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1 This article was downloaded by: [University of Colorado - Health Science Library] On: September 1, At: 1:5 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1795 Registered office: Mortimer House, 37-1 Mortimer Street, London W1T 3JH, UK Architectural Science Review Publication details, including instructions for authors and subscription information: Thermal and comfort conditions in a semi-closed rear wooded garden and its adjacent semi-open spaces in a Mediterranean climate (Athens) during summer Ioannis X. Tsiros a & Milo E. Hoffman b a Faculty of Sciences, Agricultural University of Athens, Athens, Greece b Faculty of Architecture and Town Planning, Technion-Israel Institute of Technology, Israel Published online: Sep 13. To cite this article: Ioannis X. Tsiros & Milo E. Hoffman (1) Thermal and comfort conditions in a semi-closed rear wooded garden and its adjacent semi-open spaces in a Mediterranean climate (Athens) during summer, Architectural Science Review, 57:1, 3-8, DOI: 1.18/ To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at

2 Architectural Science Review, 1 Vol. 57, No. 1, 3 8, Thermal and comfort conditions in a semi-closed rear wooded garden and its adjacent semi-open spaces in a Mediterranean climate (Athens) during summer Ioannis X. Tsiros a and Milo E. Hoffman b a Faculty of Sciences, Agricultural University of Athens, Athens, Greece; b Faculty of Architecture and Town Planning, Technion-Israel Institute of Technology, Israel (Received 3 May 1; final version received 13 July 13) The cooling effect in a courtyard s garden and in the adjoining ground- and first floor verandas, attached to the NNE side of a two-storey building is evaluated with measurements performed during a hot weather summer period in Athens. Results revealed a well defined and strong daytime cool island between the buildings rear garden (with about 85% canopy covering) and an air temperature reduction for the ground floor veranda, as compared with an urban square with low canopy coverage (about 15%), reaching a maximum air temperature reduction of.5 K during daytime. Compared with a nearby densely wooded park, the garden and the veranda were found K cooler during and 8 hours during daytime, respectively. Using the physiologically equivalent temperature thermal index with appropriate adjustments to local conditions, it was found that those two sites, compared with the urban square, were able to mitigate the extreme thermal stress conditions and to decrease the daily number of hours associated with strong thermal stress conditions. It is concluded that appropriately designed semi-open spaces in residential buildings, well known from vernacular architecture for their qualitative benefits, may be considered as positive bioclimatic pedestrian transitional elements in sustainable urban design for Mediterranean climates. Keywords: urban microclimate; thermal comfort; vegetation; passive cooling; sustainable urban design Introduction It is well documented that high air temperature values in the Urban Heat Island (UHI) phenomenon along with the associated vehicles heat and air pollutant releases have important implications in terms of summertime energy consumption in buildings, human thermal comfort, incidence of heat-related illness and air pollution (e.g. Pearlmutter, Berliner, and Shaviv 7; AboulNaga, Al-Sallal, and El Diasty ; Givoni 1998; Shaviv 199). The UHI and consequential effects are mainly aggravated because of reduced density of green areas, the trend in urban architecture towards high buildings with wide spacing usually without vegetation among the built-up units and anthropogenic heat releases (e.g. Yuan and Chen 11). There is thus a need for mitigating the heat island-related problems and such means are ranging from innovative building designs (e.g. Emmanuel 1997, 1993) to advanced passive cooling techniques (e.g. Erell, Pearlmutter, and Williamson 1; Santamouris 7). Parallel to such efforts as in the above studies, and from a more general thermal comfort management point-of-view, there is also a need to re-visit and re-evaluate the passive cooling potential of traditional designs (e.g. Shaviv, Yezioro, and Capeluto 1; Szokolay ; Auliciems and Szokolay 7). Their qualitative effects and benefits of such designs are well known from the vernacular architecture (e.g. Mazouz and Zerouala 1999), but in many cases rarely have been investigated in quantitative terms. Such effects are, in principle, microclimate control-based design methods and include inter alia shading, thermal mass and orientation (Auliciems and Szokolay 7). Some quantitative results related to the influence of shading, thermal mass and orientation, on the between the buildings (in streets, and in the open and surrounded by building spaces) air temperature may be found in the early studies of Sharlin and Hoffman (198) and Swaid and Hoffman (199/91a, 199/91b). In many cases, such effects are integrated altogether into urban design elements and features such as courtyards (with or without vegetation), patios, rear gardens, galleries, colonnades and verandas (porticos). Despite the fact that some of these design features are common elements in several vernacular buildings in various climatic regions (e.g. Givoni 199), in general, a limited number of studies have focused on their combined microclimatic bioclimatic behaviour, e.g. courtyard (Potchter and Tepper ), vegetated courtyard (Shashua-Bar, Pearlmutter, and Erell 11), galleries (Ali-Toudert and Mayer 7; Swaid, Bar-El, and Hoffman 1993); atrium and also small vegetated clusters in University campuses (Charalampopoulos et al. 13; Lin, Matzarakis, and Hwang 1) and small gardens inside the urban fabric (Cohen, Potchter, and Matzarakis 1). Corresponding author. s: itsiros@aua.gr; yohan.tsiros@gmail.com 13 Taylor & Francis

