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

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.

Disrupted flow

“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.

Downdraught effect