Theor. Appl. Climatol. 87, 223–237 (2007)DOI 10.1007/s00704-005-0194-4

Meteorological Institute, University of Freiburg, Germany

Thermal comfort in an east–west oriented street canyonin Freiburg (Germany) under hot summer conditions

F. Ali-Toudert and H. Mayer

With 10 Figures

Received March 21, 2005; revised July 29, 2005; accepted November 4, 2005Published online April 27, 2006 # Springer-Verlag 2006

Summary

Field-measurements were conducted in an urban streetcanyon with an east–west orientation, and a height-to-widthratio H=W¼ 1 during cloudless summer weather in 2003in Freiburg, Germany. This experimental work adds to theknowledge available on the microclimate of an urban can-yon and its impact on human comfort. Air temperature Ta,air humidity VP, wind speed v and direction dd were mea-sured continuously. All short-wave and long-wave radiationfluxes from the 3D surroundings were also measured. Thedegree of comfort was defined in terms of physiologicallyequivalent temperature (PET). Furthermore, the data gath-ered within the canyon were compared to data collectedby a permanent urban climate station with the aim of fur-thering the understanding of microclimatic changes due tostreet geometry.

Changes in the meteorological variables Ta, v and dd inthe canyon in comparison to an unobstructed roof levellocation were found to be in good agreement with pre-vious studies, i.e., a small increase of Ta in the canyonadjacent to irradiated surfaces, and a good correlation of vand dd between canyon and roof levels. The dailydynamics of canyon facet irradiances and their impactson the heat gained by a pedestrian were strongly depen-dent on street geometry and orientation. Thermal stresswas mostly attributable to solar exposure. Under cloudlesssummer weather, a standing body was found to absorb,on average, 74% of heat in the form of long-wave irradi-ance and 26% as short-wave irradiance. Shading the pe-destrian as well as the surrounding surfaces is, hence, thefirst strategy in mitigating heat stress in summer under hotconditions.

1. Introduction

With respect to urban climate, the urban streetcanyon is commonly considered as the basicstructural unit of the urban canopy layer (e.g.Oke, 1988; Arnfield, 1990). Basic knowledge ofstreet climate has been provided by several stud-ies which include thermal and energetic charac-teristics, air flow and air pollution (e.g. Nunezand Oke, 1977; Hussain and Lee, 1980; Oke,1981, 1988; de Paul and Shieh, 1986; Nakamuraand Oke, 1988; Arnfield, 1990; Todhunter, 1990;Yoshida et al., 1990=91; Eliasson, 1993; Arnfieldand Mills, 1994; Sini et al., 1996; Kim and Baik,1999; Santamouris et al., 1999; Uehara et al.,2000; Coronel and Alvarez, 2001).

In contrast, only a limited number of studieshave been undertaken to quantify the effects ofthe urban thermal environment on human com-fort. Whereas most of these studies have focusedon urban land use differences (e.g. Mayer andH€ooppe, 1987; Mayer, 1993; Svensson et al.,2003), or behavioural aspects (e.g. Nagara et al.,1996; Nikolopoulou et al., 2001; Spagnolo andde Dear, 2003a, 2003b; Stathopoulos et al., 2004;Nikolopoulou and Lykoudis, 2005), studies di-rectly addressing the role of street geometry,which are explicitly design-oriented, still remain

scarce (e.g. Swaid et al., 1993; Pearlmutter et al.,1999; Grundstr€oom et al., 2003; Ali-Toudert,2005; Ali-Toudert and Mayer, 2006; Ali-Toudertet al., 2005).

On both issues of street microclimate andoutdoor comfort, there has been a greater ten-dency to use numerical modelling methodsrather to conduct experimental studies. The pop-ularity of numerical modelling, over the lastdecades (Arnfield, 2003), is largely attributableto the costly and time consuming exercise of di-rectly recording all the relevant meteorologicalvariables using accurate measurement methods.In particular, continuous observations of radia-tion fluxes surrounding a human body in openspaces are lacking. For convenience, this iscommonly replaced by a globe thermometer asan integral instrument (e.g. Nikolopoulou et al.,2001; Nikolopoulou and Lykoudis, 2005), eventhough it is only accurate indoors (ASHRAE,2001). Yet, in order to validate results obtainedfrom the modelling of urban microclimate,there is need to collect extensive data (Arnfield,2003). In this study, therefore, emphasis isplaced on the experimental method, which isbased on a comprehensive measure of all vari-ables influencing human outdoor comfort withanalysis of their dependence on urban geome-try. The results include an analysis of the streetmicroclimate and, more crucially, the radiantenvironment induced by canyon geometry andtheir incidence on the thermal sensation of astanding person.

2. Site and measurements

In-situ measurements were conducted in an ur-ban street canyon (‘‘Erbprinzenstraße’’) in thedowntown area of Freiburg (48� 000 N, 7� 500 Eand 280 m a.s.l.), a medium-sized city in the south-ern upper Rhine plain in southwest Germany,on the western slopes of the Black Forest moun-tainous region.

