What is urban heat island effect?

How progress led us to dependency on systems

After the Industrial Revolution and introduction of mechanical systems into buildings, the direct influence of the local climate was diminished using technology. As a side effect, we had to deal with higher energy consumption.

The reduced dependence on local weather conditions lead to more freedom. We could choose from wider variety of architectural forms and materials. This was the time of modern international architecture in the last century and the period before the impact of higher energy consumption on the local and the global climate became evident.

Not surprisingly, an almost global style of architecture for certain building types emerged. An analogous development occurred with cities. At the beginning of the 21st century, the impact of anthropogenic input of heat, aerosols, and greenhouse gases into the world’s atmosphere becomes evident.

The cities and their direct hinterland are responsible for 80% of those globally. Therefore, all these cities share similar experiences: air pollution leading to higher mortality; higher urban temperature due to the creation of local climate zones; in addition, becoming subject to global temperature effects.

The exponential growth of population and energy demands over the last decades created a noticeable problem. The energy consumed by each and every individual and CO2 produced both accounts for what manifests as heat island effect.

Heat island effect

In one sentence, heat island effect is a phenomenon where the air temperature in cities and built environment is higher than the natural temperature of the area without the built environment.

There are several factors which, when combined, raise the air temperature in cities.

Low albedo surfaces. If the surface absords large percentage of energy from sun rays, it is said to have low albedo. Asphalt, pavement and other dark surfaces, which are very common in urban environment, contribute significantly to worsen the heat island effect.

Residual heat. Building systems, such as ventilation, which lower temperatures inside the buildings have to get rid of the excess heat. This heat is thus released into the urban environment.

Form of the cityscape. Massing of the cities usually works against the flow of air, which would normally sweep through and take the hot air and redistribute it around wider area.

The effect of higher air temperature is welcome during winter months in some areas of the world, but it does decrease the comfort level and the livability of the city in hot and tropical climates.

The local and global effect of the increased release of aerosols and greenhouse gases has a negative effect on livability in all cities, independent from their location.

Electricity should become the predominant source of energy in the city, because it does not release the local emissions and provides the highest efficiency for most of the needs we have.

The reduction of air pollution has become a primary goal of many cities and countries. As the production of energy for end use causes a more or less significant pollution, it is a prime target of attention.

In general, the production of energy should occur outside the city, to minimize the negative effects in terms of heat and pollution on the citizens.

The production of clean energy must be of highest priority. All of this leads to the conclusion that architecture and urban design need to become more regional and more local again.

We should see this as an enrichment, rather than enforcement. The more buildings and cities become responsive, they will increase the quality of life of the inhabitants.

Effects of heat island

The urban heat island is a daily reality for billions of people living in cities. Increasing temperatures decreases human comfort in hot and sub-tropic climates and raise mortality rates at temperatures outside an optimum range.

People living in urban areas exposed to the urban heat island are at greater risk compared to those in non-urban regions. Together with future temperature increases caused by global warming, higher temperatures pose particular challenges, especially in tropical cities, since they already have high background temperature.

Other issues arising from extra heat in the tropics include higher energy demand from increased use of air conditioning which is usually present throughout the entire year and changes to biological activity and disease patterns. Increased demand for air-conditioning further raises the temperature in the city, creating a positive feedback loop.

It is clear that cities are a major source of environmental problems, but they are also a potent force for more sustainable development that should be harnessed.

The urban heat island is a consequence of imbalance between urban/rural energy. A useful starting point for efforts to reduce the urban heat load is the consideration of the individual terms of this energy budget and their dependence on the urban fabric and geometry.

Mitigation

A variety of options are available for urban planners and architects to intervene and achieve the desired mitigation outcome. They include modifying the radiative, thermal, and evaporative properties of the urban surface, and its geometry and morphology.

Urban geometry has important implications for ventilation as well as incoming and outgoing radiation and, therefore, heating and cooling patterns.

The geometry of urban canyons is a major cause of the urban heat island by preventing heat stored during daytime in buildings and roads to freely escape at night. This effect is larger for narrow canyons.

The surface materials used, through their radiative and thermal properties, also greatly influence surface, and, hence, air temperatures. Bright surfaces generally have high reflectivity (albedo) and are able to reject a larger portion of the incoming solar energy back to space.

This reduces the radiative heat gain in the material, which will stay cooler compared to one with a dark-coloured surface. In terms of thermal properties, most urban building materials are characterized by high heat capacities and are therefore good reservoirs of heat contributing to higher urban temperatures.

Through choosing appropriate materials, it is possible to regulate the surface temperature and heat storage of urban materials with beneficial effects for air temperature.

Controlling surface temperature is another effective measure to mitigate urban heat considering that in most cities large areas are covered by roads and other paved surfaces. If those areas are shaded, significant portion of sun rays is reflected back into the atmosphere instead of absorbed.

Thanks to research and further understanding of complex realities of built environment, we now know what are the causes of heat island effect and how to mitigate its effects. Thus the next step is implemantation.

So use this knowledge next time you are designing cities.

Take care.