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The effect of urban morphology on urban heat island in the city of Biskra in Algeria a
a
M. Boukhabla , D. Alkama & A. Bouchair
b
a
Research Laboratory LACAMOFA, Department of Architecture , University Mohamed Khidder Biskra , Algeria b
Research Laboratory CBE, Department of Architecture , University of Jijel , Jijel , Algeria Accepted author version posted online: 19 Oct 2012.Published online: 19 Nov 2012.
To cite this article: M. Boukhabla , D. Alkama & A. Bouchair (2013) The effect of urban morphology on urban heat island in the city of Biskra in Algeria, International Journal of Ambient Energy, 34:2, 100-110, DOI: 10.1080/01430750.2012.740424 To link to this article: http://dx.doi.org/10.1080/01430750.2012.740424
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International Journal of Ambient Energy, 2013 Vol. 34, No. 2, 100–110, http://dx.doi.org/10.1080/01430750.2012.740424
The effect of urban morphology on urban heat island in the city of Biskra in Algeria M. Boukhablaa, D. Alkamaa and A. Bouchairb* a
Research Laboratory LACAMOFA, Department of Architecture, University Mohamed Khidder Biskra, Algeria; bResearch Laboratory CBE, Department of Architecture, University of Jijel, Jijel, Algeria
The objective of this study is to assess the impact of urban morphology upon the air temperature variation of urban condition in hot and dry climate of Biskra city in the south east of Algeria. A set of field measurement is performed using thermocouple equipments. A total of five measuring stations, representing different urban morphologies, are chosen in the city centre of Biskra to assess quantitatively the level of UHI and to investigate the overall impact of urban forms on this urban heat island. The results show that UHI phenomenon is becoming the character of the urban micro-climate in the city of Biskra.
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Keywords: morphology; station; urban heat island; air temperature; canyon street; dihedral street
1. Introduction In the past, vernacular architecture, such the Mzab towns, has shown to a marked degree some examples of conscious microclimatic modification by natural and man-made landscaping (Bouchair 2004). Occasionally trees were planted closely to provide shade and lower the ambient temperature (Golany 1980). Streets were sinuous and narrow, and mechanical transportation was not allowed. In the last few decades, the arid regions in the south of Algeria experience a serious decline in the micro-climatic conditions and natural environment degradation due to the man-made landscape transformation, emissions of green house gases, population activities and movements (Bouchair 2004). All these factors have significant implications on the change of cities’ climates. In urban areas of hot dry climate such as Biskra, building materials, asphalt and paved surfaces, with low albedo, have increasingly replaced preexisting natural landscape and vegetation. During summertime, solar heat is absorbed by roads, walls and roofs, causing the surrounding mean surface and air temperatures to increase significantly in comparison to the rural areas, resulting in the phenomenon of urban heat island (UHI). Several factors contribute in the formation of summer UHI, among them the replacement of natural landscapes by artificial and buildings materials, building surfaces and asphalt with low albedo and poor town-planning configurations are the prominent. Human activities such as the use of airconditioning devices, factories and transportation can also aggravate this effect. Because of global warming
*Corresponding author. Email:
[email protected] ß 2013 Taylor & Francis
and heat increase in the world climate, attention to urban forms became very important. This is due to the fact that the Urban Engineering has an impact on determining the conditions of urban micro-climate, as well as on urban climate as a whole. It was reported that there has been a relationship between the specifications of micro-climate and the physical shape of urban form and the various aspects of the solar environment, wind and energy consumption (Denis and Gerard 1976). In hot arid regions, climatic factors which must be addressed are: temperature and solar radiation, where the evolution of reconstruction is reflected in the expansion of cities, the multiplicity of buildings and increase in the heat sources that have an impact on the micro climate. The effect of reconstruction in dense built-up areas within the arid regions is the increase in temperature during summer compared to the surrounding areas – this phenomenon is known as UHI. Buildings offer many surface areas which help in catching and absorbing sun rays. During the night, building fabric releases the absorbed heat to the surrounding environment (Arecchi 1979). An environmental temperature including the impact of several physical and climatic factors was derived by Bouchair (2001). In order to protect the city from the UHI, we should look for appropriate urban shapes. In other words, we try to give answer to the following question: What is the appropriate urban morphologies that prevent the occurrence of UHI?
