Tall buildings and slow winds make urban heat worse

Dense urban infrastructure and high-rise buildings are disrupting wind flow across cities worldwide, intensifying how people experience heat. Wind speed near the ground has been dropping globally since the 1980s in a trend known as terrestrial stilling.

Causes And Effects

Amruthraj M.N., a consultant with the Sustainable Energy Lab at the Indian Institute for Human Settlements in Bengaluru, notes that reduced wind speed typically occurs around 10 metres from the floor where pedestrians move. Aaron Serre, a meteorologist and recent atmospheric science graduate from the University of Alabama in Huntsville, explains that dense buildings increase surface friction and create turbulence. The loss of vegetation along with the replacement of natural land with asphalt and concrete also reduces evapotranspiration and worsens the urban heat island effect. Furthermore, high compound walls and podiums restrict wind flow in defined wind corridors, while rougher surfaces slow down winds even more.

Wind Phenomena

When tall buildings interrupt wind flow, winds can be funnelled through narrow spaces into faster gusts in a process called the Venturi effect, observed in cities like London and Jaipur. Winds slamming into high-rise buildings can also be forced downwards in a downdraught effect. Conversely, buildings can create sheltered pockets along streets with no wind, which increases heat stress when combined with the urban heat island effect. Slower winds also take more time to disperse air pollution. Prasad Vaidya, a senior adviser at IIHS, notes that small changes in building shapes can cause eddies and swirls that reduce wind speed through breezeways.

Solutions And Planning

Prasad Vaidya points out that computational fluid dynamics simulations, which model fluid and air behaviour, are expensive and computer-intensive, making them rarely used in routine urban planning. Yugank Goyal, associate professor and chair at the J.S.W. School of Public Policy at IIM Ahmedabad, suggests building a homogenised wind record and using satellite imagery to create city ventilation maps connected to master plans.

Context

Terrestrial stilling is a global phenomenon affecting near-surface wind speeds due to changing landscapes and expanding cities. Urban planners use tools like wind records and computer simulations to understand how air moves around buildings. Better ventilation in cities helps reduce heat stress for pedestrians and improves air quality by dispersing pollution faster.