Cthe wind slows down. Skyscrapers, warehouses, suburbs and shopping malls all create obstacles to airflow and this increase in surface roughness slows down the wind. Across China, the rapid growth of cities has caused average wind speeds to decrease by 11% since the 1980s. But urbanization also increases surface temperatures within cities, causing more mixing with the air above and possibly increasing wind speeds. So which effect is dominant?
Researchers from Nanjing University in China simulated the impact of the buildings in one of China’s megacities – Shanghai – to understand what impact the cityscape has on the wind. The research, which is published in the Journal of Advances in Earth System Modeling, shows that Shanghai’s built landscape extracts a significant amount of energy from the wind, slowing it down by about 50%. However, during warm periods when the urban heat island effect is particularly strong, the slowing effect of the buildings is offset by the energy added by turbulent mixing with the air above. Under the most extreme conditions, this can result in average urban wind speeds increasing by as much as 30%.
The balance will vary from city to city, but buildings seem to mostly slow down the wind. In hot weather, though, cities can kick up quite a hoolie.
