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Temperature Cooling and Warming Rates in Three Different Built Environments within Nairobi City, Kenya

Temperature Cooling and Warming Rates in Three Different Built Environments within Nairobi City,... Hindawi Publishing Corporation Advances in Meteorology Volume 2010, Article ID 686214, 5 pages doi:10.1155/2010/686214 Research Article Temperature Cooling and Warming Rates in Three Different Built Environments within Nairobi City, Kenya George Lukoye Makokha and Chris Allan Shisanya Geography Department, Kenyatta University, P.O. Box 43844-00100, Nairobi 00100, Kenya Correspondence should be addressed to Chris Allan Shisanya, shisanya@yahoo.com Received 1 April 2010; Accepted 6 July 2010 Academic Editor: Harry D. Kambezidis Copyright © 2010 G. L. Makokha and C. A. Shisanya. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Urban canyon, urban park, and suburban surface air temperature data for hot-wet, hot-dry, cool-dry, and warm-wet periods in Nairobi city were analyzed to detect differences in the cooling and warming rates. Measurement of temperature for thirty continuous days was done at each of the three sites for each of the above periods. The cooling and warming rates were computed on an hourly basis beginning at 6.00 P.M., the approximate time of sunset. The results of the study showed that the largest cooling and warming rates were generally experienced during the hot-dry period while the lowest during the cool-dry period. Cooling and warming rates were also found to be the highest at the suburban site and the lowest at the urban canyon site. The differences in the conditions of the built environment at the three sites could explain the cause of the differential cooling and warming rates. The study recommends proper planning of the built environment to ameliorate the problem of excessive nocturnal heat loads within the built environment. 1. Introduction environment are significant factors in explaining the inten- sity of the modification of urban climate. Urban geometry is one of the major factors leading to the modification of urban climate. Specifically, urban geometry 2. Study Area that relates to the urban canopy layer (UCL) influences aspects such as increased substrate heat storage due to greater The study was carried out in the larger Nairobi area including ◦ ◦ thermal admittance of surface materials and decreased latent the city centre, extending from longitude 1 15’S to 1 25’S ◦ ◦ heat fluxes arising from the replacement of soil and vegetated and latitude 36 40’E and 37 05’E. The area extends from the surfaces with impervious material [1–5]. It also leads to Kikuyu highlands in the west to the Athi-Kapiti plains in east, increase in solar radiation absorption due to lower albedo covering approximately 690 square kilometers (Figure 1). of urban materials and reduced wind speeds caused by the Nairobi has a diversified physical environment, with altitude aerodynamically rougher urban fabric [6]. There is also ranging from an average of 1400 metres above sea level in the release of anthropogenic heat from domestic, commer- the east to approximately 1900 metres above sea level in the cial, industrial and transport energy sources and increased west. The topography, which is fairly rugged and diverse, atmospheric radiation absorption from green house gases slopes eastwards and is drained by rivers that flow from west [4, 6, 7]. to east across the city centre. It has also significant effects This paper examines the thermal behaviour of both the on the climate and vegetation. The western part of Nairobi, urban and suburban landscapes as a possible cause of the which was originally covered by forest, and the eastern part, differences in the urban heat island within Nairobi city. covered by grassland, have been modified by the growth and The study proposes that the structure and composition of expansion of the city [8]. However, there are still remnants the urban canopy as well as the density of the built-up of the original vegetation in these areas. During the census 2 Advances in Meteorology ◦  ◦ 36 40 E 37 05 E Kenya 1 15 S 1 15 S Nairobi 1 Kasarani Westlands Dagoretti Embakasi Langata (km) ◦  ◦ 08 16 1 25 S 1 25 S ◦  ◦ 36 40 E 37 05 E Provincial boundary