Urban Heat Island Mitigation: Comparing Strategies?
Urban Heat Islands: Why They Exist and Their Implications
|Figure 1. Variation in UHI Intensity in different areas of a city. Source.|
The world’s population is becoming ever more concentrated in cities. In 1960, only one third of the world lived in an urban area. By 2014, fifty-four percent of the seven billion people on Earth were city dwellers. By 2050, the UN expects that over six billion will inhabit the world’s cities (United Nations). With such projections, it is important to understand and examine urban conditions and to understand how they may affect our health.
One of the conditions that are unique to cities is elevated temperatures. These areas are called urban heat islands, and are caused by a number of factors. Firstly, the land cover of urban areas (buildings, roads etc.) tends to be darker in color, meaning that more solar radiation is absorbed by these surfaces. They warm and heat up the surrounding areas. Secondly, the geometry of downtown areas (high-rises, skyscrapers etc.) reflects the sun’s rays around, causing an even higher rate of absorption. Thirdly, cities typically have a higher concentration of pollutants, including cars and factories, which put greenhouse gases into the surrounding atmosphere. These gases help to trap any reflected and emitted radiation in the area. Lastly, a generally lower concentration of vegetation in cities means that less of the CO2 that is put into the environment can be used for photosynthesis.
With the right weather conditions (clear skies and stable atmosphere), these factors can combine to create a difference of up to 18˚F relative to surrounding rural areas (Shepherd, 2010). Additionally, heat waves are generally more intense—especially at night, when urban areas cool more slowly—and last longer as a result (Shepherd). A notable example of this was the 1995 heat wave in Chicago, during which over 700 people died from heat-related causes in less than a week. Additionally, heat islands have negative implications for air quality as well as energy and water consumption.
Overview of Mitigation Strategies
As awareness of the UHI effect has become more widespread, cities across the globe have begun to take measures to reduce the severity of their heat islands. The most common strategies are quite simple, and involve finding ways to stop urban surfaces from storing heat or finding ways to deal with the release of CO2 into the environment.
Cool roofs are built form materials that have a high albedo as well as a high rate of thermal emittance. The latter allows the buildings to release some of the heat stored inside. In warm climates, such as around the Mediterranean, roofs have commonly been made of white mortar or plaster as far back as ancient times.
|Figure 2. Green roof atop City Hall in Chicago. Source.|
Green roofs are similar to cool roofs, but instead of reflecting solar radiation, these plants use it for photosynthesis, and that energy is released through evapotranspiration. They also help to improve air quality—CO2 for photosynthesis—which, as mentioned above, can be negatively affected by the higher concentration of pollutants in these areas. Green roofs also help to lower the internal temperature of the buildings on which they sit, and can therefore reduce energy consumption.
Increased Vegetation and Trees
Perhaps the most simple mitigation strategy is to increase the amount of vegetation on the surface of these urban environments. Unfortunately, since many cities are so densely built—especially in the downtown areas, where temperatures are typically highest—it can be tough to find enough space to plant a group of trees or for a small park. Trees may also eventually have negative consequences for integral parts of urban infrastructure, such as underground pipes, which can be damaged by roots as trees continue to grow.
Increased vegetation at the surface provides the same benefits as green roofs—helping to cool buildings and improving air quality—while also improving water quality through reduced runoff, and can also preserve the life of pavements by cooling offering shade during the day.
Green/Cool Parking Lots
Many urban parking lots are covered by asphalt with limited vegetative cover, and so another strategy to mitigate elevated temperatures is to cover these parking lots with grass and trees or to use materials with a higher albedo. These parking lots cover about 10% of the surface in American cities, so they can certainly be a significant contributor to the warming of the area (Onishi 2010).
Factors to Consider For Strategy Implementation
While many cities have similar spatial patterns, none are exactly the same, and this has consequences for each city’s heat island. As Debbage and Shepherd have shown, the contiguity of a city is an important factor in determining the intensity of its heat island (2015). The density of a city is not as important as its contiguity; however it is more difficult to find space for vegetation in high-intensity urban areas, therefore making it more difficult to implement certain mitigation strategies. The distribution and location of its largest buildings also matters—skyscrapers close to the coast in Miami, for example, likely obstruct the cool breeze coming in from the sea (Debbage and Shepherd, 2015).
Another factor to take into account is the climate of a city. In arid climates, green roofs and/or increased vegetation may not be as viable an option as cool roofs; the amount of water used to maintain these plants could outweigh the benefit of having them. In wet climates, however, strategies involving vegetation—green roofs, trees, or parks—may be the better choice. There is also the possibility that a city is prone to extreme weather events, such as heat waves. In Atlanta, for example, there were 22 heat waves between 1984 and 2007, with an average length longer than 14 days (Shepherd, 2014).
