Urban building designs slow ground-level winds and worsen city heat stress

UPDATE - follow-up to Letting the wind flow freely is the secret to cooling our cities: Ground-level wind speeds in metropolitan areas continue to slow down due to dense high-rise construction, urban heat retention, and a lack of wind-aware planning, according to researchers and experts.

Urban Stilling Causes

Wind speed near the ground has been dropping worldwide since the 1980s in a phenomenon known as terrestrial stilling. Amruthraj M.N., a consultant at the Sustainable Energy Lab of the Indian Institute for Human Settlements in Bengaluru, notes that this drop happens roughly 10 meters from the floor where pedestrians move. Aaron Serre, a meteorologist from the University of Alabama in Huntsville, explains that dense infrastructure and high-rise buildings disrupt wind flow and create surface friction. He adds that replacing vegetation with asphalt and concrete cuts down evapotranspiration, which intensifies the urban heat island effect.

Building Design Factors

India's major cities feature high compound walls, podiums, and tall buildings that restrict wind flow in designated wind corridors. When winds hit these structures, they can create the Venturi effect by funnels through narrow spaces into gusts, or trigger the downdraught effect by pushing winds downward into strong gusts. Prasad Vaidya, a senior adviser at IIHS, points out that unplanned higher building forms directly contribute to lower wind speeds and extra heat stress.

Planning and Research

Experts stress that computational fluid dynamics simulations are rarely used in routine urban design because they are expensive and computer-intensive. Yugank Goyal from IIM Ahmedabad suggests using satellite imagery to build city ventilation maps and connect them to master plans. Meanwhile, cities like Stuttgart, Seoul, and Hong Kong have already assessed city-wide ventilation to improve air movement.

Context

Terrestrial stilling reduces the natural air circulation that helps cool pedestrian zones and disperse urban air pollution during hot months. Urban planners study these microclimate patterns to find ways of designing better breezeways and open spaces in densely built cities.