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In this research, first, we will demonstrate the existence of UHI within the city of Biskra, and second, we try to find a compromise on building configuration which lies between the existing urban area and the countryside. In order to achieve our aims, the city of Biskra located in southern Algeria is chosen as a case study. It is located at latitude 34.85 N and longitude 5.73 E and is characterised by hot dry climate (Evans 1980; Ecourrou 1991). The evaluation of the impact of surrounding building morphology on the UHI is made through a set of field measurement in the city centre. This work is the extension of the research work undertaken by Boukhabla (2010) in the same city. The measurement of the air temperatures were made at five measuring stations within the city and then compared to the reference air temperatures
measured at the meteorological station located at the peripheral rural area of the city of Biskra.
2. Field measurement work The evaluation of the UHI in the city of Biskra was mainly based upon the measurement of ambient air temperature and external surface temperature of the surrounding walls and streets. Temperature measurements were performed with a small handheld portable digital multimeter with LCD display, M890C connected to an accurate sensor via a thermocouple wire (Figure 1). The instrument is product conformity with IEC 1010 and CE certificate. The temperature measurement range is 20 C to 1370 C with an accuracy of 0.5%. The measurements were performed for a full day at five different locations. These were then compared to the meteorological weather data, obtained from the suburb station (open air locality) for the same period. Since the heat island appears in the thermal exchanges between the surfaces and the ambient air, we measure the temperature of the ground and the surrounding walls. Figure 2 shows a map where the measurement points were positioned.
2.1. Positions of measurement stations and criteria of their selection The location of the measuring stations was selected on the basis of three parameters as follows: Figure 1. Temperature measurement instrument.
. Geographical location within the city: the choice of the appropriate location in the city Station N°5 colonial checkered
Station N°1 Housing estate Station N°3 rehousing bungalows
Station N°2 Contiguous
Station N°4 Flats
Figure 2. General plan for the location of measurement stations in the city centre for different types of building tissues.
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M. Boukhabla et al. center was a main factor so that we can make a real evaluation of heat island and microclimate. . The absence of vegetation: so that the impact of vegetation upon the results is eliminated. . The surrounding morphology: The measurement site in the city center is relatively flat.
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With respect to the above-mentioned criteria, five measuring stations were installed in the city centre in diverse surrounding configurations. The measurements were taken for two typical days in July. The configurations used for the measurements are: Canyon Street, dihedral street and open space. Figure 3 shows a schematic diagram illustrating the selected configurations. . Station No. 1 is located at an area characterised by scattered building in the Hourriya district in the canyon street of Ibrahim Khoudja (Figure 4). The canyon has higher side walls than the separation between them. The orientation of the street is NE–SW . Station No. 2 is located in the Ben Gana district of fragmented buildings in the Meghazi Chana street characterised by the dihedral shape (Figure 5). The orientation of the street is NE–SW.
. Station No. 3 is located within an area of terraced buildings in the Z’mala street canyon (Figure 6). The orientation of the street is N–S. . Station No. 4 is located within an area characterised by dense buildings at the residence district (Figure 7). The height of the surrounding buildings is less than their separation. The orientation of the open street is E–W. . Station No. 5 is located within an area characterised by gridiron European buildings at the road station (Figure 8). It has a dihedral shape. Its orientation is NE–SW. The expressions R þ 1, R þ 2 and R þ 4 in Figures 4–8 indicate ground floor þ the number of story apartments.