The canyon axis is oriented in an east–westdirection (Fig. 1). The street is flanked by longbuildings, which preserve the canyon alignmentfor at least 150 m, despite the presence of a num-ber of gaps in the building fronts. At the measur-ing site, the canyon is symmetric with an aspectratio H=W¼ 1 and a sky view factor SVF¼ 0.26(Fig. 2).

The buildings are almost of equal height, typi-cally of two or three stories with pitched tileroofs. The street is made of asphalt and is 12 mwide. The walls of the buildings are constructedof bricks and are painted with light colours. Win-dows constitute about 30% of the walls. A smallpark with tall trees is located in the vicinity of thecanyon on the west side. The east end of thecanyon opens onto a small planted area whilethe western end connects to a main north–southorientated road. Some sparse vegetation alongthe street is also present.

The experimental work was conducted on 14and 15 July 2003: two hot, sunny days. Althoughthe period of data collection was short, the pre-vailing conditions on these two days were con-sidered representative of a typical hot summer inFreiburg.

Fig. 1. Plan view of the east–west oriented urban streetcanyon street in Freiburg, Germany, showing the locationof the street station at the northern sidewalk and the mea-suring points MP1 to MP4

Fig. 2. Fish-eye photography of the urban street canyon inFreiburg, Germany, at the location of the street station atnorthern sidewalk (top of Fig. is N wall)

224 F. Ali-Toudert and H. Mayer

A vertical mast fitted with temperature, windand radiation sensors was installed at a distanceof 1 m from the northern wall (i.e. station inFig. 1) and referred to as ‘‘street station’’. Thislocation corresponds to the pedestrian sidewalkand hence, thermal comfort is required. This isalso the most critical location with respect tocomfort for this orientation (E–W), as reportedby a previous study (Ali-Toudert and Mayer,2006). Air temperature, air humidity, wind speedand wind direction were continuously recorded atregular time intervals at two heights: 1.4 m and3.1 m a.g.l. (above ground level). The short-waveand long-wave radiation flux densities were mea-sured from the three-dimensional surroundings,i.e. upwards and downwards together with thefour lateral directions (N, E, S and W). All me-teorological variables were recorded in the formof 10-minute-averages (scan interval: 10 s) over a30-hour-period. The instrumentation used at thestreet station is listed in Table 1 and the site isillustrated in Fig. 2.

In order to obtain a spatially differentiated pic-ture of the street microclimate, supplementaryreadings were collected on 14 July and includedmanually taken measurements of air and surfacetemperatures on both sides of the street (MP1 toMP4, Fig. 1). In addition, the data obtained in thestreet were compared to those provided by a per-manent urban climate station in order to clarifythe microclimatic changes within the canyon.This is referred to as ‘‘background station’’ andis run by the Meteorological Institute, Universityof Freiburg (MIF, 2005). The background stationis located on the roof of a high-rise building at aheight of 51 m a.g.l. It is situated in the northernpart of Freiburg, approximately 1500 m from theinvestigated street. Air temperature and humiditysensors were placed 2 m above roof level (a.r.l.)according to the psychrometer method, as well as

the radiation sensor (global radiation). The windsensors (cup anemometer and wind vane) wereplaced at 10 m a.r.l. The data of the backgroundstation and the street station were compared inorder to clarify the microclimatic changes insidethe canyon due to the obstructing effects of thebuildings.

3. Thermal comfort assessment

Thermophysiologically significant indices havebeen developed in human-biometeorology to as-sess the human thermal comfort outdoors, e.g., pre-dicted mean vote PMV (Jendritzky et al., 1990),outdoor standard temperature OUT-SET� (Pickupand de Dear, 1999), physiologically significanttemperature PET (H€ooppe, 1993, 1999) or per-ceived temperature PT (Staiger et al., 1997).Despite some detailed differences in the parame-terizations employed, all these indices rely on thehuman energy balance and are applicable outdoors.

In this study, PET has been used. By defini-tion, PET is the air temperature at which, in atypical indoor setting (Tmrt¼ Ta, VP¼ 12 hPa,v¼ 0.1 ms�1), the heat balance of the humanbody, assuming light activity and a heat transferresistance of the clothing of 0.9 clo, is maintainedwith core and skin temperature equal to thoseunder actual conditions (H€ooppe, 1993). This in-dex is well documented and has been widely usedin urban climate investigations (e.g. Mayer andH€ooppe, 1987; Mayer, 1993; Svensson et al.,2003; Ali-Toudert and Mayer, 2006; Ali-Toudertet al., 2005).