Urban canyon (3) Kenyatta university Division boundary Sub-urban (1) 2 Uhuru park Makadara Division name Sampling sites 3 Central business district Urban park (2) Figure 1: Study area showing the sampled sites. of 1999, the population of Nairobi currently stands at over the urban canyon consisted of asphalt and bricks. The urban 3 million people, with an urban population growth rate of canyon observation screen was located in the middle of about5percentper annum[9]. the city centre on a paved pedestrian walk with a canyon The city centre of Nairobi lies on the foot of the Kikuyu SVFofabout 0.4, whilethe urbanparkwas locatedinan highlands but areas in all directions have become built open park with trees, grass, and an open recreational water up during the past forty years. It consists of mainly high reservoir. The suburban station situated about 20 KM north rise densely built up area with very low sky view factor east of the city centre at Kenyatta University was sited in (SVF) of canyons. The building structure of the suburban an environment that resembles a rural site. Low building residential areas varies considerably depending on the social structures surrounded the measurement screen with very status, but in unplanned residential areas, dense blocks high SVF. of multistorey buildings are common. The well-planned All the recording and measuring instruments used in residential areas have green and open spaces compared with this study were calibrated and standardized at the Kenya the inner city. Meteorological Department (KMD) at the instruments’ workshop before being taken to their respective sites for use. They included thermohygrographs type NG5538 to record 3. Data temperature and dry-bulb thermometers type 1/C BS 692 Data from three purposefully sampled observation sites were to countercheck the accuracy of thermohygrographs. The used for the analyses presented in this paper. The three sites measurements were done at screen level. were Agakhan Walk (urban canyon), Uhuru Park (urban Data for surface air temperature for the three sites park), and Kenyatta University (suburban). They are located were continuously recorded and collected for a period of 30 days in the year 2007 for the months of Febru- in areas of similar landscape morphology, that is, relatively flat areas consisting mainly of clay soils. However, in addition ary/March (hot-dry), April/May (hot-wet), July/August to differences in building geometry, the three sites have (cool-dry), and October/November (warm-wet). The four stated periods coincide with the four climatic subseasons of differences in thermal properties, as the urban park and suburban measurement screens were located on grass while Nairobi. Makadara Kamukunji Starehe Advances in Meteorology 3 3 3 2 2 1 1 0 0 −1 −1 −2 −2 −3 −3 −12 −10 −8 −6 −4 −2 0 246 810 12 −12 −10 −8 −6 −4 −2 0 246 810 12 Time after sunset (hours) Time after sunset (hours) (a) (b) −1 −1 −2 −2 −3 −3 −12 −10 −8 −6 −4 −2 0246 810 12 −12 −10 −8 −6 −4 −2 0 24 6 810 12 Time after sunset (hours) Time after sunset (hours) Urban canyon station Urban canyon station Urban park station Urban park station Sub-urban station Sub-urban station (c) (d) Figure 2: Hourly cooling and warming rates at three sites in Nairobi during different periods of the year. The temperature data were collected on a weekly basis. that is, 5.00 P.M. was assigned a value of −1, the next hour This was because the recording instruments and charts used before 5.00 P.M. a value of −2, andsoonupto −11 hours in the study were designed to record continuously for a before sunset. period of one week. For every observation period of 30 days, Starting at 0 (time of sunset), the cooling and warming the instruments were initially set on a Friday at 9.00 A.M. ratesateachhourwerecomputed[10] This computation was until the following Monday at 9.00 A.M. This period of three done by subtracting the temperature value at 7.00 P.M. from days was meant to check the accuracy of the instruments and the preceding one at 6.00 P.M., at 8.00 P.M. from at 7.00 P.M., make adjustments where necessary. Thereafter, until the end at 9.00 P.M. from at 8.00 P.M., and so on until reaching the of each observation period, the instruments were cleaned temperature value at 5.00 P.M. The cooling and warming and the charts replaced after every week on a Monday at rates were then plotted against time after and before sunset. 