The impact of Urban Heat Island (UHI) effects on Hong Kong and other cities was explored at the Second Workshop on Earth Observation in Urban Planning and Management, which was organized by the Department of Land Surveying and Geo-Informatics (LSGI) of The Hong Kong Polytechnic University (PolyU) from 20 to 21 May.

Leading experts in remote sensing, geographic information system (GIS) and urban climatology presented their latest findings, exchanged ideas and discussed various issues related to urban planning and management at this two-day workshop. Professor Tim Oke, who first put forward a theory to explain the formation of UHI in 1982, has also made a special trip from the University of British Columbia in Canada to attend this conference.

The UHI effect means an urban area is significantly warmer than its rural surroundings. The temperature difference is usually larger at night and in winter. There are several causes leading to UHI, according to Prof. Oke. These include high-rise buildings which block thermal radiation in the night, materials with thermal bulk properties such as asphalt and concrete, and the lack of vegetation in urban areas.

With its densely populated urban area, Hong Kong provides a typical example on the UHI effect. In studying the territory’s UHI intensity last winter, PolyU LSGI Associate Professor Dr Janet Nichol and her research team have reviewed satellite images and collected ground data by making some 20 trips on special mobile vehicles in dusk and dawn. Each vehicle was equipped with two temperature sensors and a GPS receiver.

After analyzing the data, PolyU researchers found that there was an average temperature difference of 7° to 8°C between urban and rural areas in a winter night, and the maximum difference could be as high as 12°C. On a summer night the difference between urban and rural areas was 5° to 6°C. Although in most cases the land was cooler than the sea at night, Kowloon and the northern part of the Hong Kong Island were much warmer. In summer the land is significantly warmer than the sea, offering the potential for cooling sea breezes.  In both summer and winter the hottest urban area is Mong Kok, closely followed by Causeway Bay. The coolest area is the rural area around Ta Kwu Ling.

Dr Nichol said the large urban centre of Kowloon, with a population of over two million, is dominated by a strongly developed, regional scale urban boundary layer, and its urban canopy layer climate is reinforced by heating from both above and below. Reduced ventilation, high temperatures and the blocking of sea breezes by tall building on the newly reclaimed land, or the so called “wall effect” are contentious issues.

The UHI effect has the potential to directly influence the health and well-being of city dwellers. UHI is particularly damaging during a heat wave, as it deprives urban residents of the cool relief found in rural areas during the night. As a result, the relationship between UHI and public health has become a hot research topic in the US.

Another conference speaker Dr Dale Quattrochi, Senior Research Scientist of US National Aeronautics and Space Administration (NASA) Earth Science Department, said NASA and the US Centre for Disease Control and Prevention are jointly working on a project to use remote sensing data for estimating airborne particulate matter over the Atlanta, Georgia Metropolitan Area.

He said this project is known as the “Health and Environment Linked for Information Exchange (HELIX-Atlanta). Its goal is to examine the feasibility of building an integrated electronic health and environmental data network in five counties of Metropolitan Atlanta. An additional challenge is estimating exposure to environmental hazards such as particulate matter with size less than or equal to 2.5 micrometers (PM2.5).

An evening field trip was also arranged during the two-day workshop for the participants to observe and experience first-hand the effects of UHI in Hong Kong.

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