3 I.X. Tsiros and M.E. Hoffman This study examines the summertime microclimatic and thermal comfort conditions of open and semi-open spaces in the urban fabric of Athens (Greece), a city for which the UHI is well documented (e.g. Santamouris et al. 1999). Although studies on passive cooling options (vegetation and cool materials) do exist for Athens streets (e.g. Santamouris, Synnefa, and Karlessi 11; Shashua-Bar, Tsiros, and Hoffman 1), there is a lack of studies on small-scale clusters such as open spaces between buildings and similar design features or elements that might have a potential for both passive cooling and thermal comfort enhancement inside the urban fabric. A recent study examined the benefits that small backyards without vegetation may have on the improvement of thermal conditions of the street side building rooms (Tsianaka ). Such backyards, on the urban block scale of the Athens city, are connected and form an uncovered central space inside a block of buildings (courtyard). In this study, the focus is on the vegetated part of such a courtyard (courtyard s garden) and its adjoining semi-open spaces. First, a microclimate analysis of the selected sites is performed based on in situ observations and measurements. The results are also compared with the corresponding conditions in two typical public open spaces, an urban square and an urban park, both located close to the study area. Thermal comfort is then assessed by the use of a well known up-to-date biometeorological index, the physiologically equivalent temperature (PET) index (Matzarakis and Mayer 199; Höppe 1999), appropriate for use in outdoor environments. Prior to this study, a PET range for comfort levels for people in the Mediterranean region had not been developed and was only available for people in Western/Central Europe (Matzarakis and Mayer 199) and in South-eastern Asia (Lin and Matzarakis 8). To overcome this limitation, we use an existing database of microclimatic and thermal sensation data for Athens (Tseliou et al. 1; Nikolopoulou and Lykoudis 7) to estimate the thermal comfort range for the PET index for the Athens locals. Then, the considered spaces were evaluated in terms of human comfort amelioration for the local people, allowing thus the identification of potential beneficial effects that may have the examined sites as passive cooling solutions for thermal comfort enhancement in the urban environment. Study sites and data Area description The main monitored sites are the open and semi-open areas of a private two-storey high mass building located in the wider area of Patissia close to downtown Athens. The urban fabric of Athens is, in general, quite typical: small blocks, narrow streets and a large number of apartment blocks covering most of the ground except the courtyard area, usually a backyard at the core of each block (Tsianaka ). Courtyards, as it is well known, are common architectural solutions, traditionally associated with the Mediterranean semi-arid regions ranging from the Middle East through Northern Africa and Southern Europe (e.g. Pérez de Lama and Cabeza 1991; Edwards et al. ). They can be positive or negative climatic elements of the urban environment, depending on their detailed design including geometry, vegetation presence and materials (e.g. Shashua-Bar and Hoffman ; Potchter and Tepper ; Pérez de Lama and Cabeza 1999; Cadmia 1998; Meir, Pearlmutter, and Etzion 1995; Etzion 199, 199; Mosseri 199). Other studies investigated the potential of courtyards for passive cooling (Canton, Ganem, and Llano 11; Rajapaksha, Nagal, and Okumiya 3), on their benefits that can be used to thermally improve the street side of the building (Tsianaka ) and on their role to promote convective cooling through the transitional spaces of courtyard houses (Ernest and Ford 1). In the case of Athens, it should be noted that courtyards are mainly the result of regulations due to high building density construction in the period where the old family houses have been replaced by small apartment blocks (Tsianaka ). The result was a so-called courtyard area, an irregularly shaped space, usually without plants and trees, mainly designed for daylight and ventilation to the backside rooms (Tsianaka ). In this study, the monitored sites include the garden of such a courtyard and also its adjacent semi-open spaces: a ground floor veranda, formed by structurally extending the roof beyond the building s walls and the first floor veranda of the building, just above the ground floor veranda. All the areas have NNE orientations since they are attached to the NNE side of the two-storey building. The garden of the courtyard is a densely wooded area with high coverage of irrigated vegetation (more than 85% of the floor area). It is delimited by the building s verandas along its SSW side and by the nearby buildings walls along its ESE side and also along its WNW side. The border of the northern side of the vegetated area is partially the wall of a threestorey school building and partially the school s courtyard. A general view of the site is shown in Figure 1a, whereas in Figure 1b the garden and the verandas are shown. Weather conditions, instrumentation and data Measurements were carried out from July 7 to August 8 in 7. The examined experimental period was exceptionally hot for south-eastern Europe and Greece. A number of heat waves hit the area at the end of June, in July and in August and Greece experienced an all time record breaking hot summer with nocturnal air temperatures also to remain at high levels (Founda and Giannakopoulos 9). Some statistics of the air temperature are given in Table 1, whereas representative days, based on air temperature, air vapour pressure (VP) and wind speed and direction are shown in Table. Continuous measurements of both air temperature and relative humidity were carried out using Hobo Pro type sensors, combined sensors of air temperature/ relative humidity and data loggers. The sensors were

4 Architectural Science Review 5 Table 1. Air temperature ( C) statistics for the measurement period (July 7 to August 8, 7), the summer of 7 and the summer from a Measurement period (7 July Air Summer Summer 7 to 8 temperature ( C) August 7) Daily average Daily mean maximum Daily mean minimum Absolute maximum Figure 1. (a) View of the urban block with the courtyard and the garden (in circle). (b) Above: The courtyard s garden; Centre: the building in front of the garden along with the first floor veranda. Below: The ground floor veranda. a Statistics is based on air temperature values recorded at the NOA meteorological station. appropriately housed in makeshift mini meteorological multi-plate screens constructed especially for the purposes of this study. The screens consist of six, 15-mm diameter, aluminium foil reflective radiation protective shields (plates) that were integrated by means of threaded metal rods. To allow sufficient natural ventilation, small-diameter plastic pipes were used as spacers to maintain an about 