Abstract
The rise in urbanization has been blamed for a deterioration in the urban environment in terms of increases in temperatures. This study will focus on urban heat island (UHI), which involves a rise in air temperature in cities, as compared to the surrounding rural areas. UHI effects are common in the Middle East cities, where a large segment of population resides in cities. This study will also cover the effects of urbanization on the increase of air temperature in Jeddah and Riyadh in Saudi Arabia. The congestion of buildings in cities, and lack of vegetation cover causes UHI, as the building materials absorbs solar energy, thus creating UHI effect. UHI effect causes discomfort to humans, as it creates respiratory difficulties, exhaustion, and heat-related mortality. Rooftop surface temperature can trigger stormwater runoff while increased demand for energy in cooling encourages air pollution and greenhouse gas emissions. Increased demand for energy is a burden to many city residents, who have to install cooling systems in their houses. Urban developers, environmentalists, and governments, are working on the mitigation strategies that can prevent steady rise in temperature, which include enlargement of surface reflectivity, enhanced vegetation cover, and use of cool roofs and green roofs. City parks and tree canopies along city streets have helped in regulating heat in city, thus making urban life more comfortable.
Mitigation of the Urban Heat Island of the Jeddah
Introduction
The rise in urbanization and industrialization has resulted in deterioration of the urban environment through rise in temperatures. This has made the study of urbanization essential for urban developers and governments, who incur exorbitant costs through energy consumption and installation of cooling systems in city buildings. Buildings in the urban areas are constructed using materials that are waterproof and non-reflective, thus capable of absorbing a large quantity of the sun’s radiation, which they release as heat. This action occurs where building are clustered, creating the urban heat island (UHI) effect. UHI has numerous negative effects, which include increased energy requirements for cooling apparatus, rise in air pollution and greenhouse gas (GHG) emissions as well as spoiling water quality. UHI effects are common in the Middle East, and some parts of North Africa, where a large part of population resides in the urban areas. UHI effects are essential to urban designers and governments, since they have been discursively connected to climate change through the greenhouse effects, and consequently to global warming.
Urban Heat Islands (UHI)
The urban climate is usually influenced by lack of proper development control that results in changes in the balancing of heat. Urban developers are experiencing challenges in their endeavor to make urban residents comfortable as they undertake their daily activities within the cities. When air temperatures around densely built urban areas become higher than the average temperatures in the surrounding region, this incident is referred to as the urban heat island(UHI) consequence (Santamouris, 2009). UHI is a manifestation of the microclimatic changes resulting from man-made adjustments of the urban surface. Heat intensity varies in different areas of the city, with the greatest intensity being recorded where buildings are very close to each other. In rural areas, a significant part of solar energy is involved in evaporation of water from vegetation and soil. On the other hand, less vegetation and soil is exposed to solar energy in cities, thus buildings and asphalt absorb much of the incoming radiation energy.
UHIs Causes
Buildings in the urban areas are the main cause of UHI effects, as their masses accelerate the thermal capacity, which create an impact on the city temperature. The difference in temperatures is due to energy flows as well as surfaces and structures. In energy flows, the cumulative effects of UHI are revealed in basic physical processes. In early evening, air temperature starts to decline in cities and rural areas, but rural areas tend to cool faster than urban areas, thus creating the difference in air temperature (Forman, 2015). After sunrise, the rural areas heat faster than the urban areas because the process of transpiration regulates air temperature. Lower wind speeds in cities also restrict evaporative cooling.
Surfaces and structures in both urban and rural areas reveal the difference in air temperature. Lack of adequate vegetation in cities results to low evaporation rates, in addition to high absorption of heat energy. Rural areas have considerable vegetation cover with a lot of moisture in the soil. Hence, evapo-transpiration, in addition to latent heat flows remains higher in vegetated regions than less vegetated cities. In cities, there is a small upward latent heat flow while a considerable latent heat proceeds upward without necessarily warming the air in rural areas. Extensive impervious surfaces, such as steel, concrete blocks, and asphalt absorb a lot of heat, thus affecting the concentration of a heat island. The heat island intensity usually rises with the increase in height-to-width ration of buildings. The narrow arrangement of city buildings creates urban canyons, which obstruct the escape route of the reflected radiation, causing high absorption of heat by buildings, boosting the urban heat release. Figure 1 shows arrangement of building in cities and thermal storage.
