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Climate change

High-rise heat trap revealed

PUBLISHED : Tuesday, 13 March, 2007, 12:00am
UPDATED : Tuesday, 13 March, 2007, 12:00am

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Tall buildings in the city are causing temperatures to rise dramatically


The high-resolution satellite image above reveals how Hong Kong's high-rises are heating up the city- with urban areas up to seven degrees Celsius hotter than rural areas.


Scientists fear the difference - the 'urban heat island effect' - could be 10 degrees by 2050.


The image was taken by the Nasa satellite Terra at 10.42pm on January 31. It shows that large areas of the city - including Central, Mong Kok, Causeway Bay and Kwun Tong - had nighttime temperatures of more than 18 degrees. But many rural areas were 13 degrees or lower.


An area of Kwun Tong next to the old airport was especially hot because of a lack of space between closely packed industrial buildings.


It is believed to be the world's first high-resolution, nighttime image of the urban heat island effect.


The effect is caused by the greater heat absorbency of man-made materials such as concrete, and reduced air circulation around high-rises.


Researchers from Hong Kong Polytechnic University were allowed to use Terra as part of the US space agency's Mission to Planet Earth research programme. The image is accurate to within 10 metres - nine times the resolution of normal thermal satellite images.


Janet Nichol, associate professor in Polytechnic University's department of land surveying and geo-informatics, said: 'The image shows that the urban heat island in Hong Kong is concentrated in the areas that are most densely built up with high-rise buildings.


'The temperatures of these areas are six or seven degrees higher than the surrounding rural areas.


'There is an uninterrupted north-south [band] of warm temperatures extending from the southern tip of Kowloon peninsula through Jordan and Mong Kok to Sham Shui Po.


'If, as scientists predict, temperatures will increase by a further three degrees Celsius up to 2050, this means that the ... air temperature in the urban areas will be 10 degrees higher than the natural level due to the impact of human activity.'


Dr Nichol said the image was best evidence yet of the urban heat island effect in Hong Kong.


'This is because it was taken at night - and urban heat island is a nighttime phenomenon.'


Lam Ka-se, from the department of civil and structural engineering, had earlier found a difference of just two or three degrees.


But Dr Nichol said this did not give the full picture because only a few sampling points were used across the city.


While the satellite was overhead, a 10-member research team took air and ground temperatures at 16 places, including the summit of Tai Mo Shan, Nathan Road in Mong Kok and the Star Ferry Pier in Tsim Sha Tsui.


Dr Nichol said that the high level of detail means temperature effects could be calculated for a housing estate, individual building and even a single row of trees. Planners could use this information to decrease the heat island effect.


'It has been shown that the more extensive the urban heat island, the greater the magnitude. If you can break it up into smaller packets using vegetation, green corridors and small patches of park, it will decrease the magnitude quite considerably.


'We could also have more roof-top gardens - and even [having] small parks and planting trees along streets would help. One large tree has the impact of two large air conditioners going at full blast.'


This is an edited version of an article by Liz Heron that appeared in the South China Morning Post on March 11.


Factbox


* Last year was the eighth warmest in Hong Kong since records began in 1884, with an average temperature of 23.5 degrees, half a degree higher than normal.


* Globally, 2006 was the sixth warmest year on record.


* Total electricity consumption last summer was 50,634 terajoules, an increase of 10.65 per cent from last year, of which domestic consumption comprised 26.64 per cent. Commercial consumption rose 3.76 per cent, to 28,008 terajoules.


* In 2004, Hong Kong emitted about 37,600 kilotonnes of carbon dioxide, 10 per cent more than in 1990, while its contribution to global greenhouse gas emissions in 2004 was about 0.2 per cent. Power stations were responsible for about 64 per cent of the greenhouse gases.


Action Blue Sky


What is it? The government's campaign to promote energy conservation


When was it launched? July 2006


What has it recommended? The public has been urged to set air conditioners at 25.5 degrees, switch off engines when vehicles are at a standstill and dress down for work to reduce energy use and help combat air pollution.