9.00 A.M. The data were extracted from the weekly recording This was done for each of the observation periods at each of charts at one-hour interval. the three sites. The results were presented using graphs and tables. 3.1. Data Analysis. To compute the temperature cooling and warming rates at each of the three sites, the beginning time 4. Results and Discussions at sunset was taken as 6.00 P.M. (18 hours local time or 1500 GMT). This time was assigned a value of zero on the The differences in the cooling and warming rates between horizontal scale. On the positive side of the scale, the time at the three sites, urban canyon, urban park, and suburban 7.00 P.M. (19 hours local time or 1600 GMT) was assigned a are shown in Figures 2(a) to 2(d). In all the four periods, value of 1 hour after sunset, the next hour a value of 2 hours warming took place during the day and cooling during the after sunset, and so on up to a value of 12 hours after sunset. night. However, both the warming and cooling rates were On the negative side of the scale, the hour before 6.00 P.M., influenced by the conditions around the station and the ◦ ◦ Cooling/warming per hour ( C) Cooling/warming per hour ( C) ◦ ◦ Cooling/warming per hour ( C) Cooling/warming per hour ( C) 4 Advances in Meteorology Table 1: Highest cooling rates ( C/hr) after sunset for three sites in Nairobi city. Site Period Site type Cooling rates ( C) Hours after sunset Agakhan Walk Urban Canyon −1.32 Feb-Mar Uhuru Park Urban Park −1.41 Kenyatta University Suburban −1.91 Agakhan Walk Urban Canyon −1.21 Apr-May Uhuru Park Urban Park −1.12 Kenyatta University Suburban −1.62 Agakhan Walk Urban Canyon −1.12 Jul-Aug Uhuru Park Urban Park −1.02 Kenyatta University Suburban −1.01 Agakhan Walk Urban Canyon −1.00 Oct-Nov Uhuru Park Urban Park −1.31 Kenyatta University Suburban −1.61 Table 2: Highest warming ( C/hr) rates before sunset for three sites in Nairobi city. Site Period Site type Warming rates ( C) Hours before sunset Agakhan Walk Urban Canyon 2.1 5 Feb-Mar Uhuru Park Urban Park 2.7 8 Kenyatta University Suburban 2.6 9 Agakhan Walk Urban Canyon 1.6 6 Apr-May Uhuru Park Urban Park 1.7 8 Kenyatta University Suburban 1.8 7 Agakhan Walk Urban Canyon 1.7 5 Jul-Aug Uhuru Park Urban Park 1.8 6 Kenyatta University Suburban 1.9 8 Agakhan Walk Urban Canyon 1.7 6 Oct-Nov Uhuru Park Urban Park 1.7 7 Kenyatta University Suburban 1.8 8 season of the year. In all the three sites, the cooling rates urban fabric and consequently leads to higher nocturnal were largest from 1 hour before until 3 hours after sunset, minimum temperature, and eventually the development of with a maximum at 1 to 2 hours after sunset (Table 1). the urban heat island [3, 11, 12]. The highest difference in Thewarming rateswerelargest from 9hours to4hours the cooling rates between the urban canyon site and the before sunset, with a maximum at 5 to 9 hours before suburban site was −0.6 C per hour experienced during the sunset (Table 2). The largest cooling and warming rates were February-March and October-November periods (Table 1). experienced during the dry-hot period of February-March These were also the periods when the urban heat island was (Figure 2(a)), whereas the lowest cooling and warming rates found to be highest in Nairobi [8]. were experienced during the cool-dry period of July-August Table 2 shows that the warming rates are highest at the (Figure 2(c)). The rainfall periods of April-May (Figure 2(b)) suburban site and lowest at the urban canyon site during the and October-November (Figure 2 (d)) experienced moderate four periods of study. The highest difference in the warming cooling and warming rates. Thus during the hot-dry period, rates between the two was 0.5 C per hour experienced during higher cooling and warming rates explain the development the hot-dry February-March period. Again, during all the of extreme urban heat island condition in Nairobi city. four periods of study, the urban canyon site warmed more Table 1 shows the highest cooling rates for each site after slowly than the suburban site (Table 2). For instance, during sunset. The highest cooling rates were experienced at