(no, it’s not just climate change)
Year after year Hong Kong sets new warmest years on record, and summer after summer we sequester ourselves in air-conditioned rooms and complain about the heat. It’s obvious, of course — Hong Kong is humid and tropical and the Earth is warming. What else is there to expect?
However, if you were to go out with a thermometer and measure the temperature in, say, the middle Tsim Sha Tsui and the temperature near the Shenzhen border, you would find the temperature in Tsim Sha Tsui to be several degrees warmer. In fact, on early winter mornings, there could be a difference of over 10°C between the two locations.
The reason for this is the urban heat island (UHI) effect, the phenomenon through which heat is trapped within cities and urban environments. And, as we know, heat is deadly. Studies have shown that, during summer months, for every degree increase above 24°C, there is a corresponding 1-4% increase in mortality rates — for reference, indoor temperatures have been reported to reach over 40°C in many of Hong Kong’s cramped, poorly-ventilated housing units.
The areas worst affected by the UHI effect are Kowloon, the northern coast of Hong Kong Island, and the airport. Several factors play a role in this:
Tall buildings
Tall buildings reduce the sky view factor — the amount of sky you can view from the ground — which makes it less likely for solar radiation that reaches the ground to be reflected back into the atmosphere. Instead, it will most likely reflect onto buildings, which then absorb and re-radiate the heat.
Additionally, tall buildings disrupt airflow, reducing the ventilation received by the city and slowing heat transfer via convection.
Urban construction materials
Clustered glass-sided buildings common in Hong Kong continually reflect radiation off of each other, leading to more being absorbed by the surroundings and the building itself. Furthermore, the concrete and asphalt of city environments are much better at absorbing and retaining heat than soil and earth.
Vegetation
In rural areas, vegetation provides shade and plays an important role in cooling through evapotranspiration, as the vaporisation of water takes away heat from the surrounding environment. Hong Kong’s harbour actually plays a similar role, mitigating the UHI effect.
Produced heat
Urban environments may trap heat, but urban activities produce heat. Transport, people, homes — traffic and congestion in particular release massive amounts of energy that gets absorbed by the surrounding environment.
The Hong Kong government has taken steps to address this issue, such as using geographic information systems to plan development in ways least disruptive to wind and airflow patterns, and incentivising the inclusion of greenery and sustainability features in new buildings. Additionally, the city has set up a heat warning system and temporary night heat shelters put up to help the population during hot weather warnings.
However, there are other things that Hong Kong can and should do:
Take better care of the vulnerable: the night heat shelters are only open at, well, night. Create free, accessible 24-hour cool spaces, set up an air-conditioning rebate system for people living in substandard housing conditions.
Build more heat resilient buildings: set buildings further away from roads, avoid large shared podiums to allow air to circulate at ground level, use permeable materials, require property owners to set up air-conditioned common spaces.
Introduce green roofs: Toronto’s widespread adoption of green roofing programme has reduced roof temperatures by as much as 5°C. Even without vegetation, using lighter materials for roofs and pavements allow for more sunlight to be directly reflected back into the atmosphere, reducing the warming effect.
Keep learning from other cities
One last thing to remember, however, is that none of these measures will help for long unless we get global warming under control. Better urban design and policy may relieve some heat and the pressure on air conditioning, but, as always, it needs to be done with the understanding that the primary aim is to conserve energy and the environment.
Sources:
Writing: Katherine Yan
Editing: Kristen Wong
Graphics: Alicia Fok
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