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Urban Heat Island Effect: What's that?

Urban development is something that we and our audience are passionate about, and the process in which our cities grow is certainly paramount to our future prosperity as a community and society. However there is a phenomenon discovered by Luke Howard in the early 19th century that gets a bit lost amongst the construction and development. This phenomenon is known as the urban heat island effect (UHI) and it occurs when urban development replaces natural permeable surfaces such as grass land or bush land with dry and impermeable surfaces such as concrete and asphalt.

UHI is defined by a metropolitan area having an increased temperature of 1-3 degrees Celsius higher compared to that of surrounding rural or vegetated areas. Further to temperature changes, UHI can also affect localised meteorology by altering wind patterns, creating fogs and clouds and changing the rates of precipitation. The main cause of UHI is that buildings block surface heat from radiating into the relatively cold night air as the changes in thermal properties of the surface materials and the lack of evapotranspiration (i.e. natural cooling effect in vegetated areas) significantly alter the heat capacity and the thermal conductivity compared to rural areas.

So why should we care about UHI? What sort of implications could there be for the inhabitants of metropolitan Melbourne? The elevated temperatures from UHIs during the summer months can affect the community's environment and quality of life. Some effects can be positive via the lengthening of plant growth seasons, the majority are negative and can include:

  • Increased energy consumption: Higher temperatures in the summer periods increase the energy demands from people trying to keep cool and in turn adds pressure to the electricity generators. Based on current energy production trends, this in turn increases the production of greenhouse emissions which promotes the formation of ground-level ozone.
  • Compromised human health and comfort levels: As UHIs are characterised by increased temperature, they can potentially increase the magnitude and duration of heat waves within cities. The warmer days and nights along with higher air pollution levels, contribute to general discomfort, respiratory difficulties, heat cramps and exhaustion, non-fatal heat stroke and heat related mortalities.
  • Impacted water quality and aquatic life: As the storm water is not absorbed naturally, the increased temperatures of pavements and rooftop surfaces transfer their excess heat to storm water after a rain event which then drains into water ways. The variations in water temperatures can be stressful to aquatic ecosystems and in turn disrupt the natural workings of fragile environments.

An example of UHI effects can be seen below in the image showing the emissivity (i.e. heat distribution) at your average shopping centre complex which clearly shows the asphalt and paved areas absorbing and radiating much more heat (indicated by the red and orange colours) than the roofs of the shopping centre complex. Although this does not appear correct, the "cool" roofs (indicated by the blue colour) are regarded as an abnormality in the data collection approach because corrugated iron has low emissivity and therefore appear to be cold.

UHI effect at a surburban shopping centre. Image courtesy AECOM.

Furthermore an additional image of a metropolitan area, which is typical of much of metropolitan Melbourne's sprawl belt comprising of detached houses, show roofs and pavement radiating plenty of heat whilst the vegetated areas, namely the gardens and the lawns, not emitting heat into the surrounding environment.

UHI effect amongst detached houses. Image courtesy AECOM.

Even though some may see these effects as somewhat vague and not clear, the impacts that they have collectively in a large metropolitan setting such as Melbourne may have the potential to cause long lasting effects if they aren't addressed appropriately. As a result of many studies, appropriate mitigation strategies are available, and local councils have been working hard to gain an understanding of the effects of UHI in their particular municipalities. These mitigation strategies and the desired effects include:

  • Increase vegetation: Encourage evapotranspirative cooling and shade built surfaces. A good example of this in a urban Melbourne context would be the provision of rooftop farms and gardens which is something Urban Melbourne touched on earlier in the year.
  • Water sensitive urban design: Retain the storm water run off to increase evaporation, and increase water availability for irrigation.
  • Increased albedo: Increase solar reflection and in turn reducing heat storage in urban areas by allowing the heat to dissipate quicker.
  • Outdoor landscaping: Protect buildings from solar radiation using deciduous vegetation where possible.
  • Building and street design: Building structures orientated in a way that allows for passive urban ventilation and to balance the thermal distribution. Also improve building design to reduce cooling requirements.
  • Parklands and open space: Many parts of Melbourne certainly maintain plenty of open space, these areas provide local cooling in built up areas.
  • Mass/public transport: The final point is something that Urban Melbourne keeps advocating and will keep advocating. The use of public transport reduces private vehicle use which results in less waste heat from exhaust and also reduces the ground level ozone effects.

Increasingly, urban development both in vertical sense and horizontal sense is happening in Melbourne. Even though we face many different challenges including the effects of UHI as Melbourne develops, it is encouraging to know that there are practical and somewhat simple ways that could be incorporated either pre- or post-development and as a result we can help mitigate these effects as we move forward into a more sustainable future.

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Discussion (1 comment)

MelbourneGuy's picture

Noticed this very strongly in New York. Manhattan was always a few degrees warmer than the surrounding countryside on my previous visits.

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