the the February-March period, the suburban site recorded suburban site in February-March period, whereas the lowest the highest warming rate 4 hours earlier than the urban at the urban site during the October-November period. canyon site (Table 2). The fast warm-up at the suburban The urban park site experienced moderate cooling rates site compared to the canyon site can be attributed to the throughout, suggestive of the moderating influence of the faster release of terrestrial radiation compared to the slower park on urban temperatures. The reduced cooling rates at release experienced in the urban atmosphere. It may also be the urban canyon site are likely to be caused by the urban attributed to the presence of shadows and low sky view factor geometry, which causes increased heat absorption by the (SVF) at the urban station [11]. Advances in Meteorology 5 5. Conclusion [10] U. Hillevi, E. Ingegard, and L. Sven, “The influence of green areas on nocturnal temperatures in a high latitude city The thermal behaviour of the three urban landscapes showed (Goteborg, Sweden),” International Journal of Climatology, vol. noticeable differences in their cooling and warming rates 18, pp. 681–700, 1998. over the four different climatic periods of Nairobi city. The [11] A. T. Buckland, “Validation of a street canyon model in two largest cooling and warming rates were generally found cities,” Environmental Monitoring and Assessment, vol. 52, no. 1-2, pp. 255–267, 1998. during the hot-dry period while the lowest during the cool- [12] T. R. Oke, Boundary Layer Climates, Routledge, London, UK, dry period. Except for the cool-dry period, all the remaining 2nd edition, 1987. three periods had the urban canyon and the urban park sites record lower cooling rates than the suburban site. The highest cooling rates were recorded at the suburban site while the lowest at the urban canyon site. The reduced cooling rates at the urban canyon site were attributed to increased heat absorption by the urban fabric, which was lacking at the suburban site. The warming rates were found to be higher at the suburban site compared to the urban canyon site. This could be attributed to faster emittance of long-wave radiation at the suburban site compared to the urban canyon site where there is more heat absorption and storage, with less emittance and lower SVF [1, 6]. Generally the urban park site showed relatively moderate cooling and warming rates, a fact that was attributed to the moderating effects of park vegetation. Therefore, to reduce excessive nocturnal heat loads and increase nocturnal cooling, built environment should have adequate open and green spaces, which will enhance air circulation and less radiation absorption during the day. References [1] S. Blankenstein and W. Kuttler, “Impact of street geometry on downward longwave radiation and air temperature in an urban environment,” Meteorologische Zeitschrift, vol. 15, no. 5, pp. 373–379, 2004. [2] S. N. Goward, “Thermal behavior of urban landscapes and the urban heat island,” Physical Geography, vol. 2, no. 1, pp. 19–33, [3] I. Eliasson, “Urban-suburban-rural temperature differences related to street geometry,” Physical Geography, vol. 15, no. 1, pp. 1–22, 1994. [4] P. E. Todhunter, “Microclimatic variations attributable to urban-canyon asymmetry and orientation,” Physical Geogra- phy, vol. 11, no. 2, pp. 131–141, 1990. [5] R. Teasler, “Urban climatological methods and data,” in Proceedings of Technical Conference on Urban Climatology and Its Application with Regard to Tropical Areas, pp. 199–236, World Meteorological Organization, Geneva, Switzerland, [6] M. Santamouris and C. Georgakis, “Energy and indoor cli- mate in urban environments: recent trends,” Building Services Engineering, vol. 24, no. 2, pp. 69–81, 2003. [7] T. R. Oke, “The energetic basis of the urban heat island,” Quarterly Journal of the Royal Meteorological Society, vol. 108, no. 455, pp. 1–24, 1982. [8] G.L.Makokha, Modification of the microclimate in a changing tropical urban environment: the case of Nairobi city, Unpub- lished Ph.D. thesis, Kenyatta University, 2003. [9] Republic of Kenya, Population and Housing Census 1999, Volume 1, Central Bureau of Statistics (CBS). Ministry of Finance and Planning, Nairobi, Kenya, 2001. 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Temperature Cooling and Warming Rates in Three Different Built Environments within Nairobi City, Kenya