5- mm gap between two consecutive shields. As in the case of the standard meteorological screen (Stevenson screen), the external surfaces of the shields were painted a glossy white colour, whereas the black on the internal surfaces were painted a matte black colour. In all cases, these mini meteorological screens were installed at a 1.7m height above the ground or the floor, a height that corresponds approximately to the average height of people considering also practical reasons inside the sites. The accuracy of the Hobo Pro sensors, in an environment where there are no errors caused by solar or thermal radiation, is ±. Cat5 C and ±3% relative humidity over to 5 C. Keeping in mind the overall accuracy of the measurements, all the sensors were calibrated over the range of ambient air temperature and relative humidity in order to minimize errors due to possible dissimilar exposure to sunlight. In addition, sporadic measurements of wind speed were also carried out to obtain representative values of the wind speed for various sites using a cup anemometer (Vector Instruments Model A1L; accuracy ±1%, threshold.15m/s). In addition, to derive first-order estimates for the mean radiation temperature (MRT) and to compare them with model-obtained values, the following parameters were also measured in selected dates: net radiation (Kipp & Zonen-Lite Net radiometer), albedo (CM7 albedometer that measures the albedo with two pyranometers combined into one instrument, with μv/(w/m ) sensitivity and ±1% error) and globe temperature using, as suggested in Nikolopoulou, Baker, and Steemers (1999), a Pt1 sensor inserted into a 38-mm diameter hollow acrylic sphere, painted with flat grey matte black paint. In the cases of selected measurement days, all the instrument readings were taken every second with the resulting data averaged and stored every 5min using a Campbell CR1 data logger.

5 I.X. Tsiros and M.E. Hoffman Table. Representative days of the monitoring 7 summer period along with the corresponding values of daily air temperature (average, maximum and minimum), prevailing wind direction, relative humidity and vapour pressure recorded at the NOA meteorological station. Air temperature ( C) Wind Prevailing Relative Date speed (m/s) wind direction humidity (%) VP (mmhg) Average Maximum Minimum July 3. NE July 31.3 S August 3.1 NNE August 7. NNE August 8. NNE August 1.1 E August ESE August 17. ENE August 18.1 ENE August 1. SW August 3 5. ENE Selection of monitored locations and reference sites To appropriately select representative measurement locations inside the examined site, initially totally three locations were measured inside the courtyard s garden. One was under a full tree canopy and its location was quite close to the building so it could also have the benefits of building s shading. The second one was again under a full tree canopy but far away from the building so that the shading of the building would not affect the sensor. The third one was in the centre of the courtyard, a location which usually is the case in similar research studies. The centre of the courtyard, however, has only low-height trees with sparse canopy. Hence, this point is a more or less sunny place and may be affected only a little from the shades of the surrounding trees. Before the final selection of the points to be analysed, a preliminary comparison was performed, in terms of maximum and minimum air temperature. The results showed that the location in the centre of the courtyard, without sufficient tree shading, presented air temperature values quite similar to a standard meteorological station (National Observatory of Athens (NOA) Meteorological Station) since, due to the season of the year (high solar altitude), this sensor is mostly under sunlit conditions. Since this study focuses on thermal comfort conditions and keeping in mind that is not typical for people in the Athens summer climate conditions to spend time in the sunlit point of various urban sites; it was considered redundant to examine further the microclimatic and bioclimatic conditions at this point, and thus it was finally not included in the analysis. By contrast, the other two locations, the one close to the building and the one most far away from the building, were found to present similar air temperature patterns and with difference in air temperature values on the order of.5 1K. Since sensitivity analyses, however, showed that in terms of comfort such differences in air temperature are not important (see sensitivity analyses paragraph in the Discussion section) and considering also that people would probably use the location that provides the most efficient shading (the location close to the building), only the latter was used for further analysis in both microclimatic and bioclimatic terms. It should be noted that considering the scale of the urban block along with the rest of the buildings, the whole area of the open spaces forms an internal courtyard as mentioned before. Since our measurements, however, were not extended to the whole area between the various buildings and were limited only to specific locations inside the area, the scale of our study is mainly a building scale and the examined site is then considered a rear garden. The monitored sites then include the building s rear garden (courtyard s garden) (site Garden ), the adjacent semi-open space of the ground floor veranda (site VerandaGF ) and the veranda of the first floor of the building (site Veranda1F ), just above the ground floor veranda. To estimate the potential cooling effect of the examined sites, the air temperature patterns had to be compared with the corresponding patterns of reference sites. Previous studies on courtyards used, in most cases, only a reference site, usually an open, unobstructed site located relatively close to the under consideration site. In this study, three different sites are used as reference sites. The first two reference sites are typical public outdoor areas, located close to the examined sites: a typical urban square (site Square ) with very low vegetation coverage (on the order of 1% of the floor area) and a typical urban green area (site Park ). The sensors were housed in mini meteorological screens as previously described in detail and they were placed under the vegetation canopy. Furthermore, additional data were obtained from the official meteorological station of the NOA located at Thission area, close to downtown Athens, and they were also used as reference data (site NOA station ). Finally, for the quantitative estimation of all the examined sites configurations, the Sky View Factor (SVF) was used. Fish-eye photographs were then taken and were adjusted properly in black and white format by the RayMan

