ABSTRACT: The urban heat island in Chicago has significant effect on the city environ. The paper explains about the climate of Chicago, the Urban fabric, vegetation, roofs, pavements, analysis on energy saving, air quality. The effect of urban heat island on human health and environment. The mitigation activities such as green and cool roofs, landscape ordinance, energy code roof top garden, city hall energy saving, impact of open space, asphalt alley reconstruction, parking lot re surfacing, greening lead in the reduction of urban heat island effect in the city.1. INTRODUCTION: Increase in the urbanization have resulted in drastic changes in the climate, economics, demography, land use. The population growth is leading to the expansion of cities, construction of roads, bridges buildings etc.
This in turn results in the micro climate which is also known as urban heat island. With increase in air temperature, the energy demand also increases and concentration of pollution in air increases. Urban heat island is a metropolitan area which is hotter that it’s surrounding rural areas due to human built environment. The main reason or cause for the UHI in the cities are due to the concentration of buildings, roads which absorb more heat than the suburb and rural areas during the day.
After the urbanization the use of materials in urban areas for roofs, pavements, concrete and asphalt, which have thermal conductance and radiative properties than the rural areas. Other major reason for UHI is lack of evapotranspiration in urban areas. This is due to the lack of vegetation, shade and cooling effect of trees and increase in the carbon dioxide levels in the atmosphere.
Chicago a metropolitan city, the heart of downtown and the centre of the entire city. It is located on the Lake Michigan. The city has the population of 7 million people and has 3750 square miles of ground cover. The city is less covered with residential community and more of tourist destinations. Skyscrapers have come up throughout the city and is also known as the Central Building District. Chicago is among the cities classified as a severe ozone nonattainment area. So there are many strategies introduced to reduce the urban heat island in Chicago.
2. LITERATURE REVIEW:2.1. Chicago’s Heat Island: Chicago was hit by a worst-weather related disaster in July, 1995, where 700 innocent died over 5 days of period. The combination of dark roof tops and parking lots are the major cause for the urban heat island. Fig.1.Heat island profile of Chicago 1990 In 1999, researchers from North-western University has identified the area in Chicago that is prone to heat island by using the data from the National Climate Data Centre.
After a long research it is plotted that the western suburbs of Chicago is prone to heat island effect than the city core. So as to study the meteorological and air quality of the heat island the researchers had to evaluate the surface characteristics, proportions of vegetation, paved areas, roads, roof surfaces that cover the total urban surface of the city using aerial photographs. Fig.2.land use in Chicago 1990 Fig.3.Urban fabric of Chicago2.2.
Chicago’s of Climate: Chicago has a flat terrain and has a lake Michigan makes weather unpredictable and extreme. It receives annual rainfall of 31.7 inches and relative humidity of 80% in the day and 60% in the noon.
Using the climatic data the researchers can estimate the energy saving and air quality impacts of the heat island mitigation strategies.2.3. Urban Fabric: The Urban fabric determines the urban surface cover in the city, the proportions of roofed, paved and vegetation. The researchers have used aerial photographs to study the neighbourhoods like urban, industrial, commercial, suburbs etc. About 40% of the urban fabric is vegetation, 27% roof area, 31% paved area.The following table is the detail information about the urban fabric based on the Aerial photograph Table.
1.Urban fabric in Chicago2.4. Vegetation cover: Over 500,000 trees were planted in the city Chicago from 1991-1998, by 1998 the city has 4.
1 million trees. The forestry of Chicago plants a 5000 new trees per year. As trees provide shade and allows the exchange of gases. The study by the researchers convey that the vegetation and canopy cover play major role in decreasing the air temperature, so the areas like industrial categories , commercial and transportation are to be focused by planting trees to decrease the heat island. Table.2.
Vegetation surface cover in Chicago 2.5. Roofed surfaces The researchers have studied the roofed surfaces and the results were given in percentages in different categories of land use. It is also observed that the density of residential and recreational buildings is very low (2-7times).The study can also record the light/white roof as they reflect.
Table.3.Roofed surface cover in Chicago2.6. Paved surfaces:Most of the Chicago’s roads and pavements are nine-inch thick concrete with three-inch asphalt overlay.
Commercial urban, commercial sub urban and transportation and industrial areas have the highest percentage of paved. The suburban areas in the west of Chicago has a large amount of new developments which increase the heat island effect.Table.4.
Paved surface cover in Chicago 2.7. Energy saving analysis: Cooling seasons in Chicago starts from mid-June to early-September. By the survey done by the department of Energy’s Lawrence Berkeley National Laboratory has calculated the air conditioned roofs in Chicago and landed up with result of 765million Mft2 and 368 Mft2 commercial.
2.8. Air Quality: As Chicago has been classified as a sever nonattainment area for ozone. As the researchers had found that the ozone noncompliance days are not effecting the downtown so they have concluded that the temperature difference not due to noncompliance. As the northern suburbs are more effected then the down town, the Chicago climate data clearly showed that there is no positive relationship between the temperature and ozone.3. Problem of Heat Island Effect: The city had lots of asphalt parking lots and black pavements which absorbed during the day and radiated lots of heat during the night.
The public could pronounced the difference in temperature when they pass by a green patch immediately by a parking lot. The paved roofs and dark roof tops added the heat which in result increased the heat by 2-5 degrees during the day and 20 at night, this increased the power consumption. The heat island warmed up the storm water runoff which effects the local waterways. Hits of heat stroke heat exaction and heat related deaths increased.4. Mitigation Strategies:There were many strategies launched in order to decrease the heat island in the city. The following activities were included:• Rooftop gardens /Green roofs• Energy Code• Landscape Ordinance• Open space impact fee Ordinance• Asphalt Alley Reconstruction• Parking Lots Resurfacing• Greening The green roofs in the city Chicago were started in year 2000 April.
About 20,000 herbaceous plants, 37 wines and 112 shrubs were planted with special mulch and compost. Materials of light weight water absorbent were laid all over to reduce the heat reflections and cool microclimate. Minimized the hardscape on roofs and increased green cover on the roof. Gravel pave was installed, the gravel was light in colour and allows rainwater to filter in the asphalt alley reconstruction.
Landscape was introduced in the parking lots, on the side pedestrians loading docks in order to decrease the heat and increase evaporative cooling to reduce the heat island effect in the city. The parking lots were replaced by the reflective surfaces and landscaping instead of concrete and asphalt.5. Conclusion: Urbanization has resulted change in the economic, climate change, land use, demography in the urban areas.
The city Chicago is now being reduced by the island effect according to the information given by the Department of the environment, Chicago. The mitigation strategies that were very effective which are included in the neighbourhood projects and also created awareness in the applicability to reduce the heat island effect in the community level. So there are many other ways to decrease the island effect where vegetation, urban fabric and urban infrastructure plays a major role. In general, to reduce the heat island effect urban surfaces have to be covered with vegetation, use of low albedo surfaces to reduce temperatures, use of light colours on the roof, considering human health and environment.