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Hindawi Publishing Corporation Advances in Meteorology Volume 2010, Article ID 686214, 5 pages doi:10.1155/2010/686214 Research Article Temperature Cooling and Warming Rates in Three Different Built Environments within Nairobi City, Kenya George Lukoye Makokha and Chris Allan Shisanya Geography Department, Kenyatta University, P.O. Box 43844-00100, Nairobi 00100, Kenya Correspondence should be addressed to Chris Allan Shisanya, shisanya@yahoo.com Received 1 April 2010; Accepted 6 July 2010 Academic Editor: Harry D. Kambezidis Copyright © 2010 G. L. Makokha and C. A. Shisanya. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Urban canyon, urban park, and suburban surface air temperature data for hot-wet, hot-dry, cool-dry, and warm-wet periods in Nairobi city were analyzed to detect differences in the cooling and warming rates. Measurement of temperature for thirty continuous days was done at each of the three sites for each of the above periods. The cooling and warming rates were computed on an hourly basis beginning at 6.00 P.M., the approximate time of sunset. The results of the study showed that the largest cooling and warming rates were generally experienced during the hot-dry period while the lowest during the cool-dry period. Cooling and warming rates were also found to be the highest at the suburban site and the lowest at the urban canyon site. The differences in the conditions of the built environment at the three sites could explain the cause of the differential cooling and warming rates. The study recommends proper planning of the built environment to ameliorate the problem of excessive nocturnal heat loads within the built environment. 1. Introduction environment are significant factors in explaining the inten- sity of the modification of urban climate. Urban geometry is one of the major factors leading to the modification of urban climate. Specifically, urban geometry 2. Study Area that relates to the urban canopy layer (UCL) influences aspects such as increased substrate heat storage due to greater The study was carried out in the larger Nairobi area including ◦ ◦ thermal admittance of surface materials and decreased latent the city centre, extending from longitude 1 15’S to 1 25’S ◦ ◦ heat fluxes arising from the replacement of soil and vegetated and latitude 36 40’E and 37 05’E. The area extends from the surfaces with impervious material [1–5]. It also leads to Kikuyu highlands in the west to the Athi-Kapiti plains in east, increase in solar radiation absorption due to lower albedo covering approximately 690 square kilometers (Figure 1). of urban materials and reduced wind speeds caused by the Nairobi has a diversified physical environment, with altitude aerodynamically rougher urban fabric [6]. There is also ranging from an average of 1400 metres above sea level in the release of anthropogenic heat from domestic, commer- the east to approximately 1900 metres above sea level in the cial, industrial and transport energy sources and increased west. The topography, which is fairly rugged and diverse, atmospheric radiation absorption from green house gases slopes eastwards and is drained by rivers that flow from west [4, 6, 7]. to east across the city centre. It has also significant effects This paper examines the thermal behaviour of both the on the climate and vegetation. The western part of Nairobi, urban and suburban landscapes as a possible cause of the which was originally covered by forest, and the eastern part, differences in the urban heat island within Nairobi city. covered by grassland, have been modified by the growth and The study proposes that the structure and composition of expansion of the city [8]. However, there are still remnants the urban canopy as well as the density of the built-up of the original vegetation in these areas. During the census 2 Advances in Meteorology ◦  ◦ 36 40 E 37 05 E Kenya 1 15 S 1 15 S Nairobi 1 Kasarani Westlands Dagoretti Embakasi Langata (km) ◦  ◦ 08 16 1 25 S 1 25 S ◦  ◦ 36 40 E 37 05 E Provincial boundary Urban canyon (3) Kenyatta university Division