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Liz Heron

Thermal image shows HK's urban areas up to 7 degrees hotter than open land - and it's getting worse

This high-resolution satellite image reveals for the first time how Hong Kong's high-rises are causing the city to heat up dramatically - with urban areas up to 7 degrees Celsius hotter than rural areas.

Scientists fear the difference, known as 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 nocturnal temperatures of more than 18 degrees, while 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 first high-resolution, nighttime image of the urban heat island effect in the world. The variation is caused by the greater heat absorbency of man-made materials such as concrete and asphalt and reduced air circulation around high-rises.

Researchers from Polytechnic University were granted a slot on Terra under Nasa's Mission to Planet Earth research programme and used a novel technique to create an image that is accurate to within  10 metres - nine times the resolution of typical 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 6 or 7 degrees higher than the surrounding rural areas. There is an uninterrupted north-south swathe 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 3 degrees Celsius up to 2050, this means that the ambient 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 provided the most comprehensive 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 2 or 3 degrees.

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

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,  as the satellite was overhead.

The high level of detail means temperature effects could be calculated for a housing estate, individual building and even a single row of trees, Dr Nichol said, and planners could use this information to mitigate 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 small parks and planting trees along streets would help. One large tree has the impact of two large air conditioners going at full blast. In new areas earmarked for development such as the West Kowloon cultural quarter and Kai Tak, there is a golden opportunity to introduce greening concepts.'

Dr Nichol said both developments would enlarge Hong Kong's extensive heat island and drive up temperatures further.

Edwin Lau Che-feng, acting director of Friends of the Earth, said he was alarmed by the findings.

'We need the government to bring forward legislation that will require developers to establish parks, gardens, trees and shrubs in any new development project,' he said.

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.



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