The mean radiant temperature Tmrt, which israther difficult to determine outdoors with ac-curacy (Ali-Toudert, 2005) is required for thecalculation of PET. H€ooppe (1992) described amethod to calculate Tmrt on the basis of measuredshort-wave and long-wave radiation flux densi-

Table 1. Meteorological instrumentation used at the station located in an E–W urban street canyon in Freiburg, Germany

Variable Instrument Height (a.g.l.) Number

Short-wave radiation Pyranometer (CM11, CM21, Kipp & Zonen) 1.4 m 6 (", #, N, E, S, W)Long-wave radiation difference between net all-wave and short-wave radiation 1.4 m 6 (", #, N, E, S, W)Net all-wave radiation Pyradiometer (Schenk) 1.4 m 6 (", #, N, E, S, W)Air temperature Pt-100 (HMP Vaisala) 1.4 m, 3.1 m 2Air humidity Humicap (HMP Vaisala) 1.4 m, 3.1 m 2Wind speed cup anemometer (Vector Instruments) 1.4 m, 3.1 m 2Wind direction wind vane (Vector Instruments) 3.1 m 1

Thermal comfort in an east–west oriented street canyon under hot summer conditions 225

ties from the three-dimensional surroundings ofhumans. The formula for Tmrt (in �C) is:

Tmrt ¼Srad

"p � �B

� �0:25

� 273:2 ð1Þ

with

Srad ¼X6

i¼1

Wi ak � Ki þ al � Lið Þ ð2Þ

where (Ki) and (Li) (i ¼ 1; . . . ; 6) represent,respectively, the short-wave and the long-waveradiation flux densities in the six directions; thetotal radiation flux density is denoted Srad. Theangle factors Wi are the percentage of these fluxesreceived by the human body in each direction. Fora standing person (assimilated to a cylinder-likeshape) Wi equals 0.22 for lateral directions and0.06 upwards and downwards. The short-waveand long-wave absorption coefficients are denotedak and al and assume the values 0.7 and 0.97, re-spectively. The emissivity of the human body "ptakes the value 0.97 and the Stefan-Boltzman con-stant: �B¼ 5.67 � 10�8 Wm�2 K�2.

4. Results

4.1. Microclimate within the urbanstreet canyon

4.1.1 Air and surface temperatures

Figure 3 shows the daily course of the air tem-perature Ta as recorded by the stations in thecanyon and above-roof, together with the sup-plementary readings measured manually at the

four additional points along the sidewalks on14 July.

The air temperature Ta within the urban streetcanyon varied between 18 �C and 35 �C indicat-ing the relatively high thermal level during themeasuring period (Fig. 3), part of the record-breaking heat-wave which affected Europe dur-ing the summer of 2003 (Fink et al., 2004). In thecanyon, there was little difference in Ta measuredat various points before 1300 LST and after 1800LST owing to the well mixed air inside the can-yon. During the afternoon of 14 July, from 1400to 1800 LST, Ta measured at the sunlit part of thestreet, i.e. at the street station, MP3 and MP4,was a few degrees higher than those recorded onthe opposite side of the street (MP1 and MP2)which were mostly in shade. Compared to Taabove roof level, almost no difference is foundduring the period between 0800 and 1300 LST,during which time the street is yet to warm up. Inthe afternoon, Ta was higher at the street station aswell as at MP3 and MP4, than above roof level.

At night, the street is cooler than the free airabove roof (at 53 m a.g.l.) with a maximum dif-ference �Ta of 3 K. This feature is apparentlyanomalous and could be attributed to the micro-climatic differences within the city of Freiburgreported by N€uubler (1979) and to the inversionforming above the mean roof level of Freiburg(approximately 20 m a.g.l.).

The urban canyon surfaces facing south wereirradiated during most of the day and experiencedhigh ground surface temperature Ts and wall sur-face temperature Tw values (Fig. 4a, calculatedfrom L with the emissivity "¼ 0.98), leading to

Fig. 3. Daily variation of the air tem-perature Ta at two heights at the streetstation (hourly mean values) and atdifferent measuring points (MP) with-in the E–W oriented street canyon(H=W¼ 1) as well as at the roof levelof a high-rise building in Freiburg,Germany, on two typical summer days(14=15 July 2003)

226 F. Ali-Toudert and H. Mayer

the increased transfer of heat to air as sensibleheat flux. Figure 4a also shows that peak surfacetemperatures Ts and Tw occurred around 1500 LSTwhich resulted in the air temperature Ta reachingits maximum one hour later (1600 LST). Com-paring both sides of the street on Fig. 3, Ta at thesouth side (MP1 and MP2) showed lower valuesthan at the north side (station, MP3 and MP4)due to the limited heat released by the adjacentsurfaces, as these had noticeably lower temper-atures (Fig. 4b). The maximum Ta differencebetween the two sidewalks was reached at 1700LST and was approximately 3 K for the sameheight (1.4 m). At the street station, Ta at 1.4 ma.g.l. was higher than at 3.1 m a.g.l. as a conse-quence of the increased proximity to the sunlitground and wall surfaces. The peak Ta differencereached 1.2 K in the afternoon, whereas it wasnegligible in the evening hours between all mea-suring points within the canyon, and did not ex-ceed 0.5 K.

The results of Ta are in good agreement withprevious studies conducted for streets withalmost the same characteristics: E–W orientationand H=W¼ 1. Nakamura and Oke (1988) foundthat Ta shows small differences between roof airand canyon air, except close to sunlit urban facetswhere the heat transferred from the heated wallsled to warmer adjacent air.