boundary Sub-urban (1) 2 Uhuru park Makadara Division name Sampling sites 3 Central business district Urban park (2) Figure 1: Study area showing the sampled sites. of 1999, the population of Nairobi currently stands at over the urban canyon consisted of asphalt and bricks. The urban 3 million people, with an urban population growth rate of canyon observation screen was located in the middle of about5percentper annum[9]. the city centre on a paved pedestrian walk with a canyon The city centre of Nairobi lies on the foot of the Kikuyu SVFofabout 0.4, whilethe urbanparkwas locatedinan highlands but areas in all directions have become built open park with trees, grass, and an open recreational water up during the past forty years. It consists of mainly high reservoir. The suburban station situated about 20 KM north rise densely built up area with very low sky view factor east of the city centre at Kenyatta University was sited in (SVF) of canyons. The building structure of the suburban an environment that resembles a rural site. Low building residential areas varies considerably depending on the social structures surrounded the measurement screen with very status, but in unplanned residential areas, dense blocks high SVF. of multistorey buildings are common. The well-planned All the recording and measuring instruments used in residential areas have green and open spaces compared with this study were calibrated and standardized at the Kenya the inner city. Meteorological Department (KMD) at the instruments’ workshop before being taken to their respective sites for use. They included thermohygrographs type NG5538 to record 3. Data temperature and dry-bulb thermometers type 1/C BS 692 Data from three purposefully sampled observation sites were to countercheck the accuracy of thermohygrographs. The used for the analyses presented in this paper. The three sites measurements were done at screen level. were Agakhan Walk (urban canyon), Uhuru Park (urban Data for surface air temperature for the three sites park), and Kenyatta University (suburban). They are located were continuously recorded and collected for a period of 30 days in the year 2007 for the months of Febru- in areas of similar landscape morphology, that is, relatively flat areas consisting mainly of clay soils. However, in addition ary/March (hot-dry), April/May (hot-wet), July/August to differences in building geometry, the three sites have (cool-dry), and October/November (warm-wet). The four stated periods coincide with the four climatic subseasons of differences in thermal properties, as the urban park and suburban measurement screens were located on grass while Nairobi. Makadara Kamukunji Starehe Advances in Meteorology 3 3 3 2 2 1 1 0 0 −1 −1 −2 −2 −3 −3 −12 −10 −8 −6 −4 −2 0 246 810 12 −12 −10 −8 −6 −4 −2 0 246 810 12 Time after sunset (hours) Time after sunset (hours) (a) (b) −1 −1 −2 −2 −3 −3 −12 −10 −8 −6 −4 −2 0246 810 12 −12 −10 −8 −6 −4 −2 0 24 6 810 12 Time after sunset (hours) Time after sunset (hours) Urban canyon station Urban canyon station Urban park station Urban park station Sub-urban station Sub-urban station (c) (d) Figure 2: Hourly cooling and warming rates at three sites in Nairobi during different periods of the year. The temperature data were collected on a weekly basis. that is, 5.00 P.M. was assigned a value of −1, the next hour This was because the recording instruments and charts used before 5.00 P.M. a value of −2, andsoonupto −11 hours in the study were designed to record continuously for a before sunset. period of one week. For every observation period of 30 days, Starting at 0 (time of sunset), the cooling and warming the instruments were initially set on a Friday at 9.00 A.M. ratesateachhourwerecomputed[10] This computation was until the following Monday at 9.00 A.M. This period of three done by subtracting the temperature value at 7.00 P.M. from days was meant to check the accuracy of the instruments and the preceding one at 6.00 P.M., at 8.00 P.M. from at 7.00 P.M., make adjustments where necessary. Thereafter, until the end at 9.00 P.M. from at 8.00 P.M., and so on until reaching the of each observation period, the instruments were cleaned temperature value at 5.00 P.M. The cooling and warming and the charts replaced after every week on a Monday at rates were then plotted against time after and before sunset. 