Development in the urban areas causes changes in the landscape. Vegetation and open space is replaced by the buildings and infrastructure and the permeable surfaces get converted into impermeable surfaces. In a typical urban area, the surfaces are darker, impermeable and the vegetation is relatively lesser. The modified land surface in cities, compared to rural environments affects the storage and transfer of both radiative and turbulent heat (Parvantis, Stigka, Fotiadi, & Mihalakakou, 2015). The leads to a phenomenon where the heat gets accumulated due to the urban construction and other human activities leading to urban heat island (UHI) effect. The most noted observation about UHI effect is that the temperature of urban region is higher than the rural region (EPA, 2008). This difference can be as much as 2.50C during the summers (Akbari et al, 2001). The urban heat island phenomenon is caused by two factors:
- intrinsic nature of cities (anthropogenic activities like vehicles, air conditioners, etc. along with buildings’ morphology, urban canopy, wind blocking, surface characters and land use planning forming the urban structure of a city)
- extrinsic factors (climate, prevailing weather circumstances and the seasons)
Fig 1: Factors responsible for Urban Heat Island Effect. (Source – Osmond, 2017)
These factors increase the energy consumption in buildings in the effort of providing thermal comfort and results in increased air pollution, eventually leading to increased greenhouse gas emissions and negative impact on health of citizens of developing cities (TERI, 2017). It has been noted that 1o C rise in temperature leads to 2-4% increase in electricity consumption (Akbari et al, 2001). Urban flooding during heavy rains, traffic congestion along with higher level of air pollution, diminishing lakes, increase in temperatures during summer are some of the negative environmental impacts on a city due to rapid urbanization (TERI, 2017). . These environmental impacts are a result of the combination of Urban Heat Island and Global Warming effect. This article talks about the Urban Heat Island effect and explores the cooling strategies adopted by Sydney to reduce the impact.
Causes and Effect of Urban Heat Island
Due to the lack of green spaces and the effect of the intrinsic and external factors discussed above, surface temperature rises (Osmond, 2017). Natural surfaces absorb more radiation in comparison to man-made structures like roads and buildings having lower albedo. As a result, natural surfaces is always cooler than an urban surface. Evaporation from water releases energy and cools the surface temperature. As the heat capacity of asphalt and concrete is lower than other types of surfaces, the solar radiations falling on the built surface causes the air temperature to rise. Therefore, rise in surface and air temperature is directly proportional to the height of the built-up areas. Conditions of the available natural resources and the climate in the urban ecological system is affected by the increased surface temperature (Ningrum, 2018).
Fig 2: Differences between day-night surface temperature and air temperature in typical land use types. (Source – Osmond, 2017)
Urban Heat Island and Climate Change
Changes and development in radiative and thermal properties of urban infrastructure are causes of Urban Heat Island. Also, the functioning of a buildings has impacts on the local microclimate, for example, the rate of cooling at night is slowed down by tall buildings. The heating effect occurring in cities or specific areas leads to a change in the climatic conditions of the region leading to local climate change. Local climate change is different from global climate change; their effects are limited to the local scale and decreases as the distance increases. Global climate change caused by increase in sun’s intensity or greenhouse gas concentrations are not locally or regionally confined (EPA, 2008).
Adaptation and Mitigation Strategies
Urban areas need better development planning and a balance between social, ecological and economic factors. The correct ratio of built-up and open spaces, control over the growth of built-up areas, more sensitivity for the open spaces are few of the many things required. This can be done through spatial development planning associated with sustainable development and creating a comfortable urban environment. Some of the adaptation strategies can be:
- Developing the green and blue areas within a city
- Managing the growth of built-up areas of the buildings
Suitable areas can be developed by forming roof gardens or more trees can be accommodated in the streets as they are a better heat-stress suppressor (Ningrum, 2018). To mitigate the urban heat island effect, the thermal environment around the buildings should be improved by using material of lower absorptivity, larger thermal conductivity and higher reflectivity (Ningrum, 2018). Durable white roofing materials and cool coloured roofing available for coating, tiles, painted metals, and fiberglass asphalt shingles are being produced by manufacturers (Akbari, 2016). To directly reduce the energy use in buildings, shading devices, trees and cool-roofs should be installed more often. In addition to cool roofs, urban vegetation and higher albedo and emissivitypavements reduce the temperature of the surroundings by a few degrees (Akbari, 2016). For water drainage, many paving materials and paving surface technologies have been characterized such as coloured concrete, white topping, chip seals, permeable pavements and grasscrete. These cool paving technologies are currently used in many specific applications (Akbari, 2016).
