'Upper limit' of heat islands still way too high
The report ('Urban heat traps could be at their worst', May 22) resulted from the Workshop on Earth Observation in Urban Planning and Management at the Hong Kong Polytechnic University.
I organised the workshop and Tim Oke, probably the world's foremost urban climatologist, was our keynote speaker. As your article stated, an urban heat island of 12 degrees Celsius, which we have observed from satellite images of Hong Kong may not get worse as there appears to be a physical limit.
However, the emphasis should be on the fact that a heat island magnitude of 12 degrees is the highest of any city in the world and is already far too high, given that the climate here is humid sub-tropical and that a further rise of up to 3 degrees before 2050 is predicted due to global warming.
The workshop noted that the heat island core in Hong Kong is centred on the older, medium-rise mixed commercial and residential district of Mong Kok, but also extends through Yau Ma Tei and Jordan to Tsim Sha Tsui. The main causes appear to be dense buildings with narrow streets creating a low sky view, which prevents loss of long-wave radiation into the sky at night.
The recent redevelopment of part of Mong Kok at Shanghai Street has failed to solve this problem as anyone who looks upwards from street level at Langham Place sees very little sky. In Mong Kok, the penetration of fresh westerly sea breezes from the nearest coastline at Tai Kok Tsui is prevented by wall-like residential developments there.
From our research at Polytechnic University, the worst areas of heat accumulation are the older medium-rise districts and not necessarily the newer high-rise areas where the building footprint is often lower. This means that the same floor space can be maintained by building upwards, as long as the space around buildings is increased, which would appear to be the lesser of two evils in climatic terms. Thus the relaxation of height restrictions may not be bad, as long as more open space at ground level is created. Again, emphasis here is on the ground, not podium level.