9.00 A.M. The data were extracted from the weekly recording This was done for each of the observation periods at each of charts at one-hour interval. the three sites. The results were presented using graphs and tables. 3.1. Data Analysis. To compute the temperature cooling and warming rates at each of the three sites, the beginning time 4. Results and Discussions at sunset was taken as 6.00 P.M. (18 hours local time or 1500 GMT). This time was assigned a value of zero on the The differences in the cooling and warming rates between horizontal scale. On the positive side of the scale, the time at the three sites, urban canyon, urban park, and suburban 7.00 P.M. (19 hours local time or 1600 GMT) was assigned a are shown in Figures 2(a) to 2(d). In all the four periods, value of 1 hour after sunset, the next hour a value of 2 hours warming took place during the day and cooling during the after sunset, and so on up to a value of 12 hours after sunset. night. However, both the warming and cooling rates were On the negative side of the scale, the hour before 6.00 P.M., influenced by the conditions around the station and the ◦ ◦ Cooling/warming per hour ( C) Cooling/warming per hour ( C) ◦ ◦ Cooling/warming per hour ( C) Cooling/warming per hour ( C) 4 Advances in Meteorology Table 1: Highest cooling rates ( C/hr) after sunset for three sites in Nairobi city. Site Period Site type Cooling rates ( C) Hours after sunset Agakhan Walk Urban Canyon −1.32 Feb-Mar Uhuru Park Urban Park −1.41 Kenyatta University Suburban −1.91 Agakhan Walk Urban Canyon −1.21 Apr-May Uhuru Park Urban Park −1.12 Kenyatta University Suburban −1.62 Agakhan Walk Urban Canyon −1.12 Jul-Aug Uhuru Park Urban Park −1.02 Kenyatta University Suburban −1.01 Agakhan Walk Urban Canyon −1.00 Oct-Nov Uhuru Park Urban Park −1.31 Kenyatta University Suburban −1.61 Table 2: Highest warming ( C/hr) rates before sunset for three sites in Nairobi city. Site Period Site type Warming rates ( C) Hours before sunset Agakhan Walk Urban Canyon 2.1 5 Feb-Mar Uhuru Park Urban Park 2.7 8 Kenyatta University Suburban 2.6 9 Agakhan Walk Urban Canyon 1.6 6 Apr-May Uhuru Park Urban Park 1.7 8 Kenyatta University Suburban 1.8 7 Agakhan Walk Urban Canyon 1.7 5 Jul-Aug Uhuru Park Urban Park 1.8 6 Kenyatta University Suburban 1.9 8 Agakhan Walk Urban Canyon 1.7 6 Oct-Nov Uhuru Park Urban Park 1.7 7 Kenyatta University Suburban 1.8 8 season of the year. In all the three sites, the cooling rates urban fabric and consequently leads to higher nocturnal were largest from 1 hour before until 3 hours after sunset, minimum temperature, and eventually the development of with a maximum at 1 to 2 hours after sunset (Table 1). the urban heat island [3, 11, 12]. The highest difference in Thewarming rateswerelargest from 9hours to4hours the cooling rates between the urban canyon site and the before sunset, with a maximum at 5 to 9 hours before suburban site was −0.6 C per hour experienced during the sunset (Table 2). The largest cooling and warming rates were February-March and October-November periods (Table 1). experienced during the dry-hot period of February-March These were also the periods when the urban heat island was (Figure 2(a)), whereas the lowest cooling and warming rates found to be highest in Nairobi [8]. were experienced during the cool-dry period of July-August Table 2 shows that the warming rates are highest at the (Figure 2(c)). The rainfall periods of April-May (Figure 2(b)) suburban site and lowest at the urban canyon site during the and October-November (Figure 2 (d)) experienced moderate four periods of study. The highest difference in the warming cooling and warming rates. Thus during the hot-dry period, rates between the two was 0.5 C per hour experienced during higher cooling and warming rates explain the development the hot-dry February-March period. Again, during all the of extreme urban heat island condition in Nairobi city. four periods of study, the urban canyon site warmed more Table 1 shows the highest cooling rates for each site after slowly than the suburban site (Table 2). For instance, during sunset. The highest cooling rates were experienced at the the February-March period, the suburban site