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Micro-scale thermal performance of tropical urban parks in Singapore
Building and Environment 94 (2015) 467e476
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Micro-scale thermal performance of tropical urban parks in Singapore Yun Hye Hwang*, Qin Jie Geraldine Lum, Yeow Kwang Derek Chan Department of Architecture, School of Design and Environment, National University of Singapore, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 May 2015 Received in revised form 22 August 2015 Accepted 9 October 2015 Available online 23 October 2015
As Singapore is near the equator, heat is a concern, especially given the increasing yearly average temperatures and significant urban heat island effects. National greening policies propose increasing the number of parks; this may indirectly benefit thermal conditions at a macro level, but there has been little consideration of the thermal environment within these spaces. This study examined micro-scale thermal conditions within 10 urban parks at the hottest period of the year, assessing operative temperature with three measurement variables: air temperature (ta), globe temperature (tg), and wind velocity (v). It found that 1) thermal performances between and within parks range widely, and 2) critical thermal points in the respective parks highlight the value of shade, especially in terms of volume and continuity over a length of path. The findings suggest the need for a quantitative study of design factors to guide the future planning and design of climatically adapted parks in the tropics. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Micro-scale thermal performance Tropical urban parks Park settings Shade Singapore
1. Introduction The equatorial climate of Singapore ensures high temperatures year round, with a diurnal maximum temperature of 31 Ce34 C and a minimum of 23 Ce26 C [28]. On top of this, the heat has been intensifying in the city-state, with an observed increase of 0.25 C in the average temperature every decade since the 1970s [18]. Singapore also experiences the urban heat island effect because of its urban development [31,36,45], with an overall average temperature difference of up to 4 C island-wide [48]. Airconditioning, used throughout Singapore to mitigate high temperatures, not only increases energy consumption, but worsens the urban heat island effect [25]. In the face of such conditions, the land use planning and conservation authority of Singapore is conducting island-wide studies in a bid to manage the urban heat at the macro level [46]. Discrepancies in temperatures measured on sites and by official weather stations highlight the inadequacy of relying on general regional measurements taken by fixed weather stations [12] suggesting the need to calibrate the value of the ‘urban’ temperature [19] and to use local macro-scale thermal measurements to verify the thermal experience of park users. Many studies demonstrate the effect of vegetation and green spaces in generating thermal benefits for their urban surroundings [1,9,15,17,22,24,35,37e39,49,53]. Such spaces are found to intercept
* Corresponding author. 4, Architecture Drive, 117566, Singapore. E-mail address: [email protected] (Y.H. Hwang). http://dx.doi.org/10.1016/j.buildenv.2015.10.003 0360-1323/© 2015 Elsevier Ltd. All rights reserved.
radiation, facilitate evapotranspiration, and even reduce energy consumption [2,3,5,8]. In the same vein, several studies of the local Singaporean context reach the following conclusions: the locations of large and dense green areas in Singapore correspond with lower measurements of temperature, indicating the function of vegetation quantity in reducing temperatures at the macro level [23,48,51]; temperature measurements are lower not only within parks compared to an external reference point but also in the parks' adjacent built-up environments, highlighting the ability of parks to cool their surroundings [13]; vegetation, especially mature trees, can improve microclimatic conditions within pedestrian canyons through shading [50]. Singapore has been called a “City in a Garden”. Vegetation and vegetated settings are a common sight, with almost half of the city covered in green (as cited in Refs. [44]; pp. 14e15), including manmade spaces such as parks, park connectors, and roadside greenery, and natural areas such as nature reserves. Public urban parks currently constitute about 40% of all man-made green space in the city [29]. There are large-scale plans to boost recreation and biodiversity in these spaces [30] and to ensure an increased quantity of, and accessibility to, key outdoor gathering and activity nodes up to 900ha by 2020 [27]. While green policies may increase functional opportunities and indirectly benefit city-scale thermal conditions, it is believed that the park users' overall experience can be further enhanced with the concurrent consideration of microscale thermal aspects during the implementation of these plans. Accordingly, this paper considers heat at the micro level and
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how it may be moderated or exacerbated by immediate park surroundings. It hypothesizes that quantifying localized thermal conditions in parks will shed light on of the experience of heat for the park user. Comparing these conditions across park settings may reveal important relationships between design and thermal aspects, setting the stage to generate guidelines useful for park design and management to regulate thermal performance. The specific objectives are: 1. To investigate the range of micro-scale thermal performance of existing parks in Singapore through on-site measurements, and 2. To identify related design parameters and examine their influence on thermal performance by identifying extreme thermal points.
Equation (1), requiring air temperature. We calculate mean radiant temperature (tmrt) from Equation (2), an equation from ISO 7726:1998 [21] that was calibrated based on a Singaporean study; simply stated, it increases the accuracy of the measurements taken with a flat grey globe thermometer in a tropical outdoors setting [43]. In all, the measurement variables used to assess the thermal performance of parks in this study are air temperature (ta), globe temperature (tg), and wind velocity (v).
pffiffiffiffiffiffiffiffi tmrt þ ta 10v pffiffiffiffiffiffiffiffi to ¼ 1 þ 10v
(1)
The first part of the paper explains the methods employed during a field study of 10 parks in Singapore. It analyzes data gathered during the observation exercise to understand the thermal performance of these particular parks. It then assesses the different park settings by setting them against the thermal analysis, creating the basis for further quantitative study. The paper concludes by noting the limitations of the research and briefly discussing the potential for future studies.
