pattern. The temperature at 2.30pm, usually the hottest time of day, was 18.4 degrees Celsius, eight degrees below the normal maximum. At 8.30am, the temperature was lower than it was at 5.30am and at 11.30am it was the same as at 5.30am but the wind was stronger and so the effect chillier. (See chart below)

“I was convinced that the city was in the grip of a cold wave,” said Subroto Mookherjee, a senior advocate of Calcutta High Court. “We probably suffered more as the high court building is so close to the river.”

Technically, Calcutta missed making the cold-wave cut. A cold wave is declared only when the minimum temperature drops five degrees or more below normal in winter. But Wednesday’s minimum was 13.4 degrees Celsius, just a notch below normal.

There is another meteorological categorisation called cold day but Calcutta could not make that grade either. For that, the temperature has to stay at 16 degrees Celsius or less throughout the day. On Wednesday, the temperature crossed 16 degrees after 1pm when the sun came out.

Weather scientists said a heavy, moist mass of air that had been formed in north India, had headed east along the Ganga. As it travelled it picked up more moisture and then turned to fog early on Wednesday, creating to a tall column condensed air.

When the winds from the north hit this moist mass it was dragged closed to the ground and blew at high speeds, adding to the chill.

dawn celsius same as noon celsius!

WHY DID YOU FEEL SO COLD?

ADVECTION FOG

Calcutta experienced advection fog on Wednesday. Advection means horizontal movement. It is different from the normal fog in that it is taller and spread over a larger area and keeps the sunlight out for a longer period after daybreak.

A moist mass of air originating in north India reached Bengal early on Wednesday. It travelled unhindered along the Ganges valley, gathering more moisture. It was steered by the north-westerly wind. When the mercury was at its lowest just before dawn, the water vapour in this moist air mass condensed and became advection fog.
According to meteorologists, the sun’s rays could not penetrate this layer of fog that hung over the city at a height of 3km. As the ground did not receive solar radiation, the mercury stayed low.

NORTH-WESTERLY WIND

Cold winds from the north and north-west made it chillier on Wednesday. At times, wind speeds in the city reached 15kmph. According to meteorologists, although the minimum temperature recorded at Alipore was 13.4 degrees Celsius, the cold breeze made the “real feel” more like 11.4 degrees.