6 Architectural Science Review 7 software (Matzarakis and Mayer 1) to calculate the SVFs of the examined sites. The SVF decreases progressively among the examined sites with values.37,.1, 7,.5 and. for the square (the less shaded), the first floor veranda, the park, the courtyard and the ground floor veranda (the mostly shaded), respectively. Thermal comfort and heat stress levels in the Athens urban areas Thermal indices For the thermal comfort and thermal stress assessments, appropriate, well-documented thermal bioclimatic indices were applied. The first choice was the index SET* (Gagge, Fobelets, and Berglund 198), a typical, well-documented index, routinely applied to building environment thermal comfort problems which has also an outdoor variant OUT_SET* (Pickup and de Dear ). SET* is designed for indoor use but it is also used in urban open areas (e.g. Lin, De Dear, and Hwang 11), also in semi-open environments (e.g. Hwang and Lin 7). In addition, the PET was considered in this study (Höppe 1999). PET is designed for outdoor use and, as SET* and OUT_SET*, is based on the climatechamber analysis of the human energy balance and takes into account the four most important meteorological parameters influencing thermal comfort (air temperature, relative humidity, mean radiant temperature and wind speed). PET has a widely known unit ( C) and has been shown to fit quite adequately to urban design studies (e.g. Shashua-Bar, Tsiros, and Hoffman 1; Cohen, Potchter, and Matzarakis 1; Yang, Lau, and Qian 11; Lin and Matzarakis 8; Lin, Matzarakis, and Hwang 1; Ali-Toudert and Mayer 7; Johansson and Emmanuel ; Johansson ; Spagnolo and De Dear 3). PET, in this study, is used only as an assessment tool of the thermal environment, i.e. to compare the effects of different urban designs of the examined sites on the thermal comfort conditions. The PET index is the main index for comfort evaluation of the examined sites since it is better fitted to outdoor conditions; this is the index which will be discussed and its results will be used for comfort assessment. PET and the MRT are calculated using the Ray-Man software (Matzarakis, Rutz, and Mayer 7) since recent testing studies of this model showed that it is able to calculate reasonably values of radiation fluxes within typical urban complexes (Matzarakis, Rutz, and Mayer 1; Lin and Matzarakis 8; Lin, Matzarakis, and Hwang 1). Thermal acceptable range for the locals and assessment of thermal classification for the PET index The next step was to provide an assessment of thermal conditions in which the local people in Athens feel comfortable and experience various levels of thermal stress. This assessment is based on the works of Nikolopoulou and Lykoudis (7) and Tseliou et al. (1). It is well known that every thermal bioclimatic index has a scale describing the thermal sensation and/or comfort conditions. The thermal comfort range of an index developed for a particular region may not be applicable, however, to another region due to differences in behavioural adjustment, short- and/or long-term acclimatization effects and also psychological habitation or expectations (e.g. Nikolopoulou and Lykoudis 7; De Dear and Brager 1). The PET classification developed in Matzarakis and Mayer (199), then, may be considered valid for people living in Western/middle European countries. Lin and Matzarakis (8) presented a PET full classification for people living in Taiwan. Johansson and Emmanuel () determined PET values for the comfort range for people in Sri Lanka whereas, more recently, Cheng et al. (1) determined a PET range for comfort levels for people in Hong Kong. A PET classification scale for the Mediterranean region, however, was not available. To estimate thus the thermal comfort range for the selected index for the Athens locals, this study makes use of the database of microclimatic and thermal sensation data for Athens reported in Nikolopoulou and Lykoudis (7) along with the associated values of the PET index for Athens recently reported in the work of Tseliou et al. (1) who also have used the same database. A total number of 153 interviews for Athens (18 in summer, 3 in autumn, 18 in winter and 37 in spring) was available from two different Athens outdoor urban areas (Nikolopoulou and Lykoudis 7). Interviewees have been asked to evaluate subjectively their thermal sensation and also to directly assess their thermal acceptability, i.e. to indicate whether they considered the current thermal environment as comfortable or uncomfortable (Nikolopoulou and Lykoudis 7). To determine the comfort range for the chosen thermal index, this study makes use of the acceptable votes in the questionnaires available from the above database. According to ASHRAE Standard 5 (ASHRAE ; Hwang and Lin 7), comfort conditions are the thermal environmental conditions which are acceptable to 8% or more of the occupants of the space under consideration. In addition, since thermal sensations usually differ among individuals, even in the same environmental setting, de Dear and Bragger () suggested the use of the bin mean vote. Following this suggestion, adopted also in similar studies by other researchers (e.g. Hwang and Lin 7; Lin, Matzarakis, and Hwang 1), the mean thermal votes were used to evaluate the percentages of unacceptability of the thermal conditions. Results of the unacceptability of the thermal conditions by local people in Athens are shown graphically in Figure. Note that, in this figure, the 8% acceptability limits are the intersections of the fitted curve and the % unacceptability line. These limits correspond to the index PET values of 13 and 31 C that can be then considered as acceptable comfort range limits for Athens, with acceptable warm winter or light summer clothing. Note that this range, assuming linearity, and since the 8% criterion was used here, includes, in terms of thermal sensation, the slightly cool class (PET range: C), the neutral class (PET