Figure 1: Thermal storage of solar radiation in cities
A synergy exists between heat wave and UHI in the cause of temperature rise in cities. A heat wave can be recognized as “a sustained period of excessive hot days during which the temperature is significantly higher than the average climatological mean” (Li and Bou-Zeid, 2013, p. 2051). A heat wave typically spread over a large surface, causing a rise in air temperature while the UHI create an effect locally due to the urban terrain. Both incidents can interact where heat waves reinforce secondary circulations, and while the hot air rises from the cities, the cool air coming from the surrounding area replace it, creating a negative feedback upon the UHI strength. In another instance, at a low wind speed, the UHI effect tends to be stronger because the reduced horizontal movement cooling by low-temperature air coming from the surrounding rural areas. The low wind speed is likely to create an interaction between heat waves and UHI effects, as shown in figure 2 below.
Figure 2: Heat wave and heat island interaction
Impacts
Urbanization plays a significant role in the spread of UHIs. Although some impacts of UHI can be beneficial for lengthening the crop-growing season, in most cases, effects of UHI are negative. It influences the environment through pollution and alteration of physical, and chemical properties in the atmosphere. UHI affected both city dwellers and the ecosystems within the city. UHI effects have negative impacts on human health and animals. UHI effects are connected to heat related illnesses and casualties where thermal discomfort has an effect on the human cardiovascular and respiratory system. Increased daytime temperatures and reduced nighttime cooling cause general discomfort, heat cramps, and non-fatal heat stroke (“Heat Island Impacts,” 2013).
UHI effect has an impact on energy consumption. Elevated summertime temperatures around cities trigger the need for energy in cooling equipment. According to Environmental Protection Agency, the demand for electricity in cooling rises by 1.5 to 2.0% every time the air temperature rise by 0.6oC (“Heat Island Impacts,” 2013). The UHIs cause electricity demand rise during the hot summer afternoon, which could result in overloading the system or power outages. Increase in energy consumption in refrigeration and air-conditioning lead to high production of energy by power plants, which consequently lead to emission of GHGs and other pollutants. Both residents and energy providers have to incur extra costs to take care of the rising demand for energy.
Altering land use through constructing impervious urban surfaces results to increased runoff, low evapo-transpiration, and increased anthropogenic heat (Loughner, et al, 2012). Stormwater runoff poses a severe threat to aquatic life if no measures are taken to curb it. High pavements and rooftop can contribute to rise in air temperature, which can accelerate stormwater runoff. The heated storm water becomes runoff, which ends up in storm sewers causing a rise in water temperatures in rivers, lakes, and ponds. Water temperature has an effect on aquatic life, particularly the metabolism and reproduction. Rapid change in temperature due to storm water runoff can become stressful to many aquatic animals. High temperatures have also been found to cause urban-induced precipitation, as well as thunderstorms.
Mitigation of the UHI
It has become increasingly essential for urban developers to come up with UHI mitigation strategies to minimize consumption of energy and enhance quality of life bearing that rapid population growth is eminent in the future (Shahmohamadi, et al., 2011). In countries where large segment of the population resides in cities, the governments, environmentalists, and other agencies are working on mitigation strategies on UHI to prevent casualties resulting from heat-related incidents. Use of “cool” roofs and vegetation has been highlighted as the most suitable measures to upgrade urban climate. Two strategies that can assist in alleviating UHI effects are:
- Increase surface reflectivity: This method can be utilized to reduce radiation absorption on urban surfaces. Reflective surfaces can be enhanced through painting white color to urban walls, or constructing urban structures using light colored materials. Light colored materials can be used to construct roofs and pavements, which can significantly lower the temperature within the city region. Increasing the albedo on the surfaces will discourage heat absorption and consequently decrease the level of temperature.
- Increase the level of vegetation cover: Through planting more trees and constructing urban parks, vegetation cover can enhance evapo-transpiration in controlling the rise in temperature. Vegetation’s evapo-transpiration contributes in cooling the atmosphere in a natural process where heat is removed from the air. Trees can provide shade and places to hide from the scotching heat. Vegetation should consist of ground-level plants that have a fewer restrictive growth and trees. Urban green spaces have been critical in offering cool atmosphere in hot urban areas (Norton, et al., 2015).
Use of cool roofs and green roofs are essential in controlling stormwater runoff. Green roofs are rooftops covered with vegetation, which is planted on a waterproofing membrane to absorb rainwater, and to lower the UHI effect. According to the Natural Resources Defense Council, green roofs have the capacity to utilize solar radiation through evapo-transpiration process, which results in latent heat loss and lower the air temperature in the surrounding area (Garrison, Horowitz and Lunghino, 2012). Figure 3 below illustrate a green roof city.
Figure 3: Green Roof City
UHI in Saudi Arabia (Jeddah)
Urbanization and Human Activities in Jeddah
Arab countries are vulnerable to UHI effect, where nighttime temperatures in most cities exceed the surrounding rural areas. A couple of studies have been carried out on the UHI effects in the Kingdom of Saudi Arabia (KSA). One study focused on air temperature trends and concluded that an increase in annual temperature in Saudi cities was because of the UHI effect. Another study that examined the relationship between urbanization and air temperature trends revealed that population growth in cities has little effect on temperature change.
