HKU Bulletin April 2010 (Vol. 11 No. 2)

7 April 2010 Health and heat waves The fact that cities are hotter than the countryside comes as no surprise to most but should we be concerned about this heating effect? According to Dr Hart yes we should, and there are a number of reasons why. The most important, perhaps, is human health. Ample evidence links hot weather to increased mortality, particularly in the elderly, or those suffering heart disease. “In 2003 tens of thousands of people died in the European heat wave due to those very heat conditions,” says Dr Hart. “We have a warming climate to start with, and if you add a bit of extra temperature on top then there’s an added burden. General comfort levels are another factor. Current climate projections suggest we are going to have more hot weather, so this adds to the burden of urban heat.” Air pollution is another issue that can be severely affected by air flow throughout a city, as tall buildings tend to trap pollutants. Dr Hart’s research has highlighted a number of urban heat islands in high density areas like Mong Kok, Sheung Wan, Central and parts of Causeway Bay. During the summer of 2008 her team, in collaboration with the Hong Kong Observatory, fixed fast response temperature and humidity sensors to vehicles and drove them through the city, measuring temperature, humidity and location every five seconds. “We linked our data to GIS information on land use, and land surface characteristics, to see which characteristics are associated with the hottest parts of the city,” she says. What she found was that during the daytime, temperatures were 1.3 to 3 degrees higher in urban areas. Overnight it was up to six degrees warmer in the city than in rural Ta Kwu Ling. Further research revealed that building height was the most important factor associated with the hottest regions. “Height to width canyon is also important,” she says. “I did a similar analysis for Portland, Oregon in the US, which is a very different city to Hong Kong both climate-wise and in urban morphology and, in Portland, what it came down to was trees. Anywhere there were “Overnight it was up to six degrees warmer in the city than in rural Ta Kwu Ling.” trees it was much cooler. So, in Portland, the urban heat factor is much easier to mitigate, you just plant more trees.” Reducing city heat “In Hong Kong one of the ways we can mitigate the problem is by retrofitting buildings and changing construction materials, adopting green roofs and vertical greening, using lighter building materials so they do not absorb so much heat, and taking into account the surrounding environment when you build. “More efficient buildings do not release so much waste heat. By changing what we call the albedo of the surface of a building, that is, the amount of radiation from the sun that is reflected back out, we can make a big Summers in this city are hot, sticky and not for the feint hearted, but Hong Kong’s topography, its overcrowding, tall buildings and narrow streets only add to the stress of our hottest months. In the Department of Geography, Assistant Professor Melissa Hart has been investigating our urban climate and looking at ways to mitigate the problems caused by the city’s urban heat islands. Hong Kong, she says, is unique in terms of urban climate. “The building and population density are unlike any other city and, from a climatological perspective, it’s quite complicated, because you have the topography as well as the surrounding water.” In the very act of building a city we destroy the natural environment, removing surface vegetation, constructing buildings, adding roads people and cars. All these factors combine to alter the energy balance of a city. We also tend to use materials that are very good at trapping heat. “Simple things like concrete buildings and asphalt roads which absorb more solar radiation from the sun and retain this heat much longer, particularly overnight” says Dr Hart. “If any heat tries to escape overnight, it’s trapped because we have these tall narrow street canyons.” Furthermore, by removing vegetation we lose its inherent cooling effect. Add to this anthropogenic heat – that released by human activity, like air conditioning, traffic, industry and people – and we have cities that are far hotter than the surrounding countryside.” difference. Something as simple as painting a rooftop white, or using lighter colour building materials can have a drastic cooling effect. You can reduce the surface temperature on a building by 10 to 15 degrees just by changing its surface characteristics. And, if you have cooler buildings you don’t need so much energy to cool them down, so you have lower heat emissions.” Dr Hart is currently working on another project investigating the relationship between building energy consumption and climate change. Using computer models she can alter a building’s energy consumption by changing its orientation, the construction materials, windows and insulation. “But in order to do that you need to know what the weather is like outside. Normally researchers use past meteorological data but our project is trying to come up with data files under future climate scenarios, so that buildings can be designed for a future climate not for past weather conditions. “You can reduce the surface temperature on a building by 10 to 15 degrees just by changing its surface characteristics.” Cover Story OUR CITY IS heating up our air pollution is worsening and, if UN projections are correct, our population will hit nine million by 2050. HKU scholars from a wide range of disciplines are at the forefront of tracking the changes and finding solutions to the more pressing problems.