Hot town, summer in the city


Nick Leech investigates a curious but very serious phenomenon: the urban heat island effect

What could be hotter than the desert? A city in the desert. And what could be even hotter than a city in the desert? The dark, dry, exposed surfaces that surround us every day. We’ve all felt that unbearable blast from the asphalt as we walk the final few metres to our cars; a heat that, if anything, feels even more intense after the sun has gone down, but few realise how damaging the effect of this can be. Not only does the extra heat that cities like Abu Dhabi radiate cause discomfort to their inhabitants, it has far-reaching environmental impacts and as recent urban heatwaves in Paris (2003) and Moscow (2010) have shown, it can even prove fatal.

So how can a roof in Abu Dhabi be hotter than the surrounding desert? Part of the answer lies in the difference between air temperature, the kind of heat that we normally experience and that’s also reported in weather reports, and what is know as ‘land surface temperature’, which can be much, much higher. Air temperatures are affected by wind, the amount of moisture in the air, and larger scale climatic variables like areas of high pressure and climatic depressions or troughs. According to Stuart Gaffin, a climate researcher at Columbia University and NASA’s Goddard Institute for Space Studies, “The value of the air temperature measurement, as opposed to land surface temperature, is that it is what the human body, a building, or an ecosystem are feeling. Air temperatures largely dictate our thermal comfort level.” Land surface temperatures on the other hand, are purely a product of the particular thermal properties of a particular material or patch of land. The better that material is at absorbing solar radiation, converting this to heat and then transmitting that back into the surrounding atmosphere, the more extreme the effects will be. Unfortunately, our cities are full of such materials, especially the asphalt and tar that make up our roofs and roads, and this process contributes directly to a peculiarly localised form of climate change known as the urban heat island effect (UHIE) in which cities become hotter than the natural landscape that surrounds them..

Image

According to the US Environmental Protection Agency, roads, roofs, and pavements can become anything from 27 to 50°C hotter than the surrounding air temperature on a clear, sunny day. Average temperatures might only differ by 1-3° centigrade during the day, but this can increase by up to 12° centigrade at night. Indeed, a 2011 study conducted by Columbia University compared the temperatures of traditional black tar and asphalt covered roofs with those covered in a new generation of white, EPDM (ethylene propylene diene monomer) membrane. Some of the dark, sunlight-absorbing roofs reached 170 degrees Fahrenheit while some of the roofs clad in the white EPDM achieved temperatures that were 42 degrees cooler.

Image

Heat islands can have a profound effect on the sustainability of urban communities and contribute significantly to their carbon footprint, not least through an increased demand for air conditioning at peak hours that can often push electrical grids to capacity. The UHIE is also responsible for increased greenhouse gas emissions however, for Peter Stair, a sustainability expert and Estidama assessor with Abu Dhabi’s Urban Planning Council, the simple effect of all this increased heat on daily life and the risks it poses to public health are enough to make the UHIE a matter for genuine concern. “It’s a significant issue here in Abu Dhabi. Just from the perspective of pedestrian comfort and the issue of air pollutants…The increased incidence of lung, throat, and other respiratory illnesses like asthma, is reason enough to take the issue seriously.”

So what causes the UHIE and what can be done to combat it? For Professor Ahmad Okeil, an expert in the relationship between urban climate and urban form, and chair of architecture & design at Abu Dhabi University, there are three key issues that any solution has to address: urban infrastructure, building materials, and the very structure of city itself.

In terms of infrastructure, cities are full of industrial processes, including transport, air-conditioning, and waste disposal systems that contribute to urban temperatures and these must become more energy efficient or we must reduce our reliance on them completely. Encouraging a shift from private car use to public transport is a key example of the kind of behavioural change that’s required, and is already one of the central tenets of Estidama, Abu Dhabi’s sustainable development code.

Image

Internationally, the use of green, vegetated roofs and walls is seen as a key weapon in the war against the UHIE, as plants reflect light and heat, provide shade, and also help keep streets and cities cool through a process of natural evaporative cooling called evapotranspiration. Unfortunately this is not a sustainable option for cities like Abu Dhabi, as Peter Stair explains, “This is a water scarce region, so Estidama focuses instead on shading and the reflectivity of surfaces.” As the Columbia University study shows, materials have a massive role to play in combating the UHIE. Professor Okeil agrees, “There is a huge difference between the countryside and the city. The countryside contains materials that are porous, reflective, and that breathe. The city contains darker materials like asphalt that absorb more light, convert this to thermal energy, and emit this back into the surrounding air as heat, warming the city.”

Desert sand is ten times more reflective than fresh asphalt, and it’s this quality that allows it to release and reflect heat quickly. When this reflectivity combines with open skies that allow solar radiation and heat to escape easily to the upper reaches of the atmosphere, you have an environment that cools relatively easily once the sun has set. Unfortunately, the situation in dense cities like Abu Dhabi could not be more different. As Professor Okeil explains, “The solar energy that enters the city ricochets of different surfaces, and each time it hits a surface, part of that energy is absorbed and converted into heat. Unless they are carefully designed, dense cities actually absorb more heat and make it more difficult for energy to escape into the sky.”

This situation effectively creates urban microclimates that become trapped in a zone called the urban canopy layer that’s defined by their skylines. A city with a very even skyline, in which many of the buildings are the same height, effectively becomes aerodynamic and cooling breezes skim over its surface, leaving all the heat and hot air trapped in its streets and open spaces below.

Image

Traditionally cities in arid climates were always dense and compact as this was the best way to provide the shade necessary to keep buildings and open spaces cool. Higher densities are also now championed by contemporary architects and planners as the answer to sustainable urban development, an irony that’s not lost on Okeil. “Architecture is all about paradoxes. You try and fix one thing and then something else goes wrong! If you increase density, you make a positive contribution by reducing the need for transport and processes that create heat, but at the same time, you decrease natural ventilation.”

For Professor Okeil, the answer is to balance density with an urban form that also maximises natural ventilation, something he would achieve by making the skylines and façades of cities more textured and varied, with tall buildings located at key points across the city and streets with greater variety in their shape and design. This would create key points around which turbulence would develop that would force cooling breezes down and through the urban canopy layer, creating a form of natural ventilation that will flush heat from the city like smoke from an exhaust.

There is a beguiling simplicity to Okeil’s solution, not least because it can be retrofitted to existing city blocks and does not require any power, but even he admits that there is still a lot of research that needs to be done. “We cannot assume that the air flowing above the city will be cooler than the air flowing inside and not every measure that may work in a cool climate may work in an arid one. The theory for cities in hot climates is not yet clear.”

A version of this article originally appeared in The National, Abu Dhabi.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s