recorded suburban site in February-March period, whereas the lowest the highest warming rate 4 hours earlier than the urban at the urban site during the October-November period. canyon site (Table 2). The fast warm-up at the suburban The urban park site experienced moderate cooling rates site compared to the canyon site can be attributed to the throughout, suggestive of the moderating influence of the faster release of terrestrial radiation compared to the slower park on urban temperatures. The reduced cooling rates at release experienced in the urban atmosphere. It may also be the urban canyon site are likely to be caused by the urban attributed to the presence of shadows and low sky view factor geometry, which causes increased heat absorption by the (SVF) at the urban station [11]. Advances in Meteorology 5 5. Conclusion [10] U. Hillevi, E. Ingegard, and L. Sven, “The influence of green areas on nocturnal temperatures in a high latitude city The thermal behaviour of the three urban landscapes showed (Goteborg, Sweden),” International Journal of Climatology, vol. noticeable differences in their cooling and warming rates 18, pp. 681–700, 1998. over the four different climatic periods of Nairobi city. The [11] A. T. Buckland, “Validation of a street canyon model in two largest cooling and warming rates were generally found cities,” Environmental Monitoring and Assessment, vol. 52, no. 1-2, pp. 255–267, 1998. during the hot-dry period while the lowest during the cool- [12] T. R. Oke, Boundary Layer Climates, Routledge, London, UK, dry period. Except for the cool-dry period, all the remaining 2nd edition, 1987. three periods had the urban canyon and the urban park sites record lower cooling rates than the suburban site. The highest cooling rates were recorded at the suburban site while the lowest at the urban canyon site. The reduced cooling rates at the urban canyon site were attributed to increased heat absorption by the urban fabric, which was lacking at the suburban site. The warming rates were found to be higher at the suburban site compared to the urban canyon site. This could be attributed to faster emittance of long-wave radiation at the suburban site compared to the urban canyon site where there is more heat absorption and storage, with less emittance and lower SVF [1, 6]. Generally the urban park site showed relatively moderate cooling and warming rates, a fact that was attributed to the moderating effects of park vegetation. Therefore, to reduce excessive nocturnal heat loads and increase nocturnal cooling, built environment should have adequate open and green spaces, which will enhance air circulation and less radiation absorption during the day. References [1] S. Blankenstein and W. Kuttler, “Impact of street geometry on downward longwave radiation and air temperature in an urban environment,” Meteorologische Zeitschrift, vol. 15, no. 5, pp. 373–379, 2004. [2] S. N. Goward, “Thermal behavior of urban landscapes and the urban heat island,” Physical Geography, vol. 2, no. 1, pp. 19–33, [3] I. Eliasson, “Urban-suburban-rural temperature differences related to street geometry,” Physical Geography, vol. 15, no. 1, pp. 1–22, 1994. [4] P. E. Todhunter, “Microclimatic variations attributable to urban-canyon asymmetry and orientation,” Physical Geogra- phy, vol. 11, no. 2, pp. 131–141, 1990. [5] R. Teasler, “Urban climatological methods and data,” in Proceedings of Technical Conference on Urban Climatology and Its Application with Regard to Tropical Areas, pp. 199–236, World Meteorological Organization, Geneva, Switzerland, [6] M. Santamouris and C. Georgakis, “Energy and indoor cli- mate in urban environments: recent trends,” Building Services Engineering, vol. 24, no. 2, pp. 69–81, 2003. [7] T. R. Oke, “The energetic basis of the urban heat island,” Quarterly Journal of the Royal Meteorological Society, vol. 108, no. 455, pp. 1–24, 1982. [8] G.L.Makokha, Modification of the microclimate in a changing tropical urban environment: the case of Nairobi city, Unpub- lished Ph.D. thesis, Kenyatta University, 2003. [9] Republic of Kenya, Population and Housing Census 1999, Volume 1, Central Bureau of Statistics (CBS). Ministry of Finance and Planning, Nairobi, Kenya, 2001. 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