Yoshida et al. (1990=91) confirmed the insig-nificant warming of canyon air in comparisonto free ambient air and the relative homogeneityof Ta across and along a street canyon. They alsoreported on large differences in the surfacetemperatures between sunlit and shaded sur-faces. Surfaces in shade are noticeably coolerthan irradiated surfaces, and the surface temper-atures can even be lower than Ta. Santamouriset al. (1999) reported almost similar findingsfor a deeper street canyon oriented close toN–S, but with moderate vertical thermal strat-ification. Air temperature differences of up to

Fig. 4. Hourly mean values of (a) air tem-perature Ta, ground surface temperature Tsand wall surface temperature Tw at thestreet station on the northern sidewalk ofthe urban canyon (H=W¼ 1) in Freiburg,Germany, on two typical summer days(14=15 July 2003) and (b) mean Ts andTw values at the points MP1 and MP2(southern sidewalk) and MP3 and MP4(northern sidewalk) on a typical summerday (14 July 2003)

Thermal comfort in an east–west oriented street canyon under hot summer conditions 227

2 K were found between irradiated and shadedsidewalks in various urban canyons under hotsummer conditions (e.g. Nakamura and Oke,1988).

4.1.2 Wind direction and wind speed

The relationship between wind flow above-roofand within the canyon is shown in Figs. 5 and 6.The following analysis should be read bearingin mind the following uncertainties: The back-ground station is about 1500 m from the streetstation and the data describing the above-roofwind conditions were recorded at 61 m a.g.l. Thisis about four times the canyon height and as aresult the wind speed directly at roof level (13 ma.g.l.) is much lower.

The air flow in the canyon is known to be asecondary circulation feature driven by theabove-roof imposed flow (e.g. Nakamura andOke, 1988; Santamouris et al., 1999). Basically,the correlation between the canyon wind andthe wind above roof level is found to be moremarked for high wind speeds, whereas the cou-pling between the upper and secondary flow islost at lower velocities, leading to much morescattering (Figs. 5 and 6b).

During the measuring period, the wind waseither parallel or oblique. Almost no perpendi-cular incidence was recorded. Three distinct andtemporarily consecutive episodes could be ob-served, with remarkably different combinationsof wind directions and speeds, which have in turninfluenced the wind flow characteristics within the

Fig. 5. Ten-minutes mean valuesof (a) wind direction dd and (b)horizontal wind speed v within theurban street canyon (3.1 m a.g.l.)and above roof level (61 m a.g.l.)on two typical summer days (14=15July 2003) in Freiburg, Germany

228 F. Ali-Toudert and H. Mayer

canyon. The wind direction in the canyon de-pended on the angle of incidence in relation tothe canyon’s axis of the upper wind (Fig. 5a).When the wind above-roof is nearly parallel tothe canyon axis (�30�), the wind in the canyonflows in the same direction due to channelling(Nakamura and Oke, 1988; Santamouris et al.,1999). In this case, it corresponded to a thermal-ly induced local circulation system known as‘‘H€oollent€aaler’’, which blows from the east (BlackForest mountainous region) with relatively highspeed (Ernst, 1995). This occurred during thenight between 2000 to 0630 LST. On the firstday, from 1200 LST to 2000 LST, the wind wasblowing at an angle of incidence with moderate

velocity: from the NW quadrant and faster than5 ms�1. This led to a wind inside the canyon flow-ing from the SW direction. This flow scheme hasbeen described as a spiral vortex induced alongthe canyon (e.g. Wedding et al., 1977; Nakamuraand Oke, 1988; Santamouris et al., 1999). Thesimple relationship suggested by Nakamura andOke (1988) for an urban canyon with H=W¼ 1applied for relatively high winds, that is ddcanyon ¼540� � ddroof for ddroof ¼ 180� � 360�, withdd¼ 360� being the north, seems to apply to thepresent case study as a first approximation.

From 1000 to 1300 LST on 14 July and after0630 LST on 15 July, weak winds with no dom-inant direction prevailed above roof level. Thisled to a large scattering in the canyon. In thiscase, the wind flow in the canyon was not onlya mechanically driven circulation but ther-mal effects may have also played a role (e.g.Nakamura and Oke, 1988; Sini et al., 1996;Santamouris et al., 1999; Uehara et al., 2000;Bohnenstengel et al., 2004) especially at thissunlit part of the street. It is quite noticeable thatlow-speed winds in the canyon tend to blowNorth-eastwards. The spacing located near thestation on the north side may have influencedthe wind direction, so that air flowed betweenthe two buildings.

