How excessive air pollution is impacting Dhaka’s rainfall pattern

When it rains only for half an hour in Dhaka city – with which we have a love-hate relationship – the roads become rivers, and carry more water than many of our dying rivers do. The city authorities have to put up signs saying "the road underneath is fine" for the sake of smooth vehicular movement.

Yet, 'city'zens, like any other Bangladeshi, cannot think of a rainless monsoon.

On the first day of Ashar, the first month of the rainy season, a young historian and writer, Aal Maruf Russell, took to social media and lamented less rainfall this year. He wrote on Facebook, "Is it global warming or Karma? Rain-less monsoon is the last thing before hell." 

I got confused a bit. I was frequently travelling out of the city in the previous weeks and enjoyed rain in Bhairab, Cumilla and Chattogram. True, I was missing rain while in Dhaka.  Later, even on Eid day, while rain washed all the dirt and bloodstain away in the afternoon, Dhaka was deprived of the blessing, although there was no shortage of clouds in the previous two weeks.

Then another Facebook post earned my attention after Eid. Zahirul Islam Akash, a young interior designer and an avid trekker, gave birth to a prolonged writeup only to shed tears on how Uttara was being deprived of the gift of the monsoon.

In Dhaka, pollution aerosols are suppressing rain in the early monsoon, and increasing it later, said a PhD candidate at the University of Saskatchewan and meteorologist Mostofa Kamal Palash. A number of studies carried out by Indian scientists in this and last decade showed that severe air pollution has caused decreased pre-monsoon rain in some parts of India

In fact, in the first week of Ashar (up to 19 June), Dhaka only had 61 mm of rain, according to the Flood Forecasting and Warning Centre data. All other districts in the Brahmaputra and Meghna Basins except Sirajganj got over hundreds of millimetres of rain by then, with Kurigram surpassing a thousand millimetre mark. 

However, later last week, clouds started to turn into rain droplets in Dhaka, providing a much-anticipated relief to the city dwellers.

A woman along with her kid and other passengers suffer immensely after a bus dropped them on the BRT Expressway in Tongi, Gazipur on Sunday despite heavy rains and a lack of shelters. Photo: Syed Zakir Hossain

Yet, at the end of June, Dhaka received much less rainfall than many other districts in the Brahmaputra and Meghna Basins, and the amount (279 mm) was well below the expected rainfall amount (348 mm) for the district for June. 

At the same time, Sylhet and Sunamganj received around 1,800 mm of rain.

We turned to experts to get an explanation.

They said this year's monsoon's behaviour has a lot to do with this rainfall pattern.

But that is not all. Excessive pollutants in Dhaka's atmosphere have changed the pattern.

As a result, Dhaka is seeing less pre-monsoon and early-monsoon rainfall than adjacent places, while the city is likely to get excessive rain in the later phase of the monsoon.

"Pollutants are essential for rainfall. The atmosphere has various aerosols such as dust particles around which water vapour condenses to form clouds," said Mostofa Kamal Palash, founder and lead meteorologist at abhawa.com.

"At the beginning of monsoon, when the amount of water vapour in the air is still low, they distribute among a large amount of particulate matter to condense into droplets, and as a result, they are tiny. Consequently, they are unable to produce rain," explained Mostofa, who is also a PhD candidate at the University of Saskatchewan.

"You'll see a lot of clouds in the sky, but a little to no rain," he said, adding, "I've observed that it was raining in Narsingdi, Munshiganj, Narayangaj and Gazipur – all around Dhaka – but there was no rain in Dhaka."

A NASA Earth Observatory article says that in the natural world, cloud-forming aerosols are things like sea salt, dust and pollen, all of which are large particles. However, pollution aerosols are usually smaller and more numerous than natural aerosols. With lots of particles to collect on, water coalesces into many tiny droplets instead of larger rain-sized drops. 

The article quotes climate scientist Tom Bell from NASA Goddard Space Flight Center as saying, "The impact [of pollution aerosol] on rain varies depending on where the clouds form. In some cases, urban aerosols suppress rain, but in others, they increase it."

In Dhaka's case, it is suppressing rain in the early monsoon, and increasing it later, Mostofa said. A number of studies carried out by Indian scientists in this and last decade showed that severe air pollution has caused decreased pre-monsoon rain in some parts of India.

Dhaka's air ranks among the worst in the world. Even after continuous drizzle in the last few days, Dhaka's air quality yesterday was only 'moderate' with a PM2.5 concentration – 5.2 times the WHO annual air quality guideline value.

"As more and more water vapour keeps arriving with the monsoon, they now have aerosols to form large droplets. In this situation, the clouds have a short lifetime, and precipitation is more likely," the meteorologist explained why Dhaka may see heavier precipitation later in the season.

"This will lead to waterlogging. And this will likely become a regular phenomenon," Mostofa concluded. The researcher said he was working on a paper on the change in rainfall patterns, which is expected to come out in December this year.

Mostafa also mentioned that the flow of monsoon this year was also responsible for initially less precipitation in Dhaka, which the officials of the Bangladesh Meteorological Department corroborated.

Shahinul Islam, a meteorologist at the department said, this year's monsoon is showing a different pattern. He said monsoon usually activates in mid-June, but this year it has done so earlier on 3 June. As a result, the southern districts such as Chattogram received a good amount of rain and thereafter it did not advance much.

But the northern divisions of Rangpur, Mymensingh and Sylhet – although they got rain, it was not induced by monsoon wind. It was pre-monsoon rain, the kind of rain that occurs during nor'westers.

From 3 June to 20 June, the monsoon did not advance. It was stuck near the border of Bangladesh and West Bengal. The little rain that Dhaka got was the impact of rainfall in Chattogram in the south and Mymensingh and Sylhet in the North.

"Usually, monsoon becomes active in the whole of Bangladesh by 15 June, and in India by 20 June or later, and a regular wind (monsoon) flows continuously," the meteorologist said.

"If you look at the satellite feed, you'll see clouds forming in the northeast and travelling westward. This is monsoon flow. Up to the 20th of this month, this flow was not there," he added, pointing at one of the large monitors in the room. This room of the Storm Warning Centre at the Meteorological Department is manned 24/7, led by a duty forecasting officer (DFO).

The moist wind from the Bay of Bengal, which turns into a 'easterly,' travels westward to meet dry westerly, and rain occurs in the surrounding area in great amounts, explained the DFO Shahinul.

Based on the current flow of monsoon, the meteorologist forecasted that more rain will occur in India, to the northwest of Bangladesh.

In recent months, citizens became furious about relentless logging in the city and transforming park areas into commercial plots, leading to excessive heat during summer.

Trees and forests are also known to increase rainfall intensity as they release fungal spores, pollen, bacterial cells and other particles into the atmosphere, providing the aerosol necessary for rain-forming. In Dhaka, though, pollutants play a similar role, only limited by their smaller size.