to ¼ Operative temperature ( C) tmrt ¼ Mean radiant temperature ( C) v ¼ Wind velocity (m/s)
tmrt ¼
tg þ 273:15
4
þ
3:42 109 v0:119 tg ta εD0:4
0:25
273:15
2. Method
(2)
2.1. Defining thermal performance In quantifying the experience of heat, the assessment of thermal performance in this study references the concept of “thermal comfort”. The concept was originally developed for indoor building spaces [34] but has been gaining recognition in the outdoor environment. Thermal comfort is defined as “the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation”. Its analysis relies on four microclimatic parameters e air temperature, radiant temperature, air velocity and relative humidity e and two human factors e metabolic rate and clothing insulation [4]. Of the microclimatic factors, air temperature, is the most relevant for the study of thermal performance in parks as it is one of the most commonly available and easily measured thermal variables [10]. The high humidity in Singapore, with a diurnal range of about 60%e90% all year round [28], cannot be easily decreased but can be offset with lower air and radiant temperature; similarly, wind cannot be easily increased through landscape approaches and requires manipulation at a larger scale of the city [10,14]. Therefore, the present study limits itself to air and radiant temperatures. While human factors are not immediately relevant to an examination of relationships between design and thermal conditions, it is possible to examine human acceptance of the thermal performance of parks. A local study by [52] used the thermal comfort model Predicted Mean Vote (PMV) to determine the neutral and acceptable ranges of operative temperatures in the outdoor urban context of Singapore, described as thermal conditions able to satisfy 100% and 80% of people respectively [4]. The neutral operative temperature was found to be 28.7 C, with the acceptable range between 26.3 C and 31.7 C [52]. The present study uses these numbers as a gauge to assess how the actual thermal performance within the parks measures up to an identified acceptable range of outdoor thermal conditions. Parks' thermal performances can be determined by calculating operative temperatures. Commonly used in the assessment of thermal comfort, operative temperature is the “weighted average of mean radiant temperature and dry-bulb air temperature” [47]. In other words, it combines the effects of convective and radiant heat. To obtain the operative temperatures (Spirn) of the parks, we use
tmrt ¼ Mean radiant temperature ( C) tg ¼ Globe temperature ( C) v ¼ Wind velocity (m/s) ta ¼ Air temperature ( C) D ¼ Globe diameter (mm) ε ¼ Globe emissivity 2.2. Site selection and observation period In order to assess the influence of different design parameters on thermal performance, it is important to include a variety of park settings. Therefore, we conducted a separate study before the observation period to determine suitable parks. Parks located within or near to coastal and forested areas and the high-density city core were disregarded to minimize unfavourable macroclimatic influences on subsequent measurements. Ten regional urban parks distributed evenly across the island were selected (Fig. 1), and a representative path of about 500e600 m in each park was demarcated for observation at the end of the preliminary study. Measurements and mappings were to be taken at short and regular intervals of 10 m along the paths, resulting in about 50e60 points of measurements for each park. The observation exercise was done during the warmest and driest period of the year, from March to May in 2015, and during the hottest part of the day, between 1pm and 3pm, to determine thermal performances under conditions of extreme heat and see how various park settings may augment or aggravate these. To account for slight daily weather variations, we visited each park on three different days for repeated measurements of ta, tg and v at each point. Measurements were made only on sufficiently sunny days, avoiding the occasional cloudy or rainy day; the entire observation period took about eight weeks. 2.3. Field measurement equipment and procedure As measurements for several thermal parameters had to be
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Fig. 1. Locations of 10 selected parks in Singapore.
Table 1 Thermal variables recorded and equipment specifications. Thermal variables
Equipment
Measurement range
Accuracy
Air temperature Wind velocity Globe temperature
Testo 445, with 3-function probe(Shielded air temperature probe) Testo 445, with 3-function probe(Wind velocity probe) Extech HT30 globe thermometera (Ø ¼ 40 mm)
20 Ce70 C 0.0e10 m/s 0 to 80 C
±0.4 C from 0 C to 50 C ±5% or 0.03 m/s whichever is greater ±2 C
a Extech HT30 globe thermometer has been compared with HOBO Thermocouple Data Logger, UX100-014 M with Type-T Copper-Constantan thermocouple sensors and 40 mm diameter grey ping pong ball (The same instruments equation (2) was calibrated for [42]). The amount of variation of tmrt value was within a ±2 C difference under park conditions at 1e3pm.
completed within two hours at many points for each park, it was important to select portable equipment that could be easily carried without compromising precision. To measure air temperature, ta, and wind speed, v, we chose the Testo 445 with a three-function probe, as it had been used in similar studies in Singapore requiring outdoor temperature measurements at different locations [52]. The accuracy of the instrument conforms to ISO 7726 [41]. For measuring radiant temperature, tg, we used an Extech HT30 globe thermometer with a 40 mm diameter black globe because of its rapid response and convenient size [20,42]. The detailed specifications of equipment used are listed in Table 1. Thermal measurements at each point were made at the height of 1.1 m to represent thermal effects on a standing or walking adult [26]. To account for equipment response time when moving from point to point, we recorded the final measurement of air temperature and globe temperature at each point only after the readings on the equipment had stabilized and while the velocity probe was held perpendicular to the direction of the airstream. The shade condition of each point - whether in shade or full sun e was also recorded. We took photographs to capture site details such as trees, path materials and surrounding conditions. For the first round of assessment, to obtain the operative temperature, to, for each point, we calculated the mean tmrt using Equation (2); the final figure was used to calculate to of the point using Equation (1). Points within the highest and lowest 5% range of operative temperatures, referred to in this study as “hot” spots and “cool” spots respectively, were singled out to evaluate differences in park settings with extreme thermal conditions. Next, we studied park settings of “hot” spots and “cool” spots using the photographs; we compared operative temperatures and the presence of shade, and we compared operative temperatures with adjacent park settings to determine the extent of influence design parameters on temperatures.
