Urban Heat Islands

Urban Heat Islands

Understanding

Urban Heat Islands – UHI – is defined as an increase in temperature in any man-made area resulting in a "warm island" among a "cool sea" - cooler temperatures in close natural landscape areas (Figure 1) [3]. 

[pic 1]

Figure 1 - Schematic representation of a typical urban heat island. Taken from the Urban Heat Islands website. Original source: EPA, 2008.

According to Akbari, H. et al. roofs constitute around 20% of urban surfaces and pavements about 40% [1]. Although there are several causes for UHI occur, two stand out among them: First, dark colours absorb more light than light colours and by material proprieties of actual roofs, buildings and pavements they “absorb at least 80% of incoming sum light” generating more heat and releasing it slowly throughout night. Second, anthropogenic heat, for example fuel combustion and air conditioning, released at atmosphere [11].

Perth

The US Environmental Protection Agency affirms that cities with 1 million people or more can, annually, be 3ºC warmer at day and up to 12ºC at night [10]. Bearing in mind Perth, with more than 2 million people [4], the UHIs effect are strong throughout places where canopy cover is low (see Figure 2), since albedo – relation of light reflected by an object to that received by it – decreases.

[pic 2]

Figure 2 - Canopy cover (%) for selected West Australian Local Governmental Areas. Source: University of Technology, Sydney.

Yang and Li made an analysis on “building density and height heterogeneity on average urban albedo and street surface temperature” [12], and analysing their article, it is understandable that Perth suburb areas have an urban average albedo (see Table 1):[pic 3]

Table 1 - Perth urban average albedo according to analysis of Yang and Li (2015) studies.

Note 1 – This data is an average, consequently  it is clear that a suburb can - and will - have different albedo from the other.

Places with less canopy covered area were verified with higher temperatures than those ones with higher canopy cover [7],[13]. As a result of the lack of natural surfaces and the increase of built surfaces, Dr Paul Barber – Director of ArborCarbon [2] – said that suburbs “can be up to six degrees warmer than some of your older, more established suburbs with high canopy cover - such as Wembley Downs and Subiaco”. Still according to Dr Barber, Perth Airport is the hottest location of Perth and Piara Waters, Clarkson and Butler have just 0.5ºC less than the airport. Moreover, Westminster suburb is as hot as the airport with “less than a degree cooler” [13].

Effects

It is known that constant heat is not good to public health and Zhao, L. et al. say that temperature difference in cities with humid climates is double than dry climate cities and alert that UHIs will intensify the heatwave at humid climate cities [14]. Likewise, Masumoto warns that continuously extremely hot days will increase the number of patients with heat stroke (see Table 2).

[pic 4]

Masumoto continues saying that “it is not an exaggeration to say that the lives of elderly people”…“infants with imperfect thermoregulation, are in danger”…“even in ordinary homes” [8]. Furthermore, Dr Vogel, former heat of WA Environmental Protection Authority, says that “Heat-wave related deaths in Perth are predicted to more than double by 2050” [5].[pic 5]

Another important effect of UHIs that is not largely broadcast is its influence on the measurement of global warming, since suburbs are each day closer to weather stations there is an increasing on temperature in that local, therefore misguiding those stations results, for more details look at Global Warming Solved.  

mitigation

There are two main ways to mitigated UHIs effects. First one is “using cool roofs and cool pavements in urban areas, on an average, can increase the albedo of urban areas by 0.1” [1]. A group called HEAT ISLAND GROUP [6] develops new technologies trying to increase materials albedo without compromise their resistance or effectiveness. Second one is increasing vegetation cover [3].

Perth with WA EPA in its “Perth and Peel @ 3.5 Million” program stated 36 recommendations to Perth, here are three that are related solutions to UHIs effects:

1. Add important areas of remaining native vegetation to the conservation estate;
2. Implement a long-term integrated transport plan that reduces traffic congestion, improves air quality and encourages increased uptake of walking, cycling and public transport; and
3. Take steps to counteract impacts to air quality, human health and amenity from the ‘urban heat island effect’, such as through tree planting [5].

dicussion

Although there are various approaches and techniques trying to understand the causes of UHIs and recent technology improved those methods, all studies cannot create a solid conclusion, since there are limited monitoring data or they are not able to cover all variables simultaneously [9]. Similarly, measuring day-to-day temperature is not an accuracy method to understand the causes of UHIs, it is because as there are geographical factors influencing long-term variations, there also are local phenomena that change temperature [8], and these phenomena can come from anthropogenic activities. However, a difference in temperature between urban areas and its surrounds exist and its effects can be dangerous, therefore must be mitigated in all ways possible.