Urban Heat Island Effect: Research & Collaboration
Posted in Research by Kate Archdeacon on September 8th, 2010
Source: Environmental Research Web
From “Urban cool” by Roland Ennos:
Cities are hot, noisy places with poor air quality that are prone to flash flooding during storms. In cities we are guilty of using huge amounts of energy for cooling in summer, heating in winter and transport the whole year round. Making cities more pleasant and sustainable places in which to live is therefore one of the key goals of environmental research, and it is one that physicists are ideally suited to contribute to, since most urban environmental problems are best understood in physical terms.
Physicists across the world, particularly those working in environmental physics and meteorology, are now collaborating with scientists from other disciplines to study the environmental performance of cities and establish how “green” these urban environments are. One particularly important environmental characteristic of cities is the “urban heat island”, whereby urban areas are hotter than their surrounding countryside. This is a real problem, which will be made even worse by climate change. It has therefore become a prime focus of research.
The urban heat island
Cities are typically about 4 °C hotter than the surrounding countryside and the larger they are, the bigger the difference. To understand why, we must consider the energy balance of the two areas (figure 1). Although heating, air-conditioning and transport all produce energy in cities, this is a surprisingly small component of their heat balance – only about 50 W m–2. Except for in winter, this is dwarfed by the energy we receive from the Sun, which even in the UK peaks at more than 800 W m–2. The difference between temperatures in a city and the surrounding countryside is therefore mostly due to what happens to the Sun’s energy in the two environments.
In rural areas, vegetation reflects about a quarter of the incoming short-wave radiation (visible light or shorter wavelengths). Of the three-quarters that is absorbed, much of the energy is used to evaporate water from leaves – a process known as “evapotranspiration”. This cools the vegetation, which therefore radiates little long-wave radiation (infrared), and even less energy remains to heat the air by convection and to heat the soil by conduction.
In cities, where vegetation has largely been replaced by buildings and roads, the energy balance is dramatically altered. Dark, artificial materials reflect less – and absorb more – radiation than vegetation. This lower “albedo” means that only about 10% of the Sun’s radiation is reflected; this figure is even lower in high-rise cities where light is reflected down into urban “canyons”. Almost all of this energy goes into heating the dry roads and roofs, where it is either stored in bricks and mortar or heats the air above, thus raising daytime surface and air temperatures well above those of the surrounding countryside.
At night the difference in temperature between the countryside and the urban heat island can become even more pronounced. Cities cool down more slowly because there is more heat stored in its buildings, which continues to dissipate into the night; there is more pollution to trap long-wave radiation; and within urban canyons less of the cool sky is visible, so less radiation can escape.
All this causes major problems for city-dwellers. The rise in urban air temperature above that of the surrounding countryside, which can reach 7 °C in a metropolis like London, makes cities less comfortable places to live in during the summer months. Soaring temperatures increase ill health and can even kill people during heatwaves: it is thought that more than 35,000 people died in Europe as a result of the 2003 heatwave, most of them in towns and cities. The urban heat island also makes cities less sustainable, since it increases the amount of energy used for air-conditioning – energy that is pumped into the open air and just makes the situation worse. Fortunately, physics shows that two very different methods could be used to alleviate the urban heat island: using “cool surfaces”; and using vegetation, or “green infrastructure”.
Read the rest of this informative article by Roland Ennos on Physics World.