3. Analysis and results 3.1. Thermal performances of parks We averaged three sets of air temperature measurements in each park to obtain one mean air temperature measurement for each point in each park; the final averages ranged from 32.3 C to 34.9 C.We expected the calculation of operative temperature to provide a more accurate picture of thermal conditions as this measure can account for convective effects and solar and terrestrial radiation with the use of the mean radiant temperature. The resulting operative temperatures of all points ranged from 36.3 C to 51.9 C e much higher than the overall acceptable outdoor range of 26.3 Ce31.7 C derived from the above-mentioned study by Ref. [52]. As Table 2 in their study shows, the lowest operative temperature of 36.3 C would feel thermally comfortable to less than approximately 40% of population; the percentage decreases as
Table 2 Comparison of temperatures between parks. Park name
Yishun Park Punggol Park Jurong Central Park Choa Chu Kang Park Clementi Woods Bishan Park Toa Payoh Town Park One North Park Woodlands Town Garden Tampines Eco Green
ta ( C)
to ( C)
Overall
Range
Overall
Range
32.3 33.1 33.5 33.3 33.4 33.5 33.7 34.0 34.4 34.9
31.6e33.2 32.1e33.7 32.6e35.1 32.0e34.4 31.9e35.1 32.3e34.5 33.1e34.3 32.7e35.7 32.8e37.0 32.5e36.9
40.0 40.5 41.7 42.3 43.2 43.3 43.3 43.5 44.7 46.9
36.3e44.6 37.5e46.0 36.7e46.5 37.5e47.2 38.2e48.1 38.6e47.7 38.7e50.0 38.7e48.4 39.6e50.7 40.3e51.9
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operative temperatures climb. While this indicates that none of the park settings surveyed may be able to achieve an overall acceptable level of thermal comfort at the hottest time of the year, there is room to increase the thermal acceptability of most of the parks by reducing operative temperatures. We calculated the overall operative temperature of each park by taking the average of the operative temperatures of all points within the park, resulting in a range of 40.0 Ce46.9 C (Table 2). We plotted the operative temperature of each point in a park onto a graph; the graphs of each park were placed in order of their overall operative temperature (Fig. 2). As Fig. 2 shows, thermal
performance varies between the parks, with the hotter parks displaying a tendency to have more fluctuations in temperatures and the cooler parks maintaining relatively stable temperatures and having lower readings in many cases. The calculation of the top and bottom five percent of operative temperatures resulted in 29 “hot” spots and 29 “cool” spots out of the 597 points surveyed. “Hot” spots ranged from 48.5 C to 51.9 C, and “cool” spots ranged from 36.3 C to 38.4 C. These spots were then located on the plans of each park, revealing a clustering phenomenon (Fig. 3). As expected, the overall hotter parks such as Tampines Eco Green and Woodlands Town Garden Park had the
Fig. 2. Comparison of operative temperatures between parks; y-axis represents temperature in degree Celsius, x-axis represents points on path.
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Fig. 3. “Hot” spots and “cool” spots located on plans of park.
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bulk of the “hot” spots, and cooler parks such as Yishun Park, Punggol Park, and Jurong Central Park had most of the “cool” spots. To summarize, the inconsistencies highlight that not all green spaces and park settings are equal in terms of thermal modification. Therefore, the next section considers the possible influence of
various park design parameters on temperatures. 3.2. Park settings The photographs of “hot” spots and “cool” spots in Fig. 4 show
Fig. 4. Park settings of singular, or clusters of, “hot” spots and “cool” spots; photos on the left are forward views, photos on the right are backwards views.
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that one of the most obvious differences between the park settings of “hot” spots and “cool” spots is spatial configuration, with “hot” spots occurring in more open or top-exposed areas and “cool” spots in more top-enclosed areas with tree canopies overlapping the paths. The latter are also usually side-exposed with few or no adjacent shrubs, probably a direct result of the interception of radiation by the canopies e or in other words, a result of the provision of shade on the path for the park user. This is in line with several studies advocating the modification of radiation in landscape design because of the high dependence of outdoor thermal experience on the exposure to solar radiation [11,37,43,49]. Although large green areas have been demonstrated to reduce heat at the macro level [13,22,24,38,48,51], the effect of vegetation quantity seems less significant at the micro level. “Hot” spots form even along paths with adjacent dense vegetation layers such as in Tampines Eco Green, Toa Payoh Town, and Woodland Park, while “cool” spots can occur in areas lined only with a few trees such as in Yishun, Punggol, Jurong and Choa Chu Kang Park, indicating vegetation quantity itself may be less important for the park user than the presence of shade, especially if the vegetation is not within close proximity to the path taken by the park user. In terms of path materials, we found no apparent impacts on thermal performance, with the most common materials asphalt appearing in both “hot” spots and “cool” spots. However, it is interesting to note that Tampines Eco Green has persistently high to despite having turf as a path material, building on the previous finding that vegetation quantity does not always seem to improve micro-scale thermal performance. In a similar vein, having a water feature near a path does not always correlate with temperature reduction, especially when the water feature is located in full-sun, as demonstrated by the appearance of certain “hot” spots within both Tampines Eco Green and Toa Payoh Town Park (Fig. 4). Having identified shade as a key influence on park thermal performance, we next plotted to of each point against the presence of shade during measurement on the same graphs to verify their relationship (Fig. 5). In general, operative temperatures tend to be higher when readings were taken at points without shade. However, there seems to be no direct correlation between the presence of shade and the extent of temperature increase or decrease, suggesting the need to study specific characteristics or patterns of shade. As expected “hot” spots develop along long lengths of paths without shade, while “cool” spots tend to occur along the lengthier spans of shaded paths e indicating that the continuity of shade, as opposed to short dashes of shade of the same total length, could be a factor in further reduction of temperatures. Some shaded paths did not develop “cool” spots, so the study was expanded to include shaded areas around the paths. We observed that some of the “hot” spots are, in fact, near to, but not in direct contact with large areas of shade, suggesting that implementing shade next to but not overlapping paths may be ineffective in enhancing thermal performance at the micro level (Fig. 6). Yet “cool” spots usually form in the midst of a large area of shade, implying that shade volume could induce an increased rate of temperature reduction. 4. Conclusions As the preceding analysis of thermal performance in parks shows, to achieve good micro-scale thermal conditions for the park user, merely setting aside a large green space or considering only one of the various thermal parameters or even employing random dense planting is insufficient. It is the combination of a number of factors, especially shade and planting configurations of trees in relation to paths, that determines design sensitivity to the tropical