Jeddah, the second biggest city in the KSA, has a dense building area at its central area with an open area near the old airport location. The city is within the vicinity of the Red Sea. The area surrounding the New Airport is categorized as non-urban area. In 2011, the city experienced a severe storm that resulted in heavy rainfall, which caused a flash flood, and at least eleven people died while more than 114 others were injured (Samman, 2014). After a thorough research on the cause of abnormal level of rainfall, it was discovered that warm surface temperature from the sea, which was accompanied by high humidity and a convective potential energy contributed to the storm. A strong wind from the east brought large water vapor over Jeddah, which interacted with the UHI effect to create the storm.
A study was carried out in Jeddah to establish the cause of UHI effect. The study was sponsored by King Abdulaziz University, and during the entire period of study, air temperature was taken three times each day twice a month (Abdullah Ambar, 1991). The study also utilized several instruments, which include a mobile station to traverse the region within two hours and air temperature sensors to enhance accuracy in measuring temperatures. The mobile station had to collect temperatures before sunrise when minimum temperatures were expected; mid-noon for maximum temperatures; and after sunset when the breeze was constant. The data was also collected in the winter.
The study revealed that a hot center existed around Madinah road, an area with dense buildings, while a cold center was discovered in the open area around Almatar Alqadim (Abdullah & Ambar, 1991). The situation is reversed during early afternoon, when the sea breeze influences the air temperature around dense building. In early afternoon, the urban boundary layer seemed to be capped by an inversion that was lifted to about 700 meters. Thus, the early morning temperatures proved that UHI effect was present in Jeddah. This study was essential for urban development, as well as city designers, so that they could make decisions on how to avert rise in temperatures and reduce energy consumptions among city residents.
To avert such storm in Jeddah in the future, researcher proposed a change in physiographic properties, alteration of urban heat island effect, and obstructing the concentration of clouds above the city. Much of the energy consumption in Jeddah city goes to ventilation and cooling systems. This has made real estate investors to generate power from solar panels for each building to minimize energy costs. Solar thermal collectors are being utilized to heat up water that circulates in the cooling systems. Currently, Jeddah is among the few cities in the arid areas whose temperatures are below the surrounding region. This is because the city has managed to increase the vegetation cover in the urban area, thus creating a cooling effect in the region. In 2015, Jeddah residents came together to plant more than 60,000 plants during the Tree Planting Week (Khan, 2015). This campaign has increased the vegetation cover in the city, in addition to providing cooling effect. Figure 4 depicts vegetation cover in Jeddah City.
Figure 4: Jeddah City’s vegetation cover
However, Jeddah can do more in curbing UHI effect by devising honeycomb housing layout, which can allow urban landscaping and wide roads. This method has succeeded in reducing temperature in other cities, as it creates a tree canopy over city streets, which reduces the heat island effect (Davis, Reimann & Ghazali, 2005). Honeycomb is a long-term plan, as it can take between five to ten years for the trees to mature. Short-term plans include the cool house and cool paving technologies, as well as green roofs. Cool house technology utilizes computer simulation programs to regulate temperatures. Cool paving technologies include high albedo cool pavements and permeable concrete, which decrease absorption rates if sun’s energy.
I believe that Jeddah is on the right track in mitigating strategies to minimize UHI effect, but more vegetation cover is necessary to enhance the reduction of air temperature in the city. Vegetation cover could enable the city to save on energy consumption through cooling systems. In areas where establishment of vegetation cover is difficult, painting wall in light color can help in reducing absorption rates of sun energy. Appropriate plans should be made before expanding the city to avoid congestion of buildings.
Conclusion
Congestion of building in urban areas has contributed significantly in the rise of air temperature compared to the surrounding rural areas. This phenomenon is usually referred to as UHI effect, and it occurs when extensive impervious surfaces, such as steel and tarmac, absorb much heat, thus creating a concentration of heat around city buildings. UHI has negative effects on the life of city residents in terms of energy consumption, water quality, and general health. Understanding whether urbanization contributes to large-scale warming is essential not only to human comfort, but also to other areas such as energy and water conservation. Several cities in the Middle East have experienced unusual levels of rainfall, which resulted from rise in air temperatures that meet with cool air temperatures from the surrounding regions. Mitigation strategies to reduce UHI effect include raising the surface reflectivity and increasing the level of vegetation cover in urban regions. Urban designers should consider the mitigation strategies to avoid making mistakes as they design future cities.
References
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