7 8 I.X. Tsiros and M.E. Hoffman Figure. Unacceptability (%) PET (deg C) Thermal comfort range for outdoors in the Athens built environment for the PET index. range: 19 5 C) and the slightly warm class (PET range: 5 31 C). Assuming again linearity as other similar studies (e.g. Lin and Matzarakis 8), the range of the warmer classes of warm and hot can then be determined by a C increase of the range of the slightly warm class, whereas the range of the cooler classes of cool and cold can be obtained by a C decrease of the range of the slightly cool class. Note that the most important values are the benchmark between the subjective perceptions of warm and hot, 37 C, indicating the start of strong heat stress and also the benchmark between the subjective perceptions of comfortable and warm, 31 C, indicating the start of (moderate) heat stress. A value of 3 C indicates the start of extreme heat stress. The comparison of the thermal sensation classifications for Athens, Taiwan and Western/Middle Europe shows that the thermal comfort range in Athens is higher than that of Western Europe but quite lower than that of Taiwan. People living in Athens can then tolerate better higher temperature than people in northern parts of Europe whereas they can tolerate lower temperature adequately as tolerated by western/middle Europeans and much more than people living in Taiwan. The above values of thermal sensations are used in this study to quantitatively evaluate the thermal environment of the considered spaces for the local people. These values apply to both the open and the semi-open sites, although it is known that there are differences in thermal perception and sensation between semi-open and open spaces (e.g. Hwang and Lin 7). Results Maximum and minimum air temperature Figure 3 shows the daily mean maximum and minimum air temperature values for the examined sites and the reference sites during the whole experimental period (31 days). Data for the courtyard s garden and the ground floor veranda present, in principle, remarkably lower maximum air temperature values than the corresponding values at the reference sites of the NOA meteorological station and the urban square (Figure 3a). More specifically, differences in air temperature between the examined sites and the meteorological station reference site are between 1.5 and.5k, 1.7 and 5.K, and. and 3.K, for the garden, the ground floor veranda and the first floor veranda, respectively. Considering as reference site the urban open area (square), the equivalent differences are in a range of..k for the garden,..9k for the veranda and in a range of. to.9k for the first floor veranda. The differences, having as reference site the park area, are 1. to.5k (garden), 1. to 3.K (ground floor veranda) and. to.9k (first floor veranda). The negative values here indicate that there are occasions where the various sites are warmer than the park area. Note that the highest differences (e.g. 5.K between veranda and NOA; K between garden and square, etc.) occur during a heatwave day (8 of ) with low wind speed (1.m/s) at the reference station. This fact implies that the cooling effect is more pronounced during hotter days as will be discussed further in the next section. Note also that this maximum cooling effect for both the courtyard s garden and the veranda is more than 5% of the approximately 1K air temperature daily range for Athens in summertime (Table 1). In contrast, as shown in Figure 3b, all examined sites (garden, ground floor veranda and first floor veranda) are always found to remain slightly hotter than the park area and also than the urban square at night. Considering as reference site the park area, differences in minimum air temperature are in a range of..7k for the garden,

8 Architectural Science Review 9 (a) /7 7/9 7/31 8/ 8/ 8/ 8/8 8/1 8/1 8/1 8/1 8/18 8/ 8/ 8/ 8/ Time (Month/day) NOA Station Veranda_GF Garden Veranda_1F Park Square (b) Air temperature (deg C) Air temperature (deg C) 18 7/7 7/9 7/31 8/ 8/ 8/ 8/8 8/1 8/1 8/1 8/1 8/18 8/ 8/ 8/ 8/ Time (Month/day) NOA Station Veranda_GF Garden Veranda_1F Park Square Figure 3. Daily mean maximum (a) and minimum (b) air temperature values for the examined sites and the reference sites during the whole experimental period (31 days) K for the ground floor veranda and.8.7k for the first floor veranda; considering as reference site the square, the corresponding differences are in a range of..3k, 1..7K, and..1k, respectively. Differences in air temperature between the examined sites and the NOA meteorological station reference site are 1 to 1.7K (garden),.3 to.7k (veranda) and.3 to 1.3K (veranda first floor), implying that the garden and the first floor veranda have occasionally lower minimum air temperature than the NOA station. Figure shows the correlations between the daily air temperature amplitude at the reference meteorological station at NOA and the corresponding amplitude at the examined urban areas. In all cases, a linear relationship does exist with relatively high values of the correlation coefficient (in all cases around.8). Values of the slopes are.5,.71 and.85, for the ground floor veranda, the garden and the first floor veranda, respectively, indicating thus that, in all cases, the diurnal range at various sites is always smaller than the range at the NOA station. According to this simple

9 7 I.X. Tsiros and M.E. Hoffman (a) 1 Urban site amplitude (K) 1 8 (b) 1 Urban site amplitude (K) VerandaGF Meteorological reference station amplitude (K) 1 8 (c) 1 Urban site amplitude (K) Garden Meteorological reference station amplitude (K) 1 8 Veranda1F Meteorological reference station amplitude (K) Figure. Correlations between daily amplitude of air temperature at the reference meteorological station (NOA site) and at the monitored sites: (a) ground floor veranda (VerandaGF); (b) Garden; (c) first floor veranda (Veranda1F). correlation, a daily range of 1K at the reference meteorological station of NOA, typical for the summer period in Athens as may be seen in Table 1, may correspond to a daily range of.5, 7 and 8.5K, for the ground floor veranda, the garden and the veranda of the first floor, respectively. The damping effect is then much stronger in the case of the ground floor veranda and much weaker, almost half in magnitude, in the case of the first floor veranda. This is due to the fact that the latter is not efficiently shaded (larger openness to the sky), and the ineffectively insulated roof (old building) is heated by the sun during the day. It should be noted that the damping effect is attributed to both morphology and