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climate. The clear variation in thermal conditions within and between parks underscores the importance of considering thermal attributes alongside the typical social, physical, aesthetic and other aspects in designing tropical urban parks. The following key findings may be useful to park designers and managers seeking to optimize the thermal conditions in parks: 1. Even with the presence of shade, the micro-scale thermal performance along foot paths in Singapore urban parks can be extreme during the hottest time of day and the hottest period of the year. Thermal acceptability for hotter parks may still be optimized, however, by referencing park settings with the lowest operative temperatures. 2. On top of having lower temperature readings at points along the path, it may be necessary to minimize temperature variations to create more stabilized and overall improved thermal conditions for park users. 3. Shade is the most important factor in reducing temperatures in the tropical urban context. A high quantity of vegetation may moderate heat on a larger scale but may not improve microscale thermal conditions if unaccompanied by overhead canopies that provide shade on the path for park users. This is linked to the planting configuration of trees along paths. In a similar vein, the use of turf as a path material or water features do not lead to lower temperatures, especially when there is no shade. 4. Providing shade can in itself enhance thermal performance as long as the shade overlaps paths taken by park users; the extent of the effects of shade on reducing temperature may be increased by implementing it in continuous lengths and in greater volume instead of the same quantity in truncated segments. Leaving extensive lengths of paths unshaded can exacerbate micro-scale thermal conditions. Despite these findings being expected outcomes, the significant effect of tree shade on the mitigation of urban heat remains overlooked in parks design in the tropics. BCA's (Building and Construction Authority (Singapore)) “Green Mark for new and existing parks” is a rating system to evaluate environmental impact and performance of parks and to promote a sustainable framework for design; however, its requirements focus on the percentage of greenery without consideration for the provision of tree shade coverage on foot paths, nor the continuity of tree shade [6,7]. In short, the presence and continuity of tree shade should become part of these guidelines used to assess existing parks as well as to design new ones. 5. Discussion The climate of a place is decided by its geographical location and cannot be changed, so heat will always be a factor in outdoor spaces in Singapore. That being said, the results of this study demonstrate how micro-scale thermal performance can vary even within a distance of 10 m, with the experience of tropical heat either exacerbated or alleviated by the immediate surroundings. This suggests steps can be taken to minimize the effect of yearround heat in Singapore; more should be done to assimilate micro thermal conditions into the design and management of parks as a supplement to the present development framework in Singapore. The study is certainly not without its limitations. Its focus on understanding the factors influencing the thermal conditions within parks requires a set of fixed respective thermal conditions; hence, it cannot consider diurnal aspects of thermal performance. However, morning to evening thermal changes can be considered
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Fig. 5. Comparison of operative temperatures and presence of shade; y-axis represents operative temperature in degree Celsius, x-axis represents points on path.
in subsequent studies to illuminate the complete diurnal relationship between temperatures and planting configurations of trees with respect to paths, especially since shade conditions change throughout the day. Now that the importance of shade and its various characteristics to micro-scale thermal performance have been identified, the next step should include quantifying the optimum range of these qualities to increase the effectiveness of design and improve management guidelines for parks. As this study is unable to reach a definitive conclusion on the influence of path material on thermal
performance, an additional study of various materials within and outside the presence of shade may prove useful as the albedo of materials does have a macro-scale effect [10,14,16]. In addition to all the environmental factors affecting the thermal performance of parks, psychological aspects such as human perception play a key role in the experience of these conditions [32,33,52]. Supplementary studies and surveys to quantify the subjective relationship between human perception and the thermal conditions observed in this study may complete the cycle and strengthen climate-adaptive park design in the tropical context.
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Fig. 6. “Hot” spots and “cool” spots in comparison with adjacent shaded areas.
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Acknowledgements This work was made possible by funding from MND Research Fund (MNDRF) for the built environment under Grant Number R295-000-095-490. The authors also wish to acknowledge and thank Zi En Jonathan Yue, research assistant in NUS, for technical support. Thanks also to Ms. Yuin-mae, NG, Deputy Director of Development Management in the National Parks Board Singapore, who collaborated for the park selection process and provided invaluable feedback on the overall research. References [1] H. Akbari, Cooling Our Communities. A Guidebook on Tree Planting and Lightcolored Surfacing, Lawrence Berkeley Laboratory, Berkeley, CA, 1992. [2] H. Akbari, Shade trees reduce building energy use and CO2 emissions from power plants, Environ. Pollut. 116 (2002) S119eS126. [3] H. Akbari, D.M. Kurn, S.E. Bretz, J.W. Hanford, Peak power and cooling energy savings of shade trees, Energy Build. 25 (2) (1997) 139e148, http://dx.doi.org/ 10.1016/S0378-7788(96)01003-1. 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Shariff, Phillip Jones, Elias Salleh, Ahmad M. Abdullah, A comparison of Mesua ferrea L. and Hura crepitans L. for shade creation and radiation modification in improving thermal comfort, Landsc. Urban Plan. 97 (3) (2010) 168e181, http://dx.doi.org/10.1016/ j.landurbplan.2010.05.008. [38] Limor Shashua-Bar, M.E. Hoffman, Vegetation as a climatic component in the design of an urban street: an empirical model for predicting the cooling effect of urban green areas with trees, Energy Build. 31 (2000) 221e235. [39] Limor Shashua-Bar, David Pearlmutter, Evyatar Erell, The influence of trees and grass on outdoor thermal comfort in a hot-arid environment, Int. J. Climatol. 31 (10) (2011) 1498e1506. http://onlinelibrary.wiley.com/doi/10. 1002/joc.2177/abstract. [41] Standardization International Organization for, BS EN 27726:1994, ISO 7726: 1985: Thermal Environments. Instruments and Methods for Measuring Physical Quantities, British Standards Institute, 1994. [42] C.L. Tan, N.H. Wong, S.K. Jusuf, Outdoor Mean Radiant Temperature Estimation in the Tropical Urban Environment, 2013. [43] Chun Liang Tan, Nyuk Hien Wong, Steve Kardinal Jusuf, Outdoor mean radiant temperature estimation in the tropical urban environment, Build. Environ. 