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HindustanTimes Sat,27 Dec 2014

Pollution makes Kolkata wettest in South Bengal

Joydeep Thakur, Hindustan Times  Kolkata, October 04, 2013
First Published: 11:35 IST(4/10/2013) | Last Updated: 12:04 IST(4/10/2013)

Everything in the climate seems to be going topsy-turvy — even the rainfall.

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Hard to believe, but Kolkata has received the maximum rainfall in the South Bengal region and scientists believe it could be the city’s pollution levels and concrete structures which transform the city into an “island of heat” that have led to excessive rains.

If one saw the rainfall map of this season, Kolkata would appear as a small lake in the midst of a green meadow. While green denotes normal rainfall (-19% to +19%), the blue indicates excess rain (more than 20%).

“This means that only the city area, which sprawls over 185 sq.km, received excess rain, while all the adjoining districts received normal rainfall. Beginning on June 1, Kolkata received about 1,675mm of rainfall till September 25. This is about 38% excess of the amount the city usually receives during this period in any year,” a senior official of the Indian Meteorological Department’s regional office in Kolkata said.

During the same period, all the districts of South Bengal, including the coastal belt, which usually receive heavy rain, saw much less.

While Hooghly received about 18% less rain, East Midnapore, one of the coastal districts, received around 11% extra. Normally, the South 24-Parganas receives the maximum rain in Gangetic West Bengal. But this year, it recorded less rain than Kolkata.

Environmental scientists and climate experts claim that the local heat trapped within the city and the pollution that emits aerosols have triggered more rain in the city.

“The monsoon winds bring in a lot of moisture and sea salt particles, both from the sea. These interact with the polluting particles present in the city’s air. Vehicular and industrial emissions generate a huge amount of sulphate and black carbon (soot) particles. Sulphate acts as a good ‘cloud condensation nuclei’, which means that sulphate accelerates the formation of cloud droplets from water vapour,” said Abhijit Chatterjee, a senior environment scientist and faculty member of Bose Institute.

Black carbon particles provide the surface where water vapour can condense with each other and form water (cloud) droplets.

Because of these sulphate and black carbon particles, water vapour can easily form cloud drops, which finally enhance precipitation.

With Kolkata being one of the worst polluted cities in India among all the mega-cities as reflected in a recent report published by a Delhi-based NGO, the Centre for Science and Environment, the generation of these particles in the atmosphere is much higher in Kolkata compared with that in other places in West Bengal.

“We’ve chemically analysed the rainwater in Kolkata this monsoon and found huge concentrations of sulphate particles. We also know that sulphate particles have the ability to reflect incoming radiation from sun. Hence, when sulphate is present in clouds, it reflects incoming solar radiation and lowers the temperature of the cloud system, which, in turn, favours condensation of the water vapours into large water drops, so triggering rain,” he added.

M Mandal, assistant professor at the Centre for Ocean, Rivers and Atmosphere and Land Sciences at IIT-Kharagpur, said another possible reason could be the localised heating, which turns the city into a “heat island”.

“Kolkata, because of its small green cover and more concrete structures, often behaves as a “heat island” when the sun shines brightly during the summer and monsoon months. The heat gets trapped in the concrete jungle of the city. This creates a convective current and a minilow-pressure area. During the monsoon months, when there is ample humidity in the air, it paves the way for cloud formation,” Mandal, who specialises in extreme weather events and cloud micro-physics, said.

Such convective clouds are very localised and cover only a few square kilometres.

It may be recalled that, because of such a convective phenomenon, “monster clouds” developed over the city and shrouded Kolkata in complete darkness before triggering blinding rain.

While one formed on August 26 and measured nearly 12km in height, the second formed on September 27 and extended up to 10km in height.

“The monsoon winds bring in a lot of moisture and sea salt particles, both from the sea. These interact with the polluting particles present in the city’s air. Vehicular and industrial emissions generate a huge amount of sulphate and black carbon (soot) particles.

Sulphate acts as a good ‘cloud condensation nuclei’, which means that sulphate accelerates the formation of cloud droplets from water vapour,” said Abhijit Chatterjee, a senior environment scientist and faculty member of Bose Institute.

Black carbon particles provide the surface where water vapour can condense with each other and form water (cloud) droplets. Because of these sulphate and black carbon particles, water vapour can easily form cloud drops, which finally enhance precipitation.

With Kolkata being one of the worst polluted cities in India among all the mega-cities as reflected in a recent report published by a Delhi-based NGO, the Centre for Science and Environment, the generation of these particles in the atmosphere is much higher in Kolkata compared with that in other places in West Bengal.
“We’ve chemically analysed the rainwater in Kolkata this monsoon and found huge concentrations of sulphate particles.

We also know that sulphate particles have the ability to reflect incoming radiation from sun. Hence, when sulphate is present in clouds, it reflects incoming solar radiation and lowers the temperature of the cloud system, which, in turn, favours condensation of the water vapours into large water drops, so triggering rain,” he added.

M Mandal, assistant professor at the Centre for Ocean, Rivers and Atmosphere and Land Sciences at IIT-Kharagpur, said another possible reason could be the localised heating, which turns the city into a “heat island”.

“Kolkata, because of its small green cover and more concrete structures, often behaves as a “heat island” when the sun shines brightly during the summer and monsoon months. The heat gets trapped in the concrete jungle of the city.

This creates a convective current and a minilow-pressure area. During the monsoon months, when there is ample humidity in the air, it paves the way for cloud formation,” Mandal, who specialises in extreme weather events and cloud micro-physics, said.

Such convective clouds are very localised and cover only a few square kilometres.

It may be recalled that, because of such a convective phenomenon, “monster clouds” developed over the city and shrouded Kolkata in complete darkness before triggering blinding rain.

While one formed on August 26 and measured nearly 12km in height, the second formed on September 27 and extended up to 10km in height.


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