From Fig. 6a, we find a linear relationshipbetween the speed of winds in the canyon andthat of winds above the roof. Winds above theroof move faster relative to those in the can-yon. A linear regression line fitted to the datafor winds blowing from the E (70� to 125�)and from the NW (270� to 335�) had a coeffi-cient of determination R2 ¼ 0.62. This meansthat approximately 62% of the variation in thespeed of winds, from E and NW, in the canyonis accounted for by the movement of winds atroof level. Similarly, considering winds fromother directions, the coefficient of determina-tion was R2 ¼ 0.49. The inference made hereis that a greater proportion of the variation inthe speed of winds from other directions inthe canyon cannot be explained by the dy-namics of the winds above the roof. It wasalso observed (Fig. 6a) that winds from theeast (E) were faster both in the canyon aswell as at the roof level. However, from ourstudy we could not confirm the simple linearrelationship suggested by Nakamura and Oke

Fig. 6. (a) Wind speed v and (b) wind direction dd from thebackground station (61 m a.g.l.) plotted against correspond-ing values within the urban canyon (H=W¼ 1) at 3.1 ma.g.l. on two typical summer days (14=15 July 2003) inFreiburg, Germany

Thermal comfort in an east–west oriented street canyon under hot summer conditions 229

(1988) between the wind speed in the canyonand at roof level.

Some studies suggest the existence of athreshold above which a coupling between thewind outside and inside the canyon may takeplace. In this study, a wind speed of about5 ms�1 (measured at 61 m) may be consideredas a threshold, as shown in Fig. 6b, above whicha strong correlation is found between the inter-nal and external wind direction, whereas muchmore scattering is observed below this limit.By invoking the power law of wind profile, thecorresponding wind speed directly above rooflevel (at 13 m a.g.l.) could be approximated to2 ms�1, which agrees with estimates from pre-vious studies (e.g. de Paul and Shieh, 1986;Nakamura and Oke, 1988).

4.2 Comfort analysis

4.2.1 Short-wave radiation flux densities

The aspect ratio (H=W¼ 1) and the streetorientation (E–W) together with the day ofyear and latitude are responsible for the solarexposure patterns prevailing within the canyonat street level. The north wall is almost per-manently irradiated along with approximatelyone half of the canyon floor. In contrast, thesouth wall and the remaining half of the streetsurface are mostly shaded, except during earlymorning and late afternoon where they are ir-radiated as the sun crosses over the street.These patterns help to understand the followingresults relating to human heat gain within thecanyon.

Fig. 7. Hourly mean values ofshort-wave radiation flux densitiesK (a) and (b) long-wave radiationflux densities L (b) received fromthe six directions surrounding astanding person located at thenorthern sidewalk of an E–W ori-ented street canyon (H=W¼ 1) aswell as downgoing short-waveradiation at the roof level (53 ma.g.l.) on two typical summer days(14=15 July 2003) in Freiburg,Germany

230 F. Ali-Toudert and H. Mayer

Figure 7a illustrates the short-wave radiationfluxes (K) from the six directions on a personstanding near the north wall. The importance ofthe orientation and the location within the can-yon is evident. The downward irradiation (K #)recorded in the canyon has a daily course com-parable to a location with a free horizon, i.e. thebackground station (K #roof). The only exceptionsare before 0900 LST and after 1700 LST wherethe fac�ades obstruct the direct solar beam. Inthe morning, the solar radiation comes fromthe south-east quadrant. As a result, K # is alreadyat 820 Wm�2 by 1100 LST and the radiationfluxes coming from the east K (E!) and fromthe south K (S!) are also relatively high,about 460 Wm�2 and 340 Wm�2, respectively(on 14 July). The maximum values of K # and K(S!) are recorded between 1200 and 1300 LSTas the sun reaches its highest position (63�) withthe sun rays originating exactly from the south.

The radiation fluxes (K (W!) and K (E!))measured parallel to the street axis are sym-metrically reversed as the sun moves from thesouth-east quadrant to the south-west quadrant.Explicitly, K (W!) and K (E!) undergo thesame pattern in the morning and afternoon,respectively. A sharp increase in the energyreceived from both directions is then recordedas the sun’s rays become parallel to the streetaxis, namely around 0900 LST for K (E!) andaround 1700 LST for K (W!) with a maximumvalue of 660 Wm�2. In contrast, values notexceeding 130 Wm�2 are measured before 1300LST for K (W!) and after 1300 LST for K(E!). These correspond to diffuse and diffuselyreflected solar radiation components. Similarly,the short-wave radiation upwards K " as well asfrom the sunlit wall K (N!) do not exceed125 W m�2 and correspond to reflected irradia-tion from the street and wall, (mean albedos of0.15 and 0.13, respectively).

4.2.2 Long-wave radiation flux densities

Solar exposure patterns influence the long-waveradiation fluxes (L) as shown in Fig. 7b. Theasphalt road is mostly irradiated during the day-time and constitutes the highest source of long-wave irradiance within the canyon at street level(L "). The peak value occurs between 1400 and1500 LST and reaches 630 Wm�2, whereas the

lowest value is recorded at 0600 LST and equals440 Wm�2.