64 (2013a) 118e129, http://dx.doi.org/10.1016/j.buildenv.2013.03.012. [44] S.P. Tee, Trees of Our Garden City: a Guide to the Common Trees of Singapore, National Parks Board, Singapore, 2009. [45] C.P. Tso, A survey of urban heat island studies in two tropical cities, Atmos. Environ. 30 (3) (1996) 507e519, http://dx.doi.org/10.1016/1352-2310(95) 00083-6. [46] Urban Redevelopment Authority, URA, Designing Our City: Planning for a Sustainable Singapore. Skyline [Supplement], Urban Redevelopment Authority, Singapore, 2012. [47] Richard D. Watson, Kirby S. Chapman, Radiant Heating and Cooling Handbook, McGraw Hill, New York, 2002. [48] Nyuk Hien Wong, Yu Chen, Study of green areas and urban heat island in a tropical city, Habitat Int. 29 (3) (2005) 547e558, http://dx.doi.org/10.1016/ j.habitatint.2004.04.008. [49] Nyuk Hien Wong, Steve Kardinal Jusuf, Air temperature distribution and the influence of sky view factor in a green Singapore estate, J. Urban Plan. Development-Asce 136 (3) (2010a) 261e272, http://dx.doi.org/10.1061// ASCE/UP.1943-5444.0000014. [50] Nyuk Hien Wong, Steve Kardinal Jusuf, Study on the microclimate condition along a green pedestrian canyon in Singapore, Archit. Sci. Rev. 53 (2) (2010b) 196e212, http://dx.doi.org/10.3763/asre.2009.0029. [51] Nyuk Hien Wong, Steve Kardinal Jusuf, Aung Aung La Win, Htun Kyaw Thu, To Syatia Negara, Wu Xuchao, Environmental study of the impact of greenery in an institutional campus in the tropics, Build. Environ. 42 (8) (2007) 2949e2970, http://dx.doi.org/10.1016/j.buildenv.2006.06.004. [52] Wei Yang, Wong, Nyuk Hien Jusuf, Steve Kardinal, Thermal comfort in outdoor urban spaces in Singapore, Build. Environ. 59 (2013) 426e435. [53] Letícia Zambrano, Cristina Malafaia, Leopoldo E.G. Bastos, Thermal comfort evaluation in outdoor space of tropical humid climate, in: Paper Presented at the the 23rd Conference on Passive and Low Energy Architecture (PLEA2006) 2006, Switzerland, Geneva, September 2006, pp. 6e8.
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Micro-scale thermal performance of tropical urban parks in Singapore Yun Hye Hwang*, Qin Jie Geraldine Lum, Yeow Kwang Derek Chan Department of Architecture, School of Design and Environment, National University of Singapore, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 May 2015 Received in revised form 22 August 2015 Accepted 9 October 2015 Available online 23 October 2015
As Singapore is near the equator, heat is a concern, especially given the increasing yearly average temperatures and significant urban heat island effects. National greening policies propose increasing the number of parks; this may indirectly benefit thermal conditions at a macro level, but there has been little consideration of the thermal environment within these spaces. This study examined micro-scale thermal conditions within 10 urban parks at the hottest period of the year, assessing operative temperature with three measurement variables: air temperature (ta), globe temperature (tg), and wind velocity (v). It found that 1) thermal performances between and within parks range widely, and 2) critical thermal points in the respective parks highlight the value of shade, especially in terms of volume and continuity over a length of path. The findings suggest the need for a quantitative study of design factors to guide the future planning and design of climatically adapted parks in the tropics. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Micro-scale thermal performance Tropical urban parks Park settings Shade Singapore
1. Introduction The equatorial climate of Singapore ensures high temperatures year round, with a diurnal maximum temperature of 31 Ce34 C and a minimum of 23 Ce26 C [28]. On top of this, the heat has been intensifying in the city-state, with an observed increase of 0.25 C in the average temperature every decade since the 1970s [18]. Singapore also experiences the urban heat island effect because of its urban development [31,36,45], with an overall average temperature difference of up to 4 C island-wide [48]. Airconditioning, used throughout Singapore to mitigate high temperatures, not only increases energy consumption, but worsens the urban heat island effect [25]. In the face of such conditions, the land use planning and conservation authority of Singapore is conducting island-wide studies in a bid to manage the urban heat at the macro level [46]. Discrepancies in temperatures measured on sites and by official weather stations highlight the inadequacy of relying on general regional measurements taken by fixed weather stations [12] suggesting the need to calibrate the value of the ‘urban’ temperature [19] and to use local macro-scale thermal measurements to verify the thermal experience of park users. Many studies demonstrate the effect of vegetation and green spaces in generating thermal benefits for their urban surroundings [1,9,15,17,22,24,35,37e39,49,53]. Such spaces are found to intercept
* Corresponding author. 4, Architecture Drive, 117566, Singapore. E-mail address: [email protected] (Y.H. Hwang). http://dx.doi.org/10.1016/j.buildenv.2015.10.003 0360-1323/© 2015 Elsevier Ltd. All rights reserved.
radiation, facilitate evapotranspiration, and even reduce energy consumption [2,3,5,8]. In the same vein, several studies of the local Singaporean context reach the following conclusions: the locations of large and dense green areas in Singapore correspond with lower measurements of temperature, indicating the function of vegetation quantity in reducing temperatures at the macro level [23,48,51]; temperature measurements are lower not only within parks compared to an external reference point but also in the parks' adjacent built-up environments, highlighting the ability of parks to cool their surroundings [13]; vegetation, especially mature trees, can improve microclimatic conditions within pedestrian canyons through shading [50]. Singapore has been called a “City in a Garden”. Vegetation and vegetated settings are a common sight, with almost half of the city covered in green (as cited in Refs. [44]; pp. 14e15), including manmade spaces such as parks, park connectors, and roadside greenery, and natural areas such as nature reserves. Public urban parks currently constitute about 40% of all man-made green space in the city [29]. There are large-scale plans to boost recreation and biodiversity in these spaces [30] and to ensure an increased quantity of, and accessibility to, key outdoor gathering and activity nodes up to 900ha by 2020 [27]. While green policies may increase functional opportunities and indirectly benefit city-scale thermal conditions, it is believed that the park users' overall experience can be further enhanced with the concurrent consideration of microscale thermal aspects during the implementation of these plans. Accordingly, this paper considers heat at the micro level and
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how it may be moderated or exacerbated by immediate park surroundings. It hypothesizes that quantifying localized thermal conditions in parks will shed light on of the experience of heat for the park user. Comparing these conditions across park settings may reveal important relationships between design and thermal aspects, setting the stage to generate guidelines useful for park design and management to regulate thermal performance. The specific objectives are: 1. To investigate the range of micro-scale thermal performance of existing parks in Singapore through on-site measurements, and 2. To identify related design parameters and examine their influence on thermal performance by identifying extreme thermal points.