overall thermal mass of the sites. Working on a large number of long-term seasonal data for street canyons, Erell and Williamson (7) were able to differentiate the effects of the two mechanisms and found that this effect may be attributed only to the thermal mass and thus be independent of the urban morphology. In the early study of Sharlin and Hoffman (198), using results from a wide semi-empirical research carried out in climatic conditions similar to Athens these authors defined the thermal mass effect using the Cluster Thermal Time Constant (CTTC) parameter, later used in the CTTC analytical-based thermodynamic urban microclimatic model (Swaid and Hoffman 199/91a, 199/91b) and also in its variant Green-CTTC model (Shashua-Bar and Hoffman ). It is also interesting to note that the values of the slopes of the correlations between the daily air temperature amplitude at NOA and the corresponding amplitude at the examined urban areas (.5,.71 and.85, for the ground floor veranda, the garden and the first floor veranda, respectively) are well correlated (determination coefficient R =.73) with the values of the SVFs for the examined sites. It can be seen then that the cooling effect (i.e. the air temperature reduction in a site compared with the corresponding air temperature at the meteorological station) depends on the openness of the site to the sky: the larger the urban site s openness (1-SVF), the lower the air temperature reduction. Note that some discrepancies to this rule may exist since additional air temperature reduction during the day or increase during the night may be found in gardens and in parks due to evapotranspiration from the trees leaves. This effect of vegetation on the air temperature inside urban clusters is explained and quantitatively described by the use of the convective heat exchange factor of trees (Shashua-Bar and Hoffman ). Cooling and warming effects Figures 5 7 show the diurnal pattern of hourly values of air temperature differences between the examined sites and the reference sites for the entire monitoring period. Note that positive values indicate the existence of a heat island effect (i.e. the site is warmer than the reference site), whereas negative values indicate the existence of a cool island effect. It is shown that the courtyard s garden (Figure 5), presents, in principle, a weak heat island compared with all references sites: on the order of.5k compared with NOA, less than 1K compared with the square and less than 1.5K compared with the park. This warming of the courtyard s garden occurs in all cases hours after midnight but it is also evident for a number of hours before midnight: hours (square), 5 hours (NOA) and hours (park).

10 Architectural Science Review 71 (a) Garden - NOA meteorological station 5 Air temperature (deg C) q5% min median max q75% (b) Air temperature difference (K) (c) Air temperature difference (K) 5 7 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 1: 3: Time (LST, hours) Garden - square : 3: 5: 7: 9: 11: 13: 15: 17: 19: 1: 3: Time (LST, hours) Garden - park : 3: 5: 7: 9: 11: 13: 15: 17: 19: 1: 3: 7 Time (LST, hours) Figure 5. Boxplots of hourly (mean) values of air temperature differences between the garden and the reference sites, NOA meteorological station (a), urban square (b), park and (c) for the entire monitoring period. Negative values indicate the existence of the cool island effect. Positive values indicate the existence of the heat island effect, i.e. the site is warmer than the reference site. q5% min median max q75% q5% min median max q75%

11 7 I.X. Tsiros and M.E. Hoffman (a) Ground floor veranda - NOA meteorological station 5 Air temperature difference (K) q5% min median max q75% (b) Air temperature difference (K) (c) Air temperature difference (K) 7 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 1: 3: Time (LST, hours) Ground floor veranda - square 1: : 3: : 5: : 7: 8: 9: 1: 11: 1: 13: 1: 15: 1: 17: 18: 19: : 1: : 3: : Time (LST, hours) Ground floor veranda - park 5 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 1: 3: Time (LST, hours) Figure. Boxplots of hourly (mean) values of air temperature differences between the ground floor veranda and the reference sites, NOA meteorological station (a), urban square (b), park and (c) for the entire monitoring period. Negative values indicate the existence of the cool island effect. Positive values indicate the existence of the heat island effect, i.e. the site is warmer than the reference site. q5% min median max q75% q5% min median max q75% The heat island intensity becomes highest a few hours after sunset, around 3:h, and then remains, in general, quite stable. Early in the morning (7:h) the garden starts to cool down compared with NOA and the square but compared with the park continues to be hotter for 3 more hours (until noon) of about.5 1K. The cooling effect continues during a significant time course of the daytime (until 18:h compared with the park and until 19:h compared with the square and the NOA). It is quite strong with a median value of about 3K (NOA), K (square) and 1.5K

12 Architectural Science Review 73 (park) with these values to occur around 1:h. During the whole experimental period, the maximum values registered were.5k (NOA),.5K (square) and K (park). Note also that the garden is found to be cooler than the park during consecutive hours (afternoon) with a median value of air T difference about 1.5K. The ground floor veranda (Figure ), presents, in principle, a quite stronger heat island than the garden when compared with various reference sites. First, compared with the NOA station, this site exhibits a weak heat island (.5 1K) hours after sunset and a moderate heat island (.5 1.5K) during 7 hours after midnight. Around 8:h, the site starts to be much cooler than the reference site since before noon a value of 3K is registered and continues to be cooler than the NOA with the same cooling intensity for consecutive hours with a maximum value during the whole experimental period to be about 5K at 13:h. Compared with the square, a quite similar heat island as in the case of the NOA is evident. Similarly, also to the NOA differences, around 9:h the site starts to be much cooler (K) and continues to be substantially cooler than the square during more than 8 consecutive hours with a rather constant median value of 3K. The maximum value of air temperature differences (.5K) is registered at 1:h. After :h the site starts to be hotter than the square with an increasing warming rate becoming hotter about K than the square at midnight. Compared with the park site, the ground floor veranda starts to be substantially hotter after sunset and until midnight (1.5 K) whereas after midnight it continues to be hotter on the order of 3K for almost 7 consecutive hours. Early in the morning the veranda and the park exhibit similar air temperature values but after 9:h, the veranda starts to be cooler than the park and around noon the air temperature difference (median value) is between 1 and 1.5K, whereas during the afternoon hours the difference becomes somewhat higher and up to K. As a result, the veranda is quite cooler than the park for 8 consecutive hours (from 11:h until 18:h), a rather significant daytime course. The absolute maximum value of mean hourly air temperature differences registered was found to be.5k (at 1:h). The first floor veranda (Figure 7), compared with the park area, is always hotter, during both night- and daytime (Figure 7c). The warming effect is rather strong with the highest median value being 3K (late morning) whereas the absolute maximum value of mean hourly air temperature differences registered was found to be 5K. At the same time, as reported previously, the ground floor veranda is about K cooler than the park, whereas the garden is also 1.5K cooler than the park, thus making the air temperature differences between the two verandas rather large, on the order of 5K. Late afternoon (1:h and 17:h), the differences between the sites are minimal whereas after 18:h they start to increase again ( 1K) and around midnight becomes more than 1K. In contrast, compared with the NOA (Figure 7a), this site presents a rather weak heat island (.5 1K hotter than the NOA) during nighttime and also a rather weak cool island (1 1.5K) during daytime (1: 19:h). The absolute maximum value of mean hourly air temperature differences registered was found to be less than 3K. Compared with the square, this site exhibits, similar to the case of the NOA, a rather weak heat island (.5 1K) during a very significant time course of the day (:h 11h). After noontime, the site starts to be cooler and during noon and afternoon hours the site is cooler than the square with a highest median value of 1.5K to be registered late afternoon (17:h) and at the same time with an absolute maximum value of mean hourly air temperature differences of.5k. Cooling effect estimations A further quantitative analysis of the cooling effect of various sites was also performed. More specifically, for each site, the cooling effect was considered and estimated as the difference between the air temperature measured at the site and that of the standard meteorological station (NOA station). Then, a simple linear regression of the cooling effect at each site with respect to the air temperature measured at the NOA meteorological station was performed to quantitatively evaluate its dependence on weather conditions. Note that statistics was performed using (a) hourly and (b) daily average values. In addition, in both cases statistics was also performed excluding days with high wind speed (more than 3.5m/s at the NOA reference station). The values of the regression analysis based on the hourly data are given in Table 3. Table 3a shows that considering all the hourly data (7 hours), only the garden and the ground floor veranda present correlation with the latter to be quite stronger (R =.8) than the former (R =.1). Similar cooling effect patterns were obtained for the two sites, indicating that the background ambient air temperature affects the level of cooling inside the examined sites; the higher the background air temperature, the stronger the cooling effect. The cooling effect of the garden is, however, weaker than the veranda s. Applying the resulted regression, when the background temperature represented by the NOA reference site rises by 1 C, the average cooling effect values are estimated to be enhanced by.5 and. C, for the veranda and the garden, respectively. Excluding days with high wind speed (more than 3.5m/s at the NOA reference station), the values of the determination coefficient (R ) and also the values of the slopes are higher for both sites (Table 3b), indicating that the examined sites are more strongly associated with more intense cooling effects during days with less windy conditions. Values of the regression analysis using all daily values of the maximum, minimum and average air temperature (31 days) are given in Table a. It is shown that only the veranda correlates well, whereas in the case of the garden only the average temperature has correlation with R =.. Excluding days with high wind speed (3.5m/s and higher

13 7 I.X. Tsiros and M.E. Hoffman (a)!st floor veranda - NOA meteorological station 5 Air temperature difference (K) (b) Air temperature difference (K) (c) Air temperature difference (K) : 3: 5: 7: 9: 11: 13: 15: 17: 19: 1: 3: Time (LST, hours) st floor veranda - square 5 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 1: 3: Time (LST, hours) 1st floor veranda - park 3 1: 3: 5: 7: 9: 11: 13: 15: 17: 19: 1: 3: Time (LST, hours) Figure 7. Boxplots of hourly (mean) values of air temperature differences between the first floor veranda and the reference sites, NOA meteorological station (a), urban square (b), park and (c) for the entire monitoring period. Negative values indicate the existence of the cool island effect. Positive values indicate the existence of the heat island effect, i.e. the site is warmer than the reference site. q5% min median max q75% q5% min median max q75% q5% min median max q75%

