Thursday, January 22, 2009


Implications of Urban Heat Island Formation

Typical urban surfaces, such as concrete and asphalt, get much hotter than vegetated surfaces during the day. They store the energy and release it at night, thus creating a dome of warmer air over the city. Hypothesized to result, in part, from the elevated heat capacity and waterproofing effect of urban construction materials, the urban heat island effect is believed to promote ozone formation, increase energy consumption, and exacerbate human and environmental heat stress (Cardelino and Chameides 1990). The increased heat of our cities increases discomfort for everyone, requires an increase in the amount of energy used for cooling purposes, and increases pollution. However, the exact nature of implications depends upon geographical and local climate situation. The major unwanted implications of urban heat island are thermal discomfort, increased cooling demand and air pollution.

Urbanization has a tremendous impact on air quality, both over the city and the surrounding countryside. Air quality attainment becomes a critical problem and is exacerbated by urban growth. We know that usually warm air rises above and leads to the development of a low-pressure area and cold air gushes in. but in cases of urban heat island, the warm air gets trapped in by the weight of the pollutants, which affects the air quality and makes the air heavier to rise.

Urban heat islands and air pollution are closely related in an urban system. Besides discomfort, urban heat islands also heavily contribute to an increase in smog production -- a serious environmental air quality health problem which especially affects breathing for children and seniors. The additional temperature acts as catalyst to enhance photochemical reaction, which increases the particles in the air, and thus contributes to the formation of smog and clouds. The presence of ozone creates smog and is the major environmental problem in many cities around the world. Smog is formed when air pollutants such as nitrogen oxides (NOx) and Volatile Organic Compunds (VOCs) -- mainly coming from cars and power plants -- combine with high outside temperatures, usually during hot summer months. Researchers have co-related that smog event increase by ten percent for each increase of 5˚F in temperature (EPA, 1992). Simply stated, smog formation is directly related to air temperatures -- the higher the air temperature -- the more smog that will be produced. According to the U.S. Department of Energy, a drop in air temperature of just a couple of degrees in urban areas can reduce levels of smog on the order to 5 percent to 10 percent, sometimes up to 20 percent -- by slowing down the cooking rate of smog.

Apart from ozone, some bio-genic hydrocarbons from emissions of automobiles are also expected to increase due to increased temperatures. A significant amount of SOx, NOx, and CO emissions take place from the evaporative losses during tank filling and transportation of petroleum products in the mega-cities.

The UHI effect prolongs and intensifies heat waves in cities, making residents and workers uncomfortable and putting them at increased risk for heat exhaustion and heat stroke. In addition, high concentrations of ground level ozone aggravate respiratory problems such as asthma, putting children and the elderly at particular risk. During periods of extreme heat, even trained athletes have been known to die from heat stroke and/or exhaustion. The extreme temperatures caused by UHIs will undoubtedly place a burden on the country’s healthcare system.

In terms of economic and infrastructure costs, additional cooling energy demand of electricity in commercial and residential buildings during summer is expected to be tremendous. Increased demand for energy can cost consumers and cities thousands of additional money in air-conditioning bills in order to maintain comfort levels. In the city of Los Angeles alone, estimates of up to 100 million dollars are spent on energy each year. The air-conditioning equipments discharge heat (which is derived from electricity produced elsewhere) in the urban atmosphere which ultimately contribute to the raising urban temperature. Further, the air-conditioning demand and outside temperature are very closely related thus requiring additional cooling load. This additional cooling load is again provided by electricity, a large portion of which is wasted as heat to urban atmosphere. Studies have shown that for each degree Fahrenheit the daily average temperature increases, electric power demand increases by nearly two percent (Akbari).

Beyond posing a threat to human health and raising air conditioning costs, the heat island effect can also cause physiological stress in other animals, change the mix of plants and animals that live in the area, and even lead to changes in the distribution of pathogens.

Moreover, added heat in cities can destabilize and change the way air circulates around cities. Rising warm air may help produce clouds that result in more rainfall around urban areas. Mostly during the warmer months, the added heat creates wind circulations and rising air that can produce clouds or enhance existing ones. Under the right conditions, these clouds can evolve into rain-producers or storms.

Modification of the landscape through urbanization alters the natural channeling of energy through the atmospheric, land and water systems. Although large-scale atmospheric and climatic phenomena are global in scope, urban areas cannot be viewed in isolation because the local environment modifies the conditions in the thin air stratum above the ground, generally referred to as the atmospheric boundary layer. As humans alter the natural landscape in the city-building process, the local energy exchanges that take place within the boundary layer are affected. Therefore, modification of the landscape influences the local (microscale), mesoscale, and even the macroscale climate.

Implications in Metro Manila

The worsening heat environment in the mega-cities has created a clear threat to the urban sustainability which is shaded by a surge of international interest in the global warming. The implications mentioned earlier clearly show the importance of improving urban heat environment and their role in ascertaining urban sustainability.

In the past several decades, there has been a worldwide shift from rural to urban areas. In Asia, it was estimated that by the year 2000, the urban population will have grown to 35% of the total population compared to only 21% in 1975. Metropolitan Manila, considered as the 18th largest metropolitan area in the world in 2002, is predicted to reach 25 million by near 2015. The metropolitan region would have to accommodate an increment of 11 million people, 3.5 million in Metro Manila and 7.5 million in the adjoining municipalities. Consequently, Metro Manila will be facing further inflow of populations while the adjoining areas will be facing severe shortage on social infrastructure to cope with the impact of rapid suburbanization.

The rapid growth of Metro Manila poised a tremendous impact on its ecosystem. Its rapid urbanization resulted to congestion and intensification of development activities which:
(1) placed serious strains on supporting structures and rendered existing services inadequate;
(2) resulted to incompatible and conflicting land uses;
(3) encouraged growth on the urban area’s peripheries where basic infrastructure services are not available; and
(4) spoiled the quality of the urban environment

With respect to atmospheric environment, the air quality has deteriorated to a point where people are wondering whether the time has come to wear gas masks. Air quality monitoring stations in eleven strategic locations in Metro Manila showed that the concentration levels of suspended particulate matters, sulfur dioxide and nitrogen oxide are in the up trend. In areas near major thoroughfares, concentration of these three pollutants has already exceeded the levels considered safe by the World Health Organization authorities. Air quality measurements indicate that particulate matter is the overwhelming pollutant of concern followed by lead. Carbon monoxide and nitrogen dioxide levels occasionally exceed accepted standards but particulate mater concentrations consistently exceed the acceptable limits. Emission inventory showed that 70 percent of air pollutants in Metro Manila are attributable to mobile sources (motor vehicles) and 30 percent to stationary sources (industries and power plants).

Due to the presence of high-rise buildings and the concentration of housing and other infrastructures, a phenomenon known as heat island effect is discernible in Metro Manila (See satellite image). This problem is aggravated by the emission of carbon dioxide, a by-product of fossil fuel combustion by both stationary and mobile sources. The presence of carbon dioxide will produce the so called green house effect which is synergistic with the heat island effect causing the ambient temperatures in the area to rise several degrees warmer than the countryside.

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