Results & Analysis
Green and Cool Roofs
A study done by Zinzi and Agnoli examined the effects that green and cool roofs had on energy consumption in three Mediterranean cities with differing climates—Barcelona, Palermo and Cairo. The study was done for an entire year, so as to account for the various seasonal characteristics of each city (see Figures 3 and 4).
The energy consumption was measured relative to the amount used in houses that had standard roofing.
|Figure 4. Mean rainfall by season in Barcelona, Palermo and Cairo (Zinzi & Agnoli 2012).|
|Figure 3. Mean Temperature by season in Barcelona, Palermo and Cairo (Zinzi & Agnoli 2012).|
The results certainly give some insight into the effectiveness of these two mitigation strategies (see Table 1). In the hottest, driest climate of the three (Cairo), the standard cool roofs were the most effective. But in Barcelona, the city with the most seasonal variation, the cool roofs actually caused an increase in energy use, the vast majority of which was used for heating. This indicates that the roofs were doing what they were supposed to—keeping the house cool—but this actually became counterproductive due to the climate in Barcelona. The low-emission cool roofs produced the most consistent results, regardless of climate.
These results indicate that cool roofs work well in areas where it is relatively hot and dry year round, while low-emission cool roofs and green roofs would work better in climates that have more variation from season to season.
This study focused solely on energy consumption, which does depend on the temperature within the house, but now we will look at a study showing how green roofs affect temperature both inside and outside of a building.
|Row (%)||Detached (%)||Mean (%)|
Table 1. Energy consumption results by roof type for Barcelona, Palermo and Cairo (Zinzi & Agnoli 2012).
This next study was done by Pompeii II & Hawkins using two scaled building models—one with a green roof and one with a standard roof—during the course of the summer months in Pennsylvania (2011). The averaged results of the study are shown in Figure 5.
As we can see, the green roof had a significant effect on the in-building temperature, especially during the warmest hours of the day. It did not, however, have as much of an impact on the outside temperature. Interestingly, the green roof also caused the building to be warmer during the night.
|Figure 5. Average indoor and outdoor temperature in the study done by Pompeii II & Hawkins (2011).|
That urban parks are generally cooler than their surroundings is well known, but not as much may be known about the cooling effects that they have on their surroundings.
|Figure 6. Google Earth image of the park in Nagoya. For reference, the yellow line is just over a mile long.|
A study was done by Hamada & Ohta in Nagoya, Japan, a city of almost ten million people. They examined the cooling effects of a large urban park (147 ha) on its surrounding areas over the course of a year (see Figure 6). What they found was that the park had a cooling effect at up to almost 500 yards, and 300 at night (of course, the cooling effects decrease with distance from the park). They also concluded that the most effective way to mitigate increased urban heat was to have small, scattered green areas throughout the city, rather than a few large ones. This is because regardless of its size, each park can only cool up to a few hundred yards around it.
|Figure 7. The park in Lisbon (Oliveira, Andrade & Vaz 2011).|
Another study was done on a much smaller park in Lisbon, Portugal. In contrast to the park in Nagoya, this one is only about the size of a block (see Figure 7). Despite its much smaller size, the park had a cooling effect at almost the same distance as the park in Nagoya (see Figure 8). Similarly, they concluded that having many small green areas was the best way to reduce the ambient temperature.
|Figure 8. Temperature vs. distance from the park in Lisbon (Oliveira, Andrade & Vaz 2011).|
Bodies of Water
Another possibly effective mitigation strategy is to introduce bodies of water. Because of its high specific heat, water can absorb large amounts of energy without much warming. In fact, it is estimated that a body of water with surface area of 20 square yards can cool an area of 3700 cubic yards by almost 2 ⁰F. Of course, since cities are generally densely packed areas, finding the space for large bodies of water would be difficult. Introducing them in already-existing parks, however, could be a viable strategy, and would help to increase the cooling effect that the parks already have.
After look at all of these studies, it seems that small parks are the most effective—and most widely applicable—mitigation strategy. While roof strategies seem to be viable options too, they are more climate-dependent. Of course, parks are contingent on there being the space for them to be put in, but as cities continue to expand, it may be more realistic to implement in the new spaces as they grow outward. Furthermore, introducing bodies of water in already-existing parks—as well as new ones—could help to lower temperatures even more.
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