Considering that we live in an ever rapidly urbanising world, with current projections predicting that by 2050, there will be 6 billion people living in cities [1], it is important to address the issues that come with such urbanisation. One of these issues is that of the urban heat island effect.
The urban heat island effect is a term used to describe conditions when urban areas are warmer than their surrounding rural areas. It forms due to the concrete nature of a city. Rural areas have far more vegetation than urban areas and incoming solar radiation acts to evaporate water from the vegetated areas and soil. However, in cities where there is less vegetation, the incoming solar radiation is instead absorbed by the urban infrastructure [2]. Evaporative water helps to carry heat away from an area, therefore if there is less of this occurring in urban areas, the surfaces of buildings will heat up during the day, meaning there are higher surface temperatures in urban areas than the surrounding rural areas [2]. During night-time the solar energy is then released into the surrounding air, which increases the temperature, along with the effect of heating and cooling units used throughout a city [2].
It is not only the artificial materials that make up the infrastructure of a city that result in more heat being absorbed, but also the structural form of a city. Rural areas are generally characterised by smaller and shorter buildings and cities are characterised by tall skyscrapers and narrow streets. This “canyon effect” helps to inhibit the loss of heat by trapping it within the cities confines [2].
The problem of the urban heat island effect appears to be one that is only going to grow as our cities continue to sprawl and the migration rates to cities increase, especially in the developing world. It is estimated that by 2040, the Met Office may need to issue heatwave warnings four times more frequently for urban areas [1]. The Met Office also predicts that we could experience night-time temperatures of around 25oC due to the urban heat island effect [1]. In 2003, Britain had a heatwave that killed 2,000 people and it is thought this was mainly down to night-time temperatures remaining around 20oC [1].
Urban Planning
There are ways to reduce the impact and likelihood of the urban heat island effect, and some things that we can all do. One of the reasons why urban areas heat up so much is due to the nature of their surfaces. Many cities, especially in temperate climates such as the UK, are made up of dark surfaces such as tarmac. Such surfaces readily absorb heat, so by changing the nature of our surfaces we can help to reduce such an effect. By using high albedo materials, more incoming solar radiation will be reflected and therefore not absorbed by buildings. The albedo is the “proportion of radiation reflected from a surface. Surfaces such as snow have a high albedo” [3].
Changing the roof surfaces of buildings or ensuring that high albedo roofs are installed on new builds is one of the most effective ways of mitigating against the urban heat island effect due to the large surface areas that roofs occupy. A typical red dark-tiled roof has an albedo of 0.18-0.33 where as a white roof membrane has an albedo of 0.80.
The implementation of green roofs and green spaces is another mitigation strategy. Increasing the amount of vegetation in urban areas is important as incoming solar radiation evaporates water from vegetation thus taking heat away with it. Green roofs can be placed on any size of building and not only help to mitigate against the urban heat island effect but also add to the visual attraction of an area.
One of the reasons for higher air temperatures in urban areas is the heat that is produced by technology such as computers, household appliances and heating and cooling systems. It may sound simple, but if we all made an effort to turn our appliances off when they weren’t being used this would prevent a large proportion of waste heat energy being released. The need for heating and cooling devices would also be greatly reduced if we ensured that our homes were properly insulated.
The urban heat island effect is a phenomenon that is only going to grow as the world continues to urbanise. It is vital that we take measures to reduce it’s impact.
References
[1] Webster, B. (2010). 20C and rising: climate change will make cities even hotter at night. The Times Newspaper 2010.
[2] Ahrens, C. (2005). Essentials of Meteorology. Belmont: Thomas Brooks/Cole.
[3] Holden, J. (2005, p.611). An Introduction to Physical Geography and the
Environment. UK: Pearson, Prentice Hall.
[4] http://earthobservatory.nasa.gov/Features/GreenRoof/Images/atlanta_thermal.jpg