2.2. Measurement period and climatic conditions The measurement period chosen for this research work was the 27th and 28th of July 2010. This is because the heat island phenomenon occurs in summer season, especially during the month of July (Boukhabla 2010). The sky was clear, the wind speed was 3–4 m/s and the sunshine duration was estimated to 7h18 with an average evaporation of 13.5 mm and an average relative humidity of 24% (Office national de me´te´orologie pour la wilaya de Biskra).
Figure 3. Dimensions and relative proportions (height to length) of the surrounding building configurations where the stations were placed.
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Figure 4. Position of Station No. 1. (a) Photographic view of the scene and (b) vertical section through the scene.
Figure 5. Position of Station No. 2. (a) Photographic view of the buildings and the street and (b) vertical section through the buildings and the street.
Figure 6. Position of Station No. 3. (a) Photographic view of the buildings and the street and (b) vertical section through the buildings and the street.
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Figure 8. Position of Station No. 5. (a) photographic view of the buildings and the street and (b) vertical section through the buildings and the dihedral street.
TEMPERATURE (°C)
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Figure 7. Position of Station No. 4. (a) Photographic view of the buildings and the open street and (b) vertical section through the buildings and the open street.
Meteo Temperature Air Temperature S1 Air Temperature S2 Air Temperature S3 Air Temperature S4 Air Temperature S5
45 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30 8h 10h 12h 14h 16h 18h 20h 22h 24h 2h 4h 6h HOUR
Figure 9. Graphs for measured air temperatures at the five stations compared to those recorded by the national meteorology office.
3. Results and discussions 3.1. Comparison between air temperatures in the city centre and the suburbs The measured air temperatures at the five stations are shown in Figure 9 and Table 1 for the same climatic conditions. Table 1 presents numerical values for the
measured air temperatures at all stations and at the meteorological station as well as their mean values (Tm). Table 2 gives temperature discrepancies between the air at the measurement stations, Ts, in the city centre and those given by meteorological office in the suburbs, Tto, as well as the mean values for day and night periods. It can be seen that for Station Nos 1–3 the measured values are higher than those recorded by the national office of meteorology for an open site in the suburbs of the city from 8 h to 15h30 and from 19h30 to 6 h. For Station No 2, the increase of temperatures appears from 8 h to 15 h and from 21h30 to 6 h. For Station No. 4 the observed increase of temperature is from 8 h to 16 h and from 21h30 to 6 h. For Station No. 5, the increase in temperature is observed from 8 h to 15 h and from 24 h to 6 h. The fact that the temperatures recorded at Station No. 5 became higher than the meteorological temperature only after midnight, is due to the street orientation (NE–SW) and the surrounding surface characteristics.
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8
Temperature Station No. 1 ( C) Temperature Station No. 2 ( C) Temperature Station No. 3 ( C) Temperature Station No. 4 ( C) Temperature Station No. 5 ( C) Mean temperature ( C) Air temperature at Meteo. station
34 35 34 35 35 34.6 32.9
10
12
14
16
18
20
22
24
35 37 35 36 38 36.2 34.5
38 38 38 36 38 37.6 36.1
41 41 41 43 40 41.2 38.5
40 39 40 41 40 40 40.9
37 38 37 36 38 37.2 40.6
39 33 39 36 37 36.8 37.6
36 36 36 35 34 35.4 34.4
35 35 35 34 34 34.6 34
2
4
6
Tm
35 34 35 34 33 34.2 32
34 33 34 33 33 33.4 31.4
33 32 33 32 31 32.2 30.6
36.4 35.9 36.4 35.9 35.9 36.1 35.3