The radiant heat from the south facing wall L(N!) is also substantial as it is mostly irra-diated. L (N!) shows a comparable temporalevolution as L ", though of lower magnitude.Indeed, being a vertical surface, it receives lessshort-wave radiation than the horizontal groundsurface. Moreover, the asphalt pavement heatsmuch more than the light coloured brick walls.After 1700 LST, the rate of heat release is sloweddown as the canyon surfaces become shadedand thus cooler as less short-wave radiation isabsorbed.

The radiation flux densities from east L (E!)and west L (W!) are composed of heat releasedfrom the ground surface, the walls and the atmo-sphere. Yet, the influence of the north wall hasbeen dominant as the measurement was con-ducted close to it. In the morning, the amountof radiant heat from both directions is almostequal to that emitted by the north wall until1300 LST and shows approximately the sameincreasing trend. However, the east side releasesslightly more heat in the early afternoon whilefor the west side this occurs during the late after-noon, due to the sun exposure patterns previouslymentioned.

Because of the location of the radiation sensorclose to the north wall, the heat emitted by theatmosphere, together with the street surface andthe south wall, constitute a large part of L (S!).At 1100 LST, L (S!) is clearly lower than theother long-wave radiation flux densities. Themain reason for this is the relatively low long-wave atmospheric radiation L # combined withthe small amount of radiant heat released bythe opposite wall and part of the street still inshade. A rapid increase of L (S!) occurs in theafternoon. The peak value of about 580 Wm�2

was recorded at 1500 LST as a result of the ac-cumulated heat stored in the ground togetherwith the late additional exposure of the wholestreet surface and south fac�ade. The long-waveirradiance decreased from 1700 LST when allcanyon facets became shaded. The slight in-crease of L (S!) at 0800 LST can be explainedby the short exposure time of the southern side ofthe street in the early morning.

At night, the street surface and the north wallremain the main sources of heat and show an

Thermal comfort in an east–west oriented street canyon under hot summer conditions 231

almost equal nocturnal cooling rate. The influenceof these two surfaces are also perceptible in theeast L (E!) and west L (W!) fluxes, and to alesser extent in the south direction L (S!).

4.2.3 Radiant heat gained by a standingperson

In order to better understand the impact of theradiative environment described above on ahuman body, the actual short-wave and long-wave radiation flux densities absorbed by astanding person in each direction have to be anal-ysed. These flux densities take into account thehuman absorption coefficients and the humanshape as expressed by Eq. (2).

The totally absorbed short-wave radiationKabs,total (Fig. 8a) ranges from 0900 to 1700 LST

between 160 and 200 Wm�2. On 14 July 2005,the highest values were recorded at 1100 LST(190 Wm�2) and 1500 LST (200 Wm�2) whenthe sun irradiated the human body laterally fromthe south-east or south-west directions. Becauseof the aspect ratio H=W¼ 1 and the E–W ori-entation of the street, the direct solar radia-tion before and after these two time points isblocked. This explains the sharp increase or de-crease of energy gained at both times, respec-tively. At midday, i.e. between 1200 and 1300LST, Kabs,total is somewhat lower due to thesmaller body surface irradiated as the sun posi-tion is high.

Basically, the human body absorbs less than18 Wm�2 from the wall Kabs (N!) which corre-sponds to diffusely reflected solar radiation,whereas up to a maximum of 68 Wm�2 is ab-

Fig. 8. Hourly mean values ofshort-wave radiation Kabs (a)and long-wave radiation Labs

(b) absorbed by a standing per-son at the northern sidewalk ofan E–W oriented street canyon(H=W¼ 1) on two typical sum-mer days (14=15 July 2003) inFreiburg, Germany

232 F. Ali-Toudert and H. Mayer

sorbed from the opposite side Kabs (S!) facingthe sun in the early afternoon.

The absorbed short-wave radiation from thesouth (Kabs (S!)) around noon, from the east(Kabs (E!)) in the morning, and from the west(Kabs (W!)) in the afternoon contributed sub-stantially to Kabs,total, whereas the portion ofthe absorbed downward radiation (Kabs #) wasdistinctly lower than for Kabs (S!). Similar pat-terns were found for Kabs (E!) and Kabs (W!)but with the situation prevailing in the morningat the east side being symmetrically reflectedon the west side in the afternoon. The highestvalues of Kabs (E!) and Kabs (W!) reached100 Wm�2 at 0900 LST for the east and 1700LST for the west facing side. With peak valuesabout 5 Wm�2, the absorbed upward short-waveradiation (Kabs ") represents the lowest part ofKabs,total.

In contrast to the absorbed short-wave radia-tion flux densities, the absorbed long-wave radia-tion flux densities (Fig. 8b) showed, as expected,a smoother daily evolution. Due to the cylinder-like shape of a standing person, the absorbedlong-wave radiation coming from the lateral

directions was much higher than those directedupwards (Labs ") and downwards (Labs #). Thesevary from 85 Wm�2 to 125 Wm�2 for the lateraldirections as opposed to between 20 Wm�2 to37 Wm�2 in the vertical direction.

