The science behind Dhaka's 'wind pockets'

A few years ago, on a sweltering night in May, a power outage made it impossible for Delwar to stay at his rented apartment in Khilgaon. 

He shared it with six friends, all unemployed graduates. They spent days and nights memorising various employment guidebooks and worked as private tutors to make ends meet. 

When the electricity went out at night, they stepped out onto the street for some relief. But the absence of breeze or wind made it equally suffocating. 

So, they headed towards a specific street in Tilpapara. After several years of residency in the area, Delwar learned that this particular street was always breezy. 

And he was proven right. When they sat on the footpath at a specific spot where three streets converged, the breeze was blowing from the northwestern side. 

It wasn't merely due to the convergence of the three streets; a few hundred feet away, there was a junction of four streets, but there was no breeze there. 

"Back in the day, Dhaka had more power outages or load shedding. We spent many summer nights in that wind pocket– chatting and having fun with friends," Delwar told The Business Standard. 

"Even now, when it's especially hot and the electricity goes out, I spend some time in that location and then return home," he added, explaining that his married life no longer allows him to spend long nights outside the home.  

It is not just in Khilgaon, we came across several such spots in Dhaka where there is a gentle breeze blowing even when the rest of the city remains hot and humid.  

Another such wind pocket can be found in the Banglamotor area, next to Holy Family Hospital Road. It's obstructed with a gate, but if you walk through, it resembles a residential corner road. Each time we passed this road facing Banglamotor, we were greeted with a refreshing breeze. 

The security guards at the gate confirmed to us that the wind blows there most of the time. "It's fun to guard this street. I don't need a fan here, and I don't suffer from heat because of this wind," one of them told us. 

We reached out to some urban planners and architects to understand the strange phenomenon behind these wind pockets.  

Muhammad Shamsuzzaman, an urban development specialist, said that this happens for two simple reasons. 

One is called the funnelling effect– some wind accumulates there like a funnel. Two, there is air turbulence created in that area for some reason. It may happen due to the surrounding buildings and air movement.  

"It doesn't happen by design, but rather created by chance," Shamsuzzaman said. "And this is not always good. In cases of high-rise buildings, sometimes, the winds like this may blow a little too fast." 

Architect Nurunnahar Mili considers this as a scientific matter. 

"You see, the air is everywhere. It is like river water. If it is obstructed somewhere, it flows stronger in the areas where it is open," she said. 

There is a push and pull factor as she explained.  

"In a house, you don't get wind through every window. But in some places, you have better airflow. Some air tracts have a stronger pull. When and where this [push and pull] situation is created, the wind is funnelled across that channel," Nurunnahar said. 

"You can create this too with different scientific methods, we don't do that however."

Urban planner Muhammad Nazmul Ahsan, who is also an assistant general manager at Sheltech, told us that this is about aerodynamics. The air enters our country from the south or southeast or sometimes the southwestern side. 

But in Dhaka, within a narrow area, there are buildings after buildings. 

So, how would the air enter the city? What happens is that the air keeps flowing – albeit restricted by the structures – through a particular channel. 

Because of the obstruction (from the buildings), one space has good airflow whereas others don't.  

"For example, my apartment is northeast facing and it was not supposed to be very airy. But the building next to mine obstructs the usual airflow, so we get wind that flows through my house," Nazmul Ahsan said.  

"This [the airflow] plays a role in building designs too. Say a building is northeast facing, but it is designed in a way by extending the north side so that wind enters it all the time. The architects can design some interesting improvisation like this," he added. 

If you explore the different areas in Dhaka such as Mohammadpur, Mirpur or Agargaon, chances are there that you will come across one such windy street. 

"The streets in some areas bend or detour, or the bigger streets turn narrower… all these obstruct the wind and create pressure, and channel it in a particular way. 

You could also think of it as a change in air pressure. How is the cyclone created? It requires a different pressure. In this case, too, there is a certain pressure that pulls the air to a certain direction," said Architect Nurunnahar Mili. 

She added, "As a result, automatically that particular spot gets the wind flow. And it will remain like this unless significant climatic/geological changes happen."