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Table 2. Temperature discrepancies between the air in the city centre (Ts1–Ts5) and meteorological temperatures (Tto) in the suburbs. Discrepancy ( C)
8
Ts1–Tto Mean 1 Ts2–Tto Mean 2 Ts3–Tto Mean 3 Ts4–Tto Mean 4 Ts5–Tto Mean 5
1.1 1.5 2.1 2.2 1.1 1.5 2.1 1.6 2.1 2.2
10
12
14
Station No 1 2 3 4 5
41 41 40 43 40
18
20
0.5
1.9
2.5
0.9
3.6
1.4
2.5
1.9
2.5
1.9
2.6
4.6
0.5
1.9
2.5
0.9
3.6
1.5
0.1
4.5
0.1
4.6
1.4 1.9 1.6
3.5
1.9
1.5
0.9
2.6
0.6
Table 3. Time of maximum air temperatures. Tmax1 for the air at measurement stations ( C)
16
Time for Tmax1
Tmax2 at meteorological station
Time for Tmax2
14h00
40.9 C
16h00
Most of the buildings are constructed from adobe and stone. It is clearly noticeable that the measured air temperatures in the five stations are higher than those recorded at the meteorological station during the night and before the afternoon. By calculating the 24 hours mean values of these temperatures and making a relative comparison between them, we notice from Table 1 that the mean value of the meteorological temperature is 35.3 C, which is lower than all the mean values recorded at the five stations by about 1 C. Discrepancies in temperature before 14 h
22
24
2
4
6
1.6 1.9 1.6 1.4 1.6
1
3
2.6
2.4
1
2
1.6
1.4
1
3
2.6
2.4
0.6 1.03 0.4
0
2
1.6
1.4
0 0.7
1
1.6
0.4
and at night fluctuated between 0.5 C and 4.5 C (as shown in Table 2). Mean discrepancies for these periods fluctuated between 0.7 C and 2.2 C. This can be explained by the presence of traffic, air-conditioning systems and the surface absorptivity characteristics. During the night, building materials dissipated heat to the surrounding air and lead to the increase of its temperature. The air temperature at Station No. 1 oriented NE– SW is slightly higher than the temperature of the air at Station No. 3 oriented north–south (Figure 9). This can be explained by the fact that Station No. 1 is completely protected from the summer prevailing winds, whereas Station No. 3 is influenced by south– east wind in Biskra.
3.2. Effect of configuration of streets on the air temperature variation From Table 1 we can observe that the mean temperature value for Station Nos 1 and 3 is 36.4 C, whereas at Station Nos 2, 4 and 5, the mean value is 35.9 C, which is a little bit lower. The air temperature at the canyon streets (Station Nos 1 and 3) is higher than the
M. Boukhabla et al. temperature of the air at Station No. 1 is 35 C and it is 34 C at Station No. 4. The protection of the Station No. 1 from these winds contributes to the elevation of the air temperature. For a dihedral street at Station No. 2, which is oriented NE–SW, with low wind speed, air temperature may increase up to 6 C at 20 h compared to Station No. 3 located in a street canyon oriented (N–S), because Station No. 2 is sheltered from the south–east prevailing wind. This illustrates clearly the effect of geometry of the street on the intensity of the UHI. Street canyon presents higher values of air temperature than the dihedral streets and open space. There is a difference of 2 C between the canyon street and the dihedral street and 3 C between the canyon street and open space. From 2 o’clock to 6 o’clock in the morning, air temperatures for Station Nos 1 and 3 are: 35 C at 2 h, 34 C at 4 h and 33 C at 6 h (Figure 9). At the Station No. 2 this is 34 C at 2 h, 33 C at 4 h and 32 C at 6 h, with the exception of Station No. 5, which is lower by 1 C at 2 h and 2 C at 6 h in comparison to other values of the Station No. 2. However, after sunrise the air tends to warm faster, and thus the UHI is most pronounced at the beginning of the day and less pronounced in the middle of the day between 16 h and 20 h in Station Nos 1, 3, 4 and 5, and from 16 h to 22 h for Station No. 2. At this period of the day, the city can be even cooler than the suburbs, as indicated by the sets of graphs. At the beginning of the night and from 20 h or 22 h the UHI becomes more pronounced until 6 h in the morning.