Remarkably, the differences between Labs

(E!), Labs (W!), Labs (N!) and Labs (S!)do not exceed 15 Wm�2. This means that theradiant environment is relatively homogenousvertically, in spite of the complex and variableshade patterns within the canyon. This is due tothe fact that the radiant heat received from eachdirection originates from all surfaces (walls andground) and from the sky simultaneously. The larg-est contrast however, is observed for Labs (S!)which shows the lowest amount (88 Wm�2) at1000 LST as the associated surfaces are still cool,and the highest value (124 Wm�2) at 1500 LST,when the canyon surfaces have in the meantimethe stored heat and receive additional energy fromthe irradiated opposite part of the street canyon.

Altogether, the totally absorbed long-waveradiation Labs,total reaches distinctly higher val-ues than Kabs,total, which emphasises the impor-tance of the absorbed long-wave radiation for

Table 2. Percentage of short-wave radiation (SW) and long-wave radiation (LW) absorbed by a standing person at the southfacing side of an E–W oriented street canyon with an aspect ratio H=W¼ 1 on two typical summer days (14=15 July 2003) inFreiburg, Germany

LST (hrs) 11 12 13 14 15 16 17 18 19 20 21 to 5 6 7 8 9 10

SW (%) 29 16 24 26 27 27 24 3 2 1 0 2 4 16 28 29LW (%) 71 74 76 74 73 74 76 97 98 99 100 98 96 84 72 71

Fig. 9. Relationship between thelong-wave radiation absorbed bya standing person (Labs,total) at thenorthern sidewalk of an E–W ori-ented street canyon (H=W¼ 1)and the radiant heat emitted bythe ground (L ") and the adjacentsouth facing wall L (N!) on twotypical summer days (14=15 July2003) in Freiburg, Germany

Thermal comfort in an east–west oriented street canyon under hot summer conditions 233

the radiant heat gained by a standing person.Labs,total peaked between 1400 and 1500 LST(540 Wm�2 on 15 July 2005) and reached itsminimum value between 0500 and 0600 LST(405 Wm�2). At night, Labs,total provided all theradiant heat for a standing person within theurban street canyon, whereas during daylighthours (Kabs,total þLabs,total) consisted of 74%Labs,total and 26% Kabs,total on average (Table 2).This highlights the importance of shading toreduce the radiant heat gained by a standingperson, because it prevents the direct exposureof the person and keeps the adjacent surfacescooler.

The radiant heat gained by a standing personLabs,total is plotted against the heat released bythe ground (L ") and the north wall (L (N!))in Fig. 9. A strong linear relationship is found.The correlation of Labs,total was slightly higherwith L (N!) than with (L ") because the streetstation was close to the north wall and becausethe body was considered in a standing posture(vertical). The importance of the ground hasalready been reported by Watson and Johnson(1988) and is probably more relevant for de-sign purposes if these findings could be ex-tended to other locations across the street (i.e.,further from).

4.2.4 Human thermal sensation

The mean radiant temperature Tmrt, which sumsup the absorbed short-wave and long-waveradiation fluxes, is plotted in Fig. 10 together

with the comfort index PET and the air temper-ature Ta. A large daily amplitude of Tmrt wasfound, with a maximum of 66 �C occurring at1500 LST and a minimum of 20 �C at 0500LST on both days. The course of Tmrt can beeasily understood in light of the two previousgraphs (Fig. 8a and 8b). The exposure of astanding person to the short-wave irradiance ishigh from 0900 to 1700 LST, while long-waveirradiance becomes progressively high through-out the day, with a maximum occurring at 1500LST. After 1700 LST, Tmrt decreases drasticallybecause short-wave irradiance becomes negligi-ble and results in a reduction of the surface tem-peratures and hence less radiant heat. At night,Tmrt values remain high, namely between 20 �Cand 30 �C. This is attributable to the surplus heatreleased by the surfaces.

During the day, Tmrt exceeded Ta to a largeextent (Fig. 10). The peak value of the differenceTmrt–Ta (34 �C) occurred between 1400 and 1500LST on 14 July 2004. As is known from otherinvestigations, Tmrt is almost equal to Ta at night(e.g. Jendritzky et al., 1990; Mayer, 1993).

During the day, the variation of the thermalindex PET was mostly influenced by Tmrt ratherthan by Ta (Fig. 10) although peak air tempera-tures and low-speed winds accentuated the PETvalues. This confirms that Ta alone is an inap-propriate indicator for the assessment of thermalcomfort outdoors during high pressure weather.A maximum PET value of 48 �C was registeredat 1600 LST on both days, while a minimumvalue of about 15 �C was recorded.

