Letting the wind flow freely is the secret to cooling our cities

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A view of the South Mumbai skyline in 2024. When winds slam into high-rise buildings, they can be forced downwards along the structures in a phenomenon called the downdraught effect. | Photo Credit: Drone Master/Unsplash

India’s cities are bustling with taller buildings and more densely packed populations. The hotter days in many of these cities also come with high humidity, and can make the experience of heat seem unforgiving. One of the reasons is often a perceived lack of winds, which often have to make their way through the nooks and crannies of crowded streets before reaching a window.

Slower winds can affect how people experience heat. Normally, the air immediately above the ground becomes saturated with water vapour due to evaporation. When winds blow faster, the saturated air is replaced with drier air. Slower winds, however, also limit evaporation.

Wind speed near the ground has been dropping worldwide, in a phenomenon called terrestrial stilling, since the 1980s.

“Multiple factors lead to reduced wind speed specifically in the near surface level,” according to Amruthraj M.N., a consultant with the Sustainable Energy Lab at the Indian Institute for Human Settlements (IIHS), Bengaluru.

“When we say surface level, it’s typically around 10 m from the floor, where a lot of pedestrians move around the city and that’s the place where typically there’s reduced wind speed that has been seen over a few decades.”

Aaron Serre, a meteorologist and recent graduate in atmospheric science from the University of Alabama in Huntsville, U.S., said dense urban infrastructure and high-rise buildings tend to disrupt wind flow, creating turbulence, and increase surface friction that results in slower winds near the surface.

He also said the loss of vegetation and the introduction of asphalt and concrete instead drastically reduce evapotranspiration, intensifying the urban heat island effect.

“Urbanisation and buildings … taking into account the analysis of wind speeds across these cities, can be built to ensure that there is wind flow at that level,” Mr. Amruthraj noted. “Unfortunately, not a lot of people follow that approach.”

He added that many structures like higher compound walls and podiums restrict wind flow in previously defined wind corridors.

India’s metropolitan cities are replete with high compound walls, podiums, and high-rise buildings of varying heights, often clumped together without adequate spacing, affecting the wind’s speed — rougher surfaces have a greater slowing effect — and direction.

When a wind’s flow is interrupted by tall buildings, it can instead be funnelled through narrow spaces, where it accelerates and builds up into a gust before reaching the pedestrians walking around the buildings. This phenomenon is called the Venturi effect and has been observed in London and Jaipur, among other cities.

As well, when winds slam into high-rise buildings, they can be forced downwards along the structures in a phenomenon called the downdraught effect, which further pushes the wind lower down into stronger gusts.

However, Prasad Vaidya, a senior adviser at IIHS and lead of its Sustainable Energy Lab, said that while there have been studies showing stilling is underway, there are no specific studies that compare rural versus urban stilling to reveal the impact of urbanisation.

He also said wind speeds are changing even as cities are warming, losing trees, and losing water bodies — and it has been hard to establish causal relationships between them.

Depending on their geometry, other physical characteristics, and their environs, tall buildings can also create pockets sheltered from winds along the streets surrounding them. If the people there are also experiencing an urban heat island, the lack of wind may distress them further.

“While there are no specific studies that correlate it,” Mr. Vaidya said, “there is enough experience to show that higher building forms that are unplanned will contribute to [lower] wind speeds and additional heat stress.”

“We need to build a homogenised wind record firstly because, right now, the placement of an anemometer may be inconsistent,” Yugank Goyal, associate professor and chair at the J.S.W. School of Public Policy at IIM Ahmedabad, said. (An anemometer is a device to measure wind speed.)

Slower winds can also take more time to disperse air pollution. “We have done some [computational fluid dynamics] simulations. What we found is that maintaining good air movement through a breezeway or corridor is not easy, because small changes in building shape  … can actually cause eddies and swirls and reduce the wind speed through the length of the breezeway,” Mr. Vaidya said. 

Computational fluid dynamics is a computer-based modelling technique to help simulate and analyse the behaviour of fluids in different conditions.

In other words, “the reality is that cities are so complex that little movements — somebody putting out a window or canopies — is going to affect the wind’s movement.”

Mr. Amruthraj said that while research is ongoing on how wind flows through cities, computational fluid dynamics modelling of how buildings’ shapes, wind corridors, open spaces, and other features affect wind speeds is not routinely incorporated into urban design and planning.

One reason is computational fluid dynamics is expensive to implement. The computer simulation models and tracks motions within a turbulent fluid going from small to large, from fleeting to long-lived. To resolve them all, the simulation needs increasingly fine spatial grids and correspondingly shorter time-steps. As a result, the total computational cost of resolving a turbulent flow scales roughly as the cube of the Reynolds number, a number measuring the amount of turbulence.

Mr. Vaidya cited assessments by the governments of Stuttgart, Seoul, and Hong Kong to improve city-wide ventilation, and said, “Cities like Chennai and Mumbai can actually do something similar, but we haven’t seen specific wind-related studies tied to heat and pollution happening in India, as much as we would like to see them.”

According to him, “part of the reason is that these studies require [fluid dynamics] simulations, which are extremely computer-intensive and require expertise.”

“We need to use satellite imagery to evaluate each city’s changing form,” Dr. Goyal added. “We could potentially create city ventilation maps and connect all this to master plans.”

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