air temperature at dihedral and open streets (Station Nos 2, 4 and 5). Open streets are more exposed to air movement and long wave radiation loss than narrow streets. The maximum air temperature at the five measurement stations occurs at 14 h whereas at the meteorological station this occurs at 16h00 (Table 3). This gives a time lag of two hours between the two locations. This observation confirms well the negative impact of the heat island on the urban environment. From Figure 10, it can be noticed that surface temperature of the walls and the ground follow the same pattern as air temperature measured at Station No. 1, canyon Street. Before 5 pm and after 7:30 pm, their values are higher than the meteorological station values. The maximum temperature of the ground surface (50 C) seems to be higher than all temperatures. However, the calculated mean temperature for the air and the surrounding surfaces at Station No. 1 are found to be similar, 36.4 C (air temperature) and 36.6 C (surface temperature), as shown in Table 4. The temperature of the air at Station No. 1 (canyon street), which is oriented NE–SW, is higher than that recorded at Station No. 4 (dihedral or open street) located in a street oriented east–west, because of its exposure to night summer winds (20–6 h) that blow from S–E. At 20 h the air temperature is 39 C at Station No. 1 and 36 C at Station No. 4. At 24 h the
TEMPERATURE (°C)
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52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30
Meteo Temperature Air Temperature S1 Wall Temperature S1 Soil Temperature S1
3.3. Effect of configuration of street on the surface temperature For Station No. 2, from Figure 11, before 15 h and after 21 h, the temperatures of the air and the surrounding surfaces follow similar pattern with different values. The mean air temperature of the station, 35.9 C, approaches the mean wall surface temperature, 36 C, but less than that of the ground surface, 36.7 C (Table 5). The highest maximum temperature is recorded at ground surface, 39 C. Station No. 3 is placed in a canyon street. Its ground and the surrounding wall surfaces are
8h 10h 12h 14h 16h 18h 20h 22h 24h 2h 4h 6h
HOUR
Figure 10. Temperature variation for the air, the wall surfaces and the ground surface in comparison to meteorological temperatures at Station No. 1.
Table 4. Measured temperatures for the air, the surrounding surfaces and the mean, Tm, at Station No. 1 (canyon street). Hour (h)
8
Air temperature ( C) Wall temperature ( C) Ground temperature ( C) Meteo. temperature ( C)
34 33 33 32.9
10
12
14
16
18
20
22
24
35 35 36 34.5
38 39 39 36.1
41 44 50 38.5
40 44 38 40.9
37 38 37 40.6
39 37 38 37.6
36 34 35 34.4
35 34 34 34.0
2
4
6
Tm
35 34 34 32.0
34 33 33 31.4
33 33 33 30.6
36.4 36.5 36.7 35.3
107
than both the walls and the air by about 1.3 C. With reference to Figure 15 and Table 9, we can observe that the mean ground surface temperature value for Station No. 4 (open space) has the highest one which is 39.9 C, followed by Station No. 5 (Dihedral street) with 37.7 C, then comes Station No. 1 and 2 with a value of 36.7 C, and finally, the lowest one is recorded at Station No. 3 with a mean value of 33.1 C. For all stations, the mean ground surface temperature, 37.7 C, is higher than the mean air temperature which is 36.1 C. Ground surface temperatures (Figure 15) are remarkably high for Station No. 4 because of its large emissivity due to the presence of asphalt which heats up quickly during the day time, its exposure to direct sun radiation with the absence of shading obstacles as well as the orientation of the open space which is west–east. As you may observe that the peak temperature of the ground surface is 51 C at 14 h, Station Nos 1, 2 and 5 have relatively dusty ground. A surface of medium conductivity 1.1 w/m K1 (like concrete), as in the case of Station No. 3, decreases the deviation between the surface temperature of the ground and air temperature, which is attributed to the hot prevailing winds in summer blowing from SE. As can be seen in Tables 4–8, the mean air temperature is 36.4 C, Tm, for the canyon
