Urban Heat Island Effect

 by Maximilian Bergmair

The Urban Heat Island Effect. What exactly is this phenomenon, and why is it relevant to H.E.A.T.’s mission? A heat island may simply refer to any area that is hotter than its surroundings. An urban heat island specifically refers to a metropolitan area which has a higher average temperature than its surrounding rural areas due to human activities.


Hong Kong’s design and architecture exemplify a number of factors that contribute strongly to this heating effect. Although Hong Kong’s total proportion of urban or built-up land is only 25.1% [1],  its densely populated districts are highly urbanised, as shown in this interactive map by the Planning Department. [1] In these districts, dark pavements and tarmac dominate Hong Kong’s floorspace. The use of these surfaces contributes to the problem in the following ways: First off, darker surfaces absorb solar radiation much more readily thanks to their lower albedos, and less of the heat energy carried by the sun’s light is reflected away. In addition, the materials used for urban construction, such as concrete and asphalt, have significantly different thermal properties from the rural surfaces they replaced. As a result, the urban area’s energy budget (the balance between the energy Earth receives from the sun and the energy it radiates back into outer space) significantly increases, resulting in higher temperatures in the affected area.


Another reason for the urban heat island in Hong Kong’s densely-packed districts is the deficiency in cooling from evapotranspiration. During evaporation, fast-moving liquid molecules break away from the surface of said liquid to enter a gaseous phase. Since only fast-moving, and thus more energetic, molecules can break away, the average energy of remaining molecules decreases as a result. Since temperature is a measure of average energy, a decrease thereof is represented on a larger scale by a cooling effect. 


Two main causes can be identified for the lack of this phenomenon in Hong Kong’s metropolitan environment. The first is the city’s extremely high humidity. For about half the year, relative humidity levels in Hong Kong don’t drop below 80%. [2] When the air is so saturated with moisture, it becomes increasingly difficult for water to evaporate from other sources, limiting the action of evaporation. The second main cause is a lack of vegetation, or natural land cover. Plants are a major source of evaporation. This is because they are suffused with rich stores of water which they can’t contain perfectly; there is always bound to be some leakage. This leakage is known as “transpiration”, and occurs mostly from the surface leaves as part of gas exchange—a process necessary for the plants to stay alive. As discussed previously, however, most of Hong Kong’s urban surfaces are plastered with asphalt and concrete, leaving our urban centres with little to no vegetation. These two factors seriously hamper the potential cooling effect that could be garnered from evapotranspirative processes.


It’s clear that Hong Kong’s urban heat island effect is rooted in inefficient land management. But how specifically has it impacted Hong Kong, and what effect will it continue to have on our city’s most vulnerable stakeholders? Stay tuned for next week’s article.

  1. "Planning Department - Land Utilization In Hong Kong". Pland.Gov.Hk, 2021, https://www.pland.gov.hk/pland_en/info_serv/statistic/landu.html. Accessed 12 Oct 2021

  2. “Climate of Hong Kong.” Hong Kong Observatory, 2021, www.hko.gov.hk/en/cis/climahk.htm.