Hong Kong struggling to breathe under weight of ‘maximum’ urban density, academic says
Academic argues against city building more high-rises as already weaker winds are leaving heat and pollutants to shroud the concrete jungle
Urban parts of Hong Kong cannot cope with any more high-rises as street-level airflow has become more stagnant and unable to disperse heat and pollutants, an academic says, warning of an "urban dome".
The winds are weakening as more tall towers are built close to one another, Observatory data shows. Average wind speeds at the King's Park urban weather station in Hung Hom dropped from 3.5 metres per second in 1968 to two metres per second last year. With the still air came a 2 degrees Celsius rise in average temperatures in the same period.
"Further densification of our city is highly not recommended," Professor Li Yuguo, who heads the mechanical engineering department at the University of Hong Kong, said. "Future land reclamation may also need to be more disciplined."
Li said poor ventilation might be a factor behind days of very high pollution as the winds could barely disperse the pollutants.
The urban lethargy contrasts with the Observatory's remote Waglan Island weather station, where wind speeds have been stable over the last 50 years.
The Observatory says dense developments increase roughness at the surface, exerting a drag on winds near the ground.
Li warned of an "urban dome" effect based on ventilation studies conducted by his team. They found that, in the absence of winds, convective heat from individual buildings rose and formed a dome-shaped accumulation of warm air and pollutants above the city.
Few studies have been conducted locally on the urban dome, which differs from the "heat island" effect - which describes cities that are much warmer than surrounding rural areas - as it demonstrates airflow trapped within an inverted layer.
If anything, the urban dome was a result of the heat island effect, Li said. The warm breezes felt on some afternoons tended to be man-made "city winds" induced by the heat island effect rather than natural winds.
But there was "no removal strategy for the urban dome", Li said. He suggested that secondary streets along northern Hong Kong Island be kept "wide and short" to facilitate downslope windflow from the hills.
Buildings could also provide openings of about 10 metres at their bases - as exemplified by the HSBC headquarters in Central - to improve low-level airflow, he said. "But of course, there are development challenges," Li admitted. "The price to pay is not having a 7-Eleven on the ground floor of every building."
Urban planner Paul Zimmerman, of Designing Hong Kong, said the government could give incentives to developers to provide better ventilation and make land pricing policies more flexible. He said Li's study suggested the government could be more aggressive in improving ventilation in public spaces.
"Unlike a city like New York, where streets run towards the water and air can come in on all sides, our streets run along the water and heat is trapped," he said. "That's why waterfront development is so important, because people can go there to enjoy cooler weather."