Equation (1), requiring air temperature. We calculate mean radiant temperature (tmrt) from Equation (2), an equation from ISO 7726:1998 [21] that was calibrated based on a Singaporean study; simply stated, it increases the accuracy of the measurements taken with a flat grey globe thermometer in a tropical outdoors setting [43]. In all, the measurement variables used to assess the thermal performance of parks in this study are air temperature (ta), globe temperature (tg), and wind velocity (v).
pffiffiffiffiffiffiffiffi tmrt þ ta 10v pffiffiffiffiffiffiffiffi to ¼ 1 þ 10v
(1)
The first part of the paper explains the methods employed during a field study of 10 parks in Singapore. It analyzes data gathered during the observation exercise to understand the thermal performance of these particular parks. It then assesses the different park settings by setting them against the thermal analysis, creating the basis for further quantitative study. The paper concludes by noting the limitations of the research and briefly discussing the potential for future studies.
to ¼ Operative temperature ( C) tmrt ¼ Mean radiant temperature ( C) v ¼ Wind velocity (m/s)
tmrt ¼
tg þ 273:15
4
þ
3:42 109 v0:119 tg ta εD0:4
0:25
273:15
2. Method
(2)
2.1. Defining thermal performance In quantifying the experience of heat, the assessment of thermal performance in this study references the concept of “thermal comfort”. The concept was originally developed for indoor building spaces [34] but has been gaining recognition in the outdoor environment. Thermal comfort is defined as “the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation”. Its analysis relies on four microclimatic parameters e air temperature, radiant temperature, air velocity and relative humidity e and two human factors e metabolic rate and clothing insulation [4]. Of the microclimatic factors, air temperature, is the most relevant for the study of thermal performance in parks as it is one of the most commonly available and easily measured thermal variables [10]. The high humidity in Singapore, with a diurnal range of about 60%e90% all year round [28], cannot be easily decreased but can be offset with lower air and radiant temperature; similarly, wind cannot be easily increased through landscape approaches and requires manipulation at a larger scale of the city [10,14]. Therefore, the present study limits itself to air and radiant temperatures. While human factors are not immediately relevant to an examination of relationships between design and thermal conditions, it is possible to examine human acceptance of the thermal performance of parks. A local study by [52] used the thermal comfort model Predicted Mean Vote (PMV) to determine the neutral and acceptable ranges of operative temperatures in the outdoor urban context of Singapore, described as thermal conditions able to satisfy 100% and 80% of people respectively [4]. The neutral operative temperature was found to be 28.7 C, with the acceptable range between 26.3 C and 31.7 C [52]. The present study uses these numbers as a gauge to assess how the actual thermal performance within the parks measures up to an identified acceptable range of outdoor thermal conditions. Parks' thermal performances can be determined by calculating operative temperatures. Commonly used in the assessment of thermal comfort, operative temperature is the “weighted average of mean radiant temperature and dry-bulb air temperature” [47]. In other words, it combines the effects of convective and radiant heat. To obtain the operative temperatures (Spirn) of the parks, we use
tmrt ¼ Mean radiant temperature ( C) tg ¼ Globe temperature ( C) v ¼ Wind velocity (m/s) ta ¼ Air temperature ( C) D ¼ Globe diameter (mm) ε ¼ Globe emissivity 2.2. Site selection and observation period In order to assess the influence of different design parameters on thermal performance, it is important to include a variety of park settings. Therefore, we conducted a separate study before the observation period to determine suitable parks. Parks located within or near to coastal and forested areas and the high-density city core were disregarded to minimize unfavourable macroclimatic influences on subsequent measurements. Ten regional urban parks distributed evenly across the island were selected (Fig. 1), and a representative path of about 500e600 m in each park was demarcated for observation at the end of the preliminary study. Measurements and mappings were to be taken at short and regular intervals of 10 m along the paths, resulting in about 50e60 points of measurements for each park. The observation exercise was done during the warmest and driest period of the year, from March to May in 2015, and during the hottest part of the day, between 1pm and 3pm, to determine thermal performances under conditions of extreme heat and see how various park settings may augment or aggravate these. To account for slight daily weather variations, we visited each park on three different days for repeated measurements of ta, tg and v at each point. Measurements were made only on sufficiently sunny days, avoiding the occasional cloudy or rainy day; the entire observation period took about eight weeks. 2.3. Field measurement equipment and procedure As measurements for several thermal parameters had to be
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Fig. 1. Locations of 10 selected parks in Singapore.
Table 1 Thermal variables recorded and equipment specifications. Thermal variables
Equipment
Measurement range
Accuracy
Air temperature Wind velocity Globe temperature
Testo 445, with 3-function probe(Shielded air temperature probe) Testo 445, with 3-function probe(Wind velocity probe) Extech HT30 globe thermometera (Ø ¼ 40 mm)
20 Ce70 C 0.0e10 m/s 0 to 80 C
±0.4 C from 0 C to 50 C ±5% or 0.03 m/s whichever is greater ±2 C
a Extech HT30 globe thermometer has been compared with HOBO Thermocouple Data Logger, UX100-014 M with Type-T Copper-Constantan thermocouple sensors and 40 mm diameter grey ping pong ball (The same instruments equation (2) was calibrated for [42]). The amount of variation of tmrt value was within a ±2 C difference under park conditions at 1e3pm.