14 Architectural Science Review 75 Table 3. Linear regression results for the garden s and ground-floor veranda s cooling effect (the difference between the air temperature measured at the site and that of the NOA meteorological station) with respect to the NOA air temperature showing that the ambient air temperature affects the level of cooling inside the urban site. a (a) August 1 and 11, above normals days Courtyard Veranda1F VerandaGF Correlation (determination Site Slope, b coefficient, R ) 5 (a) All data (n = 7) Garden..1 Veranda GF.5.8 (b) Excluding days with high wind speed (n = 5) Garden.9.7 Veranda GF.5.9 a Regression is performed using the hourly air temperature data (a) including all days of the experimental period and (b) excluding days with high wind speed at the reference meteorological station (NOA site). Table. Linear regression results for the garden s and ground-floor veranda s cooling effect (the difference between the air temperature measured at the site and that of the NOA meteorological station) with respect to the NOA air temperature showing that the ambient air temperature affects the level of cooling inside the urban site. a Correlation (determination Slope, b coefficient, R ) Air temperature Garden VerandaGF Garden VerandaGF (a) All data (n = 31) Minimum Maximum Average (b) excluding days with high wind speed (n = 1) Minimum Maximum Average a Regression is performed using mean daily air temperature data (a) including all days of the experimental period and (b) excluding days with high wind speed at the reference meteorological station (NOA site). at the NOA reference station); however, the R values were found to be significantly higher for both sites (Table b). This, as in the case of hourly data, indicates that the examined sites are strongly associated with cooling effects mostly during days with less windy conditions. Note also that the minimum and maximum air temperatures at the veranda site are less affected by the wind whereas all the conditions inside the garden are mostly affected considering less windy conditions. Thermal index patterns Figure 8 shows typical diurnal patterns of the PET index for various sites. The days shown in this figure (August 1 and PET (deg C) (b) PET (deg C) Local standard time (h) August 1 and 11, above normals days Square Park Local atandard time (h) Figure 8. PET index daily patterns calculated for typical summer days (with air temperature close to normal, Table ) at various sites (a) and at the reference sites of the square and the park (b). 11) are both with air temperature above normal values, i.e. they are hot spell days (see Table 1). Since the PET is significantly influenced by the MRT, the patterns of the diurnal courses of PET and MRT are similar and thus only the PET patterns will be discussed. The results show that, for a given day with a certain type of weather, as expected, both MRT and PET values are noticeably higher in the most exposed sites than in the more sheltered sites. More specifically, there is a difference of 8 and 11K in PET between the square (the more exposed site) and the more sheltered shaded sites (ground floor veranda, park and garden) during the significant course of the day (1: 1:h). Despite the considerable differences between the examined sites with respect to their thermal comfort conditions, PET values for all sites during daytime were above the assumed acceptable range for locals (less than 31 C). For the more shaded areas, however, a significant time course is associated with

15 7 I.X. Tsiros and M.E. Hoffman PET values below the value of 37 C, the benchmark for strong heat stress start whereas the maximum values never exceeded the value of 3 C (start of extreme heat stress) even in the case of the hot spell day discussed here. On the contrary, in the case of the poorly shaded urban square, PET values by far exceed the above limit. This is due to the fact that the examined square is characterized by low vegetation coverage and extensive surfaces of hard materials in the ground level, allowing thus both much more shortand long-wave radiation from the upper hemisphere. After 1:h, differences of PET between the sites are becoming smaller and after 18:h they are diminished. Thermal conditions are then influenced by the heat released by various urban surfaces (vertical and horizontal), with PET values in a narrow range of about 5 C for several hours after sunset. Note also that the daily PET amplitudes depend on the design of the site. In the case of the less efficient shaded place (square), the daily amplitude of the PET is relatively large on the order of 5 C. For all the other (more or less) shaded places, the daily amplitude of the PET is lower, being about 15 C for the ground floor veranda, 18 C for the park and the garden, and C for the first floor veranda. Finally, examining the effects of weather conditions on thermal comfort conditions inside various sites, it was found that differences in PET values between the more shaded sites (such as the garden and the veranda) and the urban square reference site are affected by weather conditions, these differences being higher in warmer days, e.g. on the order of 9, 11.5 and 13 C, in the case of a normal day (August 7), a hot spell day (August 11) and a heatwave day (August ), respectively. Thermal comfort conditions and heat stress levels Figures 9 11 show the number of hours of human thermal sensation and stress level for various sites under some characteristic weather conditions occurring during the experimental period. Note that the PET classification for local conditions is used as discussed earlier. Note also that the three most important stress levels are examined: moderate (PET > 31 C), strong (PET > 37 C) and extreme (PET > 3 C). As can be seen, on a daily basis, a significant number of hours with heat stress occur in all cases. The most shaded sites (park, garden and ground floor veranda) are associated, however, only with moderate and strong heat stress whereas the most sunny site (paved square) is also associated with a number of hours with extreme stress conditions, even in the case of a summer day with air temperature conditions below the normal (August ). For the same day (Figure 9a), the most shaded sites are associated only with moderate stress conditions whereas in the case of the first floor veranda some hours of strong heat stress occur. Under normal summer conditions (Figure 9b), there are some hours of strong heat stress conditions in the most shaded sites, whereas in the case of the first floor (a) (b) Garden VerandaGF Veranda1F Park Square 8 8 Moderate Strong Extreme 9 Garden VerandaGF Veranda1F Park Square Moderate Strong Extreme Figure 9. Number of hours with heat stress conditions on a daily basis at the examined sites for representative weather days (Table ): (a) day below normal (August ) and (b) normal day (August 7). (a) (b) Garden VerandaGF Veranda1F Park Square 11 Moderate Strong Extreme 9 11 Garden VerandaGF Veranda1F Park Square Moderate Strong Extreme Figure 1. Number of hours with heat stress conditions on a daily basis at the examined sites for days with air temperature values above normal (Table ): (a) August 11 and (b) August