from concrete. It is clearly noticeable from Figure 12 that the measured air temperature is higher than all surrounding surface temperatures (wall and ground). The mean value for the air temperature is 36.4 C, and then comes the wall surface mean temperature by 35 C and finally the ground mean surface temperature by 33.1 C (Table 6). This can be explained by the fact that the surrounding building surfaces at Station No. 3 are constructed from concrete and the ground surface pavement having bright colour with low absorptivity. During the daytime, the discrepancy between air temperature and surface one is low. Whereas at night, the discrepancy is big as the ground temperature reduces significantly due to of the continuous spraying of the water on the ground. For the Station No. 4 (Figure 13), the surface temperature of the ground is higher than both of the air and the walls temperatures throughout the day. The mean value of its temperature is about 40 C, while the mean wall surface and air temperatures are respectively 35.6 C and 35.9 C (Table 7). A ground covered with asphalt of large conductivity, 1.90 w/m K, as in the case of Station No. 4, is the main factor in the deviation between the surface temperature of the ground and the air temperature. For Station No. 5 (Figure 14, Table 8) the mean ground surface temperature which is 37.7 C is higher 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30
Meteo Temperature Air Temperature S2 Wall Temperature S2 Soil Temperature S2 TEMPERATURE (°C)
TEMPERATURE (°C)
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8h 10h 12h 14h 16h 18h 20h 22h 24h 2h 4h 6h
42 41 40 39 38 37 36 35 34 33 32 31 30 29 28 27
Meteo Temperature Air Temperature S3 Wall Temperature S3 Soil Temperature S3
8h 10h 12h 14h 16h 18h 20h 22h 24h 2h 4h 6h
HOUR
HOUR
Figure 11. Temperature variation for the air, the wall surfaces and the ground surface in comparison to meteorological temperatures at Station No. 2.
Figure 12. Temperature variation for the air, the wall surface and the ground surface in comparison to meteorological ones at Station No. 3.
Table 5. Measured temperatures for the air, the surrounding surfaces and the mean, Tm, at Station No. 2 (dihedral street). Hour (h)
8
Air temperature ( C) Wall temperature ( C) Ground temperature ( C) Meteo. temperature ( C)
35 34 34 32.9
10
12
14
16
18
20
22
24
37 36 35 34.5
38 39 39 36.1
41 42 49 38.5
39 38 39 40.9
38 37 38 40.6
33 37 35 37.6
36 35 36 34.4
35 35 34 34.0
2
4
6
Tm
34 34 34 32.0
33 33 34 31.4
32 32 33 30.6
35.9 36 36.7 35.3
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Table 6. Measured temperatures for the air, the surrounding surfaces and the mean, Tm, at Station No. 3 (canyon street). Hour (h)
TEMPERATURE (°C)
34 33 33 32.9
10
12
14
16
18
20
22
24
35 34 33 34.5
38 34 33 36.1
41 39 38 38.5
40 38 37 40.9
37 38 39 40.6
39 35 35 37.6
36 35 31 34.4
35 34 31 34.0
52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30
2
4
6
Tm
35 34 30 32.0
34 33 29 31.4
33 33 28 30.6
36.4 35 33.1 35.3
Meteo Temperature Air Temperature S4 Wall Temperature S4 Soil Temperature S4
8h 10h12h14h16h18h20h22h24h 2h 4h 6h
HOUR
Figure 13. Temperature variation for the air, the wall surface and the ground surface in comparison to meteorological ones at Station No. 4.
Table 7. Measured temperatures for the air, the surrounding surfaces and the mean, Tm, at Station No. 4 (open street). Hour (h) Air temperatures ( C) Wall temperature ( C) Ground temperature ( C) Meteo. temperature ( C)
8 35 36 36 32.9
TEMPERATURE (°C)
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Air temperature ( C) Wall temperature ( C) Ground temperature ( C) Meteo. temperature ( C)
8
10
12
14
16
18
20
22
24
36 35 40 34.5
36 36 44 36.1
43 37 51 38.5
41 41 47 40.9
36 39 47 40.6
36 36 39 37.6
35 35 37 34.4
34 34 36 34.0
48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30
2
4
6
Tm
34 34 35 32.0
33 33 34 31.4
32 31 33 30.6
35.9 35.6 39.9 35.3
Meteo Temperature Air Temperature S5 Wall Temperature S5 Soil Temperature S5
8h 10h 12h 14h 16h 18h 20h 22h 24h 2h
4h
6h
HOUR
Figure 14. Temperature variation for the air, the wall surface and the ground surface in comparison to meteorological ones at Station No. 5.