completed within two hours at many points for each park, it was important to select portable equipment that could be easily carried without compromising precision. To measure air temperature, ta, and wind speed, v, we chose the Testo 445 with a three-function probe, as it had been used in similar studies in Singapore requiring outdoor temperature measurements at different locations [52]. The accuracy of the instrument conforms to ISO 7726 [41]. For measuring radiant temperature, tg, we used an Extech HT30 globe thermometer with a 40 mm diameter black globe because of its rapid response and convenient size [20,42]. The detailed specifications of equipment used are listed in Table 1. Thermal measurements at each point were made at the height of 1.1 m to represent thermal effects on a standing or walking adult [26]. To account for equipment response time when moving from point to point, we recorded the final measurement of air temperature and globe temperature at each point only after the readings on the equipment had stabilized and while the velocity probe was held perpendicular to the direction of the airstream. The shade condition of each point - whether in shade or full sun e was also recorded. We took photographs to capture site details such as trees, path materials and surrounding conditions. For the first round of assessment, to obtain the operative temperature, to, for each point, we calculated the mean tmrt using Equation (2); the final figure was used to calculate to of the point using Equation (1). Points within the highest and lowest 5% range of operative temperatures, referred to in this study as “hot” spots and “cool” spots respectively, were singled out to evaluate differences in park settings with extreme thermal conditions. Next, we studied park settings of “hot” spots and “cool” spots using the photographs; we compared operative temperatures and the presence of shade, and we compared operative temperatures with adjacent park settings to determine the extent of influence design parameters on temperatures.
3. Analysis and results 3.1. Thermal performances of parks We averaged three sets of air temperature measurements in each park to obtain one mean air temperature measurement for each point in each park; the final averages ranged from 32.3 C to 34.9 C.We expected the calculation of operative temperature to provide a more accurate picture of thermal conditions as this measure can account for convective effects and solar and terrestrial radiation with the use of the mean radiant temperature. The resulting operative temperatures of all points ranged from 36.3 C to 51.9 C e much higher than the overall acceptable outdoor range of 26.3 Ce31.7 C derived from the above-mentioned study by Ref. [52]. As Table 2 in their study shows, the lowest operative temperature of 36.3 C would feel thermally comfortable to less than approximately 40% of population; the percentage decreases as
Table 2 Comparison of temperatures between parks. Park name
Yishun Park Punggol Park Jurong Central Park Choa Chu Kang Park Clementi Woods Bishan Park Toa Payoh Town Park One North Park Woodlands Town Garden Tampines Eco Green
ta ( C)
to ( C)
Overall
Range
Overall
Range
32.3 33.1 33.5 33.3 33.4 33.5 33.7 34.0 34.4 34.9
31.6e33.2 32.1e33.7 32.6e35.1 32.0e34.4 31.9e35.1 32.3e34.5 33.1e34.3 32.7e35.7 32.8e37.0 32.5e36.9
40.0 40.5 41.7 42.3 43.2 43.3 43.3 43.5 44.7 46.9
36.3e44.6 37.5e46.0 36.7e46.5 37.5e47.2 38.2e48.1 38.6e47.7 38.7e50.0 38.7e48.4 39.6e50.7 40.3e51.9
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operative temperatures climb. While this indicates that none of the park settings surveyed may be able to achieve an overall acceptable level of thermal comfort at the hottest time of the year, there is room to increase the thermal acceptability of most of the parks by reducing operative temperatures. We calculated the overall operative temperature of each park by taking the average of the operative temperatures of all points within the park, resulting in a range of 40.0 Ce46.9 C (Table 2). We plotted the operative temperature of each point in a park onto a graph; the graphs of each park were placed in order of their overall operative temperature (Fig. 2). As Fig. 2 shows, thermal
performance varies between the parks, with the hotter parks displaying a tendency to have more fluctuations in temperatures and the cooler parks maintaining relatively stable temperatures and having lower readings in many cases. The calculation of the top and bottom five percent of operative temperatures resulted in 29 “hot” spots and 29 “cool” spots out of the 597 points surveyed. “Hot” spots ranged from 48.5 C to 51.9 C, and “cool” spots ranged from 36.3 C to 38.4 C. These spots were then located on the plans of each park, revealing a clustering phenomenon (Fig. 3). As expected, the overall hotter parks such as Tampines Eco Green and Woodlands Town Garden Park had the
Fig. 2. Comparison of operative temperatures between parks; y-axis represents temperature in degree Celsius, x-axis represents points on path.
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Fig. 3. “Hot” spots and “cool” spots located on plans of park.
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bulk of the “hot” spots, and cooler parks such as Yishun Park, Punggol Park, and Jurong Central Park had most of the “cool” spots. To summarize, the inconsistencies highlight that not all green spaces and park settings are equal in terms of thermal modification. Therefore, the next section considers the possible influence of
various park design parameters on temperatures. 3.2. Park settings The photographs of “hot” spots and “cool” spots in Fig. 4 show
Fig. 4. Park settings of singular, or clusters of, “hot” spots and “cool” spots; photos on the left are forward views, photos on the right are backwards views.
