The Heat Island

Hong Kong has never been so hot. The Mid-Autumn Festival used to hail the start of cooler weather, when we could spend all night outdoors, enjoying the breeze. But this year was the most sweltering holiday in 13 years. Sure, people blame global warming, but why is it affecting Hong Kong more than other places?

Janet Nichol, associate professor from the Department of Land Surveying and Geo-Informatics at Polytechnic University, says it’s all down to a phenomenon called the urban heat island effect. Though a buzzword first coined in the US during the 80s, the phenomenon it refers to explains how urban air and surface temperatures are higher than those of rural areas due to tall buildings and narrow streets that trap heat and reduce airflow.

Recently, Dr. Nichol and her team took thermal satellite images of the city to study the maximum difference in temperature between urban and rural areas (the study compared Ta Kwu Ling in the northern New Territories to urban areas like Mong Kok and Causeway Bay). On winter nights, the difference could be up to 12°C.

The blame for this clearly falls with high-density developments, says Dr. Nichol. She says that you can tell how overdeveloped an area is by looking skyward. “If you walk down Shanghai Street, I doubt if you could see any sky at all,” she says, adding that the new developments in West Kowloon are the worst, with wall-like residential estate projects doubling the difference in temperatures compared to rural areas over the past decade.

There are several reasons why massive high-rise developments heat up the city. Concrete is a heat absorber, and tall buildings stop airflow and trap pollutants. Bill Barron of the Institute for Environment and Sustainable Development at the University of Science and Technology says Hong Kong requires airflow even more than other built-up cities. “We have subtropical weather and it’s always humid,” he says. “With our long warm periods, cooling is a big issue for us.”

And our urban planning has made proper airflow simply impossible, says Barron. “The harbor ought to serve as an excellent cooling body and take heat from the urban area,” he says, “but the cooler air near the harbor can’t flow back to the city because all the high-rises block the passage and trap the  heat inside.” He gives North Point as an example. The entire area is blocked off from the harbor by lines of high-rises built along the waterfront. Podium projects, where developers build clusters of skyscrapers on top of the podiums of MTR stations, are another cause of overheating. According to Dr. Nichol, large podiums block airflow on the ground level, resulting in stagnant air on the street.