Fig. 10. Hourly mean values ofair temperature Ta, mean radianttemperature Tmrt and physiologi-cally equivalent temperature PETon two typical summer days(14=15 July 2003) at the northernsidewalk of an E–W orientedstreet canyon (H=W¼ 1) inFreiburg, Germany

234 F. Ali-Toudert and H. Mayer

5. Discussion and conclusion

In this paper, the findings of an in-situ study con-ducted in an east–west oriented urban canyon,with an aspect ratio H=W¼ 1, located in themid-latitude city of Freiburg (Germany) in sum-mer 2003, are presented. The near-surface micro-climate within the urban street canyon and itsimpact on the thermal comfort of a standingperson outdoors were investigated. The resultsare representative of typical hot and cloudlesssummer days in a mid-latitude location. Mea-surements were taken of the meteorological vari-ables required for determining the thermal indexPET, used here as a thermophysiologically sig-nificant index.

The evolution of the air temperature Ta ob-served within the canyon was in good agreementwith previous field studies. The air temperatureTa was found to be slightly affected by the can-yon geometry except close to irradiated canyonfacets where the air was warmer.

A sensitivity analysis carried out on the basisof the obtained data also revealed that PET in-creases at a rate of about 0.75 K per unit increasein Ta (�PET¼ 3

4Ta), if all other parameters were

kept constant. It is, therefore, inappropriate touse Ta as the main indicator for comfort out-doors under hot and sunny conditions. This con-trasts with the common use of Ta as a comfortindicator (e.g. Swaid et al., 1993; Shashua-Barand Hoffamn, 2000; Coronel and Alvarez,2001; Grundstr€oom et al., 2003). Despite the smalldata sample, the wind flow is found to bestrongly correlated in speed and direction withthe free air and corroborates former field studies.

The results showed the dominant effect of theexposure to the sun on the human thermal discom-fort in the daytime with Tmrt and PET attainingtheir maxima around 66 �C and 48 �C, respec-tively. This was mainly due to the large amountsof energy absorbed by an irradiated standing per-son, namely up to a total of 730 Wm�2. On thenorthern side of the investigated E–W canyon,with H=W¼ 1 and typical urban materials, thepedestrian absorbed about 74% of the total ener-gy as long-wave irradiance (405 to 545 Wm�2)compared to 26% as short-wave irradiance (160to 200 Wm�2) in the daytime. The absorption ofenergy from the sun and the surroundings dependsstrongly on the aspect ratio and orientation and

confirms former numerical results (Ali-Toudertand Mayer, 2006).

Further field investigations are required inorder to verify the generality of these resultsfor other locations and climatic conditions. Arecent one, for instance, was conducted in a sub-tropical desert city (Ali-Toudert et al., 2005).Based on a similar methodology for assessingoutdoor thermal comfort, this study corroboratedmany of the findings of the present study: thedecisive role of the aspect ratio and orientationon the resulting thermal sensation, the conserva-tive character of Ta, the proportions of short-wave and long-wave irradiances absorbed by ahuman body and the effects of building materials.This can partly be explained by the extreme hotconditions that prevailed at the time of studyin Freiburg, which is rather typical for lowerlatitudes.

Intuitively, this study quantitatively confirmsthat shading for both pedestrians and surroundingsurfaces is crucial in mitigating human heatstress. A judicious combination of high aspectratios and orientation, arranging galleries, plant-ing trees, greening the fac�ades or using othershading devices on the walls are a few possiblesolutions. Promoting ventilation through appro-priate urban plan orientation and density is afurther possibility to reduce heat stress. Thesewere verified quantitatively to be effective forthe subtropics (Ali-Toudert, 2005).

Short-wave irradiance absorbed by a pedes-trian is responsible for peak discomfort values.Its impact can be reduced by wearing lightcoloured clothes (ak<0.7). Moreover, special at-tention has to be given to the surfaces them-selves: ground pavements constructed of lightcoloured, porous materials are advisable (Caet al., 1998); thin layer pavements or pavementsmixed with green surfaces for promoting evap-oration from underground are also suitable(Asaeda and Ca, 1993; Aseada et al., 1996) espe-cially in latitudes where summers are not dry,like in Freiburg. Building materials also play arole: high thermal capacity may help to reducesurface temperatures further and thus the heatreleased. However, this could lead to the delayedcooling of deep street canyons (Ali-Toudert et al.,2005).

Finally, as the surrounding radiative environ-ment is decisive in determining outdoor human

Thermal comfort in an east–west oriented street canyon under hot summer conditions 235

thermal comfort, special emphasis was placedhere on an extensive measurement of the short-wave and long-wave radiation flux densitieswithin the urban street. Given the lack of suchinformation, the data gathered in this study couldbe used for validation purposes, such as thenumerical estimation of Tmrt, e.g. by the modelof Asawa et al. (2004) and ENVI-met model(Bruse, 2005).

Acknowledgements

The authors are grateful to Dr. Thomas Holst, Dr. JuttaRost and Dr. Florian Imbery for helping in planning andcarrying out the measurements. The text was proofreadby Dr. Argwings Ranyimbo.

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Authors’ address: Fazia Ali-Toudert (e-mail: fazia.alitoudert@meteo.uni-freiburg.de), Helmut Mayer, Mete-orological Institute, University of Freiburg, Werderring 10,79085 Freiburg, Germany.

Thermal comfort in an east–west oriented street canyon under hot summer conditions 237