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Table 8. Measured temperatures for the air, the surrounding surfaces and the mean, Tm, at Station No. 5 (dihedral street). Hour (h) Air temperature ( C) Wall temperature ( C) Ground temperature ( C) Meteo. temperature ( C)
8 35 36 37 32.9
10
12
14
16
18
20
22
24
38 36 38 34.5
38 41 45 36.1
40 41 47 38.5
40 44 47 40.9
38 38 37 40.6
37 36 34 37.6
34 34 35 34.4
34 33 34 34.0
2
4
6
Tm
33 33 34 32.0
33 34 33 31.4
31 31 32 30.6
35.9 36.4 37.7 35.3
51 Meteo Temperature
49
Soil Temperature S1
TEMPERATURE (°C)
47
Soil Temperature S2
45
Soil Temperature S3
43
Soil Temperature S4
41
Soil Temperature S5
39 37 35
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33 31 29 27 8h 10h 12h 14h 16h 18h 20h 22h 24h 2h HOUR
4h
6h
Figure 15. Ground surface temperature variation at the five stations compared to the air temperature given by the national office of meteorology.
Table 9. Measured ground surface temperatures for Figure 15 and the mean values. Hours Ground temperature Station No. 1 Ground temperature Station No. 2 Ground temperature Station No. 3 Ground temperature Station No. 4 Ground temperature Station No. 5 Mean temperature ( C) Air temperature at Meteo. station
8 ( C) ( C) ( C) ( C) ( C)
33 34 33 36 37 34.6 32.9
10
12
14
16
18
20
22
24
36 35 33 40 38 36.4 34.5
39 39 33 44 45 40 36.1
50 49 38 51 47 47 38.5
38 39 37 47 47 41.6 40.9
37 38 39 47 37 39.6 40.6
38 35 35 39 34 36.2 37.6
35 36 31 37 35 34.8 34.4
34 34 31 36 34 33.8 34
streets (Station Nos 1 and 3) is a little bit higher than for the open and dihedral streets (Station Nos 2, 4 and 5), Tm (35.9 C).
4. Conclusion The experimental work performed for the evaluation of the occurrence of heat island in the city of Biskra shows that for all measurement in the selected locations, the air temperature is higher than the one recorded by the meteorological station situated in the surrounding rural area. Air temperature in the city centre could rise by about 4.5 C and the observed time lag for the maximum temperatures (2 hours) between urban and rural areas proves the build-up of the UHI
2
4
6
Tm
34 34 30 35 34 33.4 32
33 34 29 34 33 32.6 31.4
33 33 28 33 32 31.8 30.6
36.7 36.7 33.1 39.9 37.7 37.7 35.3
within this city. The morphology of the urban surrounding fabric forming the street influences the variation in the surface and air temperatures, especially the ground surface. In the canyon streets, the air temperature is higher than that of the dihedral and open streets. Open streets are more exposed to air movement and long wave radiation loss than narrow streets. Ground surface temperatures are remarkably high where the asphalt is present because of its high absorptivity, and the absence of shading obstacles. Emissivity and conductivity relating to material covering the roads play an important role in increasing the temperature of the air. So in order to reduce the effect of UHI it is important to avoid dense buildings, to increase shading objects such as vegetation, and to use traditional building material having better thermal
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performance. In some cases the direction (N–S) offers more freshness for the canyons oriented (N–S) and (NE–SW). This is due the direction of the prevailing winds which is south–east. The temperature of the air in the streets is affected by their configurations and that the widest streets (open spaces) are the most advantageous by the air movement.
References
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