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that one of the most obvious differences between the park settings of “hot” spots and “cool” spots is spatial configuration, with “hot” spots occurring in more open or top-exposed areas and “cool” spots in more top-enclosed areas with tree canopies overlapping the paths. The latter are also usually side-exposed with few or no adjacent shrubs, probably a direct result of the interception of radiation by the canopies e or in other words, a result of the provision of shade on the path for the park user. This is in line with several studies advocating the modification of radiation in landscape design because of the high dependence of outdoor thermal experience on the exposure to solar radiation [11,37,43,49]. Although large green areas have been demonstrated to reduce heat at the macro level [13,22,24,38,48,51], the effect of vegetation quantity seems less significant at the micro level. “Hot” spots form even along paths with adjacent dense vegetation layers such as in Tampines Eco Green, Toa Payoh Town, and Woodland Park, while “cool” spots can occur in areas lined only with a few trees such as in Yishun, Punggol, Jurong and Choa Chu Kang Park, indicating vegetation quantity itself may be less important for the park user than the presence of shade, especially if the vegetation is not within close proximity to the path taken by the park user. In terms of path materials, we found no apparent impacts on thermal performance, with the most common materials asphalt appearing in both “hot” spots and “cool” spots. However, it is interesting to note that Tampines Eco Green has persistently high to despite having turf as a path material, building on the previous finding that vegetation quantity does not always seem to improve micro-scale thermal performance. In a similar vein, having a water feature near a path does not always correlate with temperature reduction, especially when the water feature is located in full-sun, as demonstrated by the appearance of certain “hot” spots within both Tampines Eco Green and Toa Payoh Town Park (Fig. 4). Having identified shade as a key influence on park thermal performance, we next plotted to of each point against the presence of shade during measurement on the same graphs to verify their relationship (Fig. 5). In general, operative temperatures tend to be higher when readings were taken at points without shade. However, there seems to be no direct correlation between the presence of shade and the extent of temperature increase or decrease, suggesting the need to study specific characteristics or patterns of shade. As expected “hot” spots develop along long lengths of paths without shade, while “cool” spots tend to occur along the lengthier spans of shaded paths e indicating that the continuity of shade, as opposed to short dashes of shade of the same total length, could be a factor in further reduction of temperatures. Some shaded paths did not develop “cool” spots, so the study was expanded to include shaded areas around the paths. We observed that some of the “hot” spots are, in fact, near to, but not in direct contact with large areas of shade, suggesting that implementing shade next to but not overlapping paths may be ineffective in enhancing thermal performance at the micro level (Fig. 6). Yet “cool” spots usually form in the midst of a large area of shade, implying that shade volume could induce an increased rate of temperature reduction. 4. Conclusions As the preceding analysis of thermal performance in parks shows, to achieve good micro-scale thermal conditions for the park user, merely setting aside a large green space or considering only one of the various thermal parameters or even employing random dense planting is insufficient. It is the combination of a number of factors, especially shade and planting configurations of trees in relation to paths, that determines design sensitivity to the tropical
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climate. The clear variation in thermal conditions within and between parks underscores the importance of considering thermal attributes alongside the typical social, physical, aesthetic and other aspects in designing tropical urban parks. The following key findings may be useful to park designers and managers seeking to optimize the thermal conditions in parks: 1. Even with the presence of shade, the micro-scale thermal performance along foot paths in Singapore urban parks can be extreme during the hottest time of day and the hottest period of the year. Thermal acceptability for hotter parks may still be optimized, however, by referencing park settings with the lowest operative temperatures. 2. On top of having lower temperature readings at points along the path, it may be necessary to minimize temperature variations to create more stabilized and overall improved thermal conditions for park users. 3. Shade is the most important factor in reducing temperatures in the tropical urban context. A high quantity of vegetation may moderate heat on a larger scale but may not improve microscale thermal conditions if unaccompanied by overhead canopies that provide shade on the path for park users. This is linked to the planting configuration of trees along paths. In a similar vein, the use of turf as a path material or water features do not lead to lower temperatures, especially when there is no shade. 4. Providing shade can in itself enhance thermal performance as long as the shade overlaps paths taken by park users; the extent of the effects of shade on reducing temperature may be increased by implementing it in continuous lengths and in greater volume instead of the same quantity in truncated segments. Leaving extensive lengths of paths unshaded can exacerbate micro-scale thermal conditions. Despite these findings being expected outcomes, the significant effect of tree shade on the mitigation of urban heat remains overlooked in parks design in the tropics. BCA's (Building and Construction Authority (Singapore)) “Green Mark for new and existing parks” is a rating system to evaluate environmental impact and performance of parks and to promote a sustainable framework for design; however, its requirements focus on the percentage of greenery without consideration for the provision of tree shade coverage on foot paths, nor the continuity of tree shade [6,7]. In short, the presence and continuity of tree shade should become part of these guidelines used to assess existing parks as well as to design new ones. 5. Discussion The climate of a place is decided by its geographical location and cannot be changed, so heat will always be a factor in outdoor spaces in Singapore. That being said, the results of this study demonstrate how micro-scale thermal performance can vary even within a distance of 10 m, with the experience of tropical heat either exacerbated or alleviated by the immediate surroundings. This suggests steps can be taken to minimize the effect of yearround heat in Singapore; more should be done to assimilate micro thermal conditions into the design and management of parks as a supplement to the present development framework in Singapore. The study is certainly not without its limitations. Its focus on understanding the factors influencing the thermal conditions within parks requires a set of fixed respective thermal conditions; hence, it cannot consider diurnal aspects of thermal performance. However, morning to evening thermal changes can be considered
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Fig. 5. Comparison of operative temperatures and presence of shade; y-axis represents operative temperature in degree Celsius, x-axis represents points on path.
in subsequent studies to illuminate the complete diurnal relationship between temperatures and planting configurations of trees with respect to paths, especially since shade conditions change throughout the day. Now that the importance of shade and its various characteristics to micro-scale thermal performance have been identified, the next step should include quantifying the optimum range of these qualities to increase the effectiveness of design and improve management guidelines for parks. As this study is unable to reach a definitive conclusion on the influence of path material on thermal
performance, an additional study of various materials within and outside the presence of shade may prove useful as the albedo of materials does have a macro-scale effect [10,14,16]. In addition to all the environmental factors affecting the thermal performance of parks, psychological aspects such as human perception play a key role in the experience of these conditions [32,33,52]. Supplementary studies and surveys to quantify the subjective relationship between human perception and the thermal conditions observed in this study may complete the cycle and strengthen climate-adaptive park design in the tropical context.
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Fig. 6. “Hot” spots and “cool” spots in comparison with adjacent shaded areas.
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Acknowledgements This work was made possible by funding from MND Research Fund (MNDRF) for the built environment under Grant Number R295-000-095-490. The authors also wish to acknowledge and thank Zi En Jonathan Yue, research assistant in NUS, for technical support. Thanks also to Ms. Yuin-mae, NG, Deputy Director of Development Management in the National Parks Board Singapore, who collaborated for the park selection process and provided invaluable feedback on the overall research. References [1] H. Akbari, Cooling Our Communities. A Guidebook on Tree Planting and Lightcolored Surfacing, Lawrence Berkeley Laboratory, Berkeley, CA, 1992. [2] H. Akbari, Shade trees reduce building energy use and CO2 emissions from power plants, Environ. Pollut. 116 (2002) S119eS126. [3] H. Akbari, D.M. Kurn, S.E. Bretz, J.W. Hanford, Peak power and cooling energy savings of shade trees, Energy Build. 25 (2) (1997) 139e148, http://dx.doi.org/ 10.1016/S0378-7788(96)01003-1. 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