Effective urban cooling in a city requires the correct strategy of cooling depending on the available factors, factors like the state of development, aspect ratio, sky view factor. Inner city/ CBD, inner and outer suburb areas have different strategies for urban cooling and should be carefully examined as per the character of the city before implementing. Local weather conditions and spatial configurations should also be carefully considered before the application of urban cooling methods. The urban context of the cities can be divided in three categories (Osmond, 2017):
- Inner cities: Tall buildings surround the public spaces. Due to the shade of the buildings, urban surfaces are partially protected from the solar radiations. Therefore, in smaller public spaces like plazas, pedestrian open air malls, using high emittance cool-paving and building envelope treatments prevents ventilation and facilitates less heat storage.
- Inner suburbs: Two to six storey buildings surround the public spaces and due to the shade of the buildings, public spaces are partially protected from solar radiations. Depending on the city’s latitude, solar radiations may also reach the public spaces. To complete the shadow over urban canyons or around plazas temporary and tree canopy shade may be used.
- Outer suburbs: The development is low density in many cities and has a high sky view factor. Typically, in this urban form, areas comprise mainly of single or double storey buildings. The public spaces are generally not protected by the solar radiations by the shade of the surrounded buildings. Therefore, the main sources of shade in the plazas are tree canopy and shading structures.
Case-Study: UHI In Sydney
Intrinsic and extrinsic factors
Summers in Sydney are typically hot and humid. Highest monthly mean temperature of the city is 25.9 degree Celsius and daily sunshine during summers is of 7.1 hours on an average. With the maximum monthly mean rainfall of 117 mm, rainfall in summer is slightly lower than in autumn but higher than spring and winter (Osmond, 2017). The studies suggest that the Urban Heat Island Intensity (UHII) ranged from a mean of about 2-4o C and average daily peaks of 7o C (Parvantis, Stigka, Fotiadi, & Mihalakakou, 2015). Sydney increasingly experiences the UHII due to its numerous urban development projects (Sharifi & Lehmann, 2014). It is estimated that the combined effect of Global Warming and UHI will increase the temperatures by 3.7o C (Argüeso, Evans, Fita, & Bormann, 2014). The temperature in the urban areas ranges between 1.1o to 3.7o C as compared to the rural areas (ranging between 0.8o to 2.6o C), the UHII is much bigger during the nights (Parvantis, Stigka, Fotiadi, & Mihalakakou, 2015).
Suitable Urban Cooling Strategies
Depending on Sydney’s intrinsic and external factors, suggested strategies are more effective (Osmond, 2017):
- When the relative humidity in Sydney is high, the effect on outdoor thermal comfort by evaporative cooling and surface water will be lower. Central Sydney and eastern suburbs benefit from regular sea breezes in the summer afternoon improve the cooling effect of water. Misting fans for temporary cooling at pedestrian scale is pretty effective.
- Sydney usually receives high level of UV radiations and solar intensity during summer. Thus, the best suitable strategy is shading and increased tree canopy especially in higher density urban region.
- During most of the summer days, maximum temperature stays below 30 degree Celsius but surpasses 35 degree Celsius on some occasions. To radiate the urban heat away, the best practice is to use the high emittance paving. While addressing storm water management, permeable paving is a good option for urban cooling as Sydney has an average annual rainfall of 1221 mm.
- In low pedestrian and car traffic areas, especially in the CBD area, high albedo paving is a possible urban cooling strategy.
Fig 3: Summary of different cooling strategies suitable for Sydney
Humans are the main contributor to the urban heat island effect and solely responsible for the intrinsic factors of the urban heat island effect. Insulated buildings design and changes in human behaviour can reduce the energy consumption utilised to cool the indoor environment at the expense of heating up the outdoors. Climate responsive building designs can be really useful in minimizing the use of air conditioning by reducing the indoor temperatures during heat wave (Osmond, 2017).
The energy efficient measures such as increased level of insulation in roofs and walls, appropriate orientation, increased shading and application of reflective painting on the building envelope can develop better heat resistance in a building. The integration of these design techniques and adaptation in existing and new building designs could effectively reduce air conditioning and increase the indoor thermal comfort of the occupant.