Study on the Micro Climate Condition Along Green Pedestrian Canyon in Singapore

Study on the microclimate condition alonga green pedestrian canyon in SingaporeNyuk Hien Wong and Steve Kardinal Jusuf*Department of Building, National University of Singapore, 4 Architecture Drive, Singapore 117566, Singapore

The research on urban canyons in the tropics is still very limited. Currently, the research focus has been on the temperate

climate, especially in the development of the urban canyon temperature prediction model. In the Singapore context, the

cooling impacts of greenery in the form of parks and rooftop gardens have been well established. However, the cooling effects

within the different green canyon forms have not yet been explored. Experimental data were therefore collected in this study to

examine the issues. Firstly, the microclimate condition and the thermal effects along two pedestrian canyons with different

greenery and building distribution conditions were investigated. The results show that the cooling effects inside the canyons

vary as a function of vegetation cover and shading effect from trees and buildings. Planting vegetation within the canyon can

be an effective passive measure to improve the microclimatic condition. Secondly, pedestrian thermal comfort was also cal-

culated. The calculated mean radiant temperature and physiologically equivalent temperature show that mature trees are able

to provide thermal comfort to the pedestrian due to their ability to provide sufficient shading.

Keywords: Green pedestrian canyon; impact of trees; microclimate condition; outdoor thermal comfort; Singapore

INTRODUCTIONRapid urbanization is taking place and 61% of the world’s

population is projected to live in cities and towns by 2030

(United Nations, 1997). Increasing urbanization has turned

cities into densely populated urban areas with less trees,

more roads and more buildings. The loss of vegetations

and their replacements with buildings or pavements increase

the heat storage in the ground layer and building fabrics, con-

tributing to the urban heat island (UHI) phenomenon (Oke,

1982). This phenomenon has become a rising concern to

the quality of the urban environment.

It is evident from the past studies that planting of vege-

tation in urban areas is the simplest and most effective

way of cooling the air temperature. From infrared photo-

graphs, it can be seen that large parks appear as ‘cold

spots’, indicating their ability to mitigate the UHI effect

(Forsyth and Musacchio, 2005). A preliminary study on 61

city parks in Taiwan has shown that urban parks have a

lower average air temperature than the surroundings of

0.81K at noon in the summer (Chang, 2007). Streiling and

Matzarakis (2003) investigated the influence of single and

small cluster groups of trees on the bioclimatic condition

of the city. Between the areas with trees and without trees,

the maximum air temperature difference was 2.2K. The

mean differences of the air temperature were 1.0 and 0.9K,

respectively. The differences between the single tree and

the cluster of trees demonstrated that, on average, there is a

0.1K higher air temperature detectable under the single tree

than under the cluster of tree crowns. The maximum differ-

ences of 30.8 and 34.1K in TMRT were found between places

that were heavy and lightly influenced by trees. The

maximum mean differences are 19.3 and 21.0K, respect-

ively. The physiologically equivalent temperature (PET)

showed maximum differences of 17.6 and 16.6K, whereas

in relation to the mean values, there were maximum differ-

ences of 7.5 and 9.9K.

In the Singapore context, Chen and Wong (2006) investi-

gated the cooling impacts of greenery in the form of parks.

Through a field measurement, the built environment,

located close to a park, has a lower average air temperature

of 1.3K. Thus, the more the parks that are built in an urban

area, the lower the urban air temperature. The results

derived from the traffic analysis and simulation showed

that every 1K air temperature reduction lowers the cooling

energy consumption by 5%. The air temperatures measured

inside the parks also have a strong relationship with the

density of plants, since plants with higher leaf area indexes

(LAIs) may cause lower ambient temperatures.

The shade of trees can also be an effective passive method

of solar control for buildings (Papadakis et al., 2001). The

radiative and thermal loads in the area shaded with high

trees were significantly lower than those of the non-shaded

area. Vegetation, when properly located and arranged, can

*Corresponding author: Email: [email protected]

ARCHITECTURAL SCIENCE REVIEW 53 | 2010 | 196–212doi:10.3763/asre.2009.0029 #2010 Earthscan ISSN: 0003-8628 (print), 1758-9622 (online) www.earthscan.co.uk/journals/asre

provide maximum cooling and reduce the outdoor air temp-

erature by up to 2.7K (Parker, 1983).

Wong et al. (2002) studied the impact of intensive and

extensive rooftop greenery on buildings and on the environ-

ment in Singapore. Rooftop greenery can provide benefits

not only to the building, but also to the environment’s

ambient temperature condition. With an intensive system,

the surface temperature may be reduced by up to 31K and

the ambient temperature at 1m may be reduced by up to

1.5K. The impact of rooftop greenery is clearer for a metal

roof. Without plants, the surface temperature of the metal

roof can be up to 60–708C during daytime and lower than

20K at night-time. With plants, it ranges only from 248Cto 328C. The benefits of reducing the surface temperature

by greenery can be observed from the mean surface tempera-

ture differences between the hard metal surface and those

below the plants. They are 4.7, 1.9 and 1.4K with the pres-

ence of dense plants, sparse plants and weed, respectively.

However, in urban areas, having large greenery areas may

be a constraint. As a result, the microclimatic conditions in

these canyons become the most crucial element in influen-

cing the city’s overall climate (Shashua-Bar and Hoffman,

2004). A city with various land uses may comprise warm

and cold areas as a result of distinct urban land use

change. The change between park and built-up area can

produce urban air temperature differences up to 7K

(Spronken-Smith, 1998).

Toudert (2005) studied different street designs to improve

outdoor thermal comfort in the hot and dry climate of

Ghardia, Algeria and in the temperate climate of Freiburg,

Germany. The study found that wide streets (H/W � 0.5)

are not favourable for either E–W or N–S street orientation.

However, N–S orientation has some advantages and its

benefit increases as the aspect ratio (H/W) increases.

Shashua-Bar and Hoffman (2004) found similar findings in

their microclimatic study of urban streets and courtyards

with trees. Increasing the building height from 12 to 24m

reduces the air temperature by 1.5K in a street or a courtyard

24m wide. The cooling effect in the N–S street orientation is

slightly stronger, about 0.64K, than that in the E–W orien-

tation in the H/W ¼ 1.0 cluster with high albedo. Another

study in a hot and dry climate (Johansson, 2006) also con-

cluded that a deep canyon is favourable to providing shade

in the summer, cooler by as much as 10K during the

hottest period, as compared to a shallow canyon.

In addition to canyon geometry, materials and orientation,

researchers have also been interested in vegetation as a

climate component in urban street design. Shashua-Bar and

Hoffman (2000) studied the cooling effect of 11 small

urban green sites with trees in Tel-Aviv, Israel. Tree

shading provided an average of a 3K cooling effect at noon-

time, while the specific cooling effect of the site due to its

geometry and tree characteristics, besides the shading, was

found to be relatively small at about 0.5K.

Many researchers have explored the climatic impact of

vegetation and small green areas on either building energy

or various canyon types. However, similar studies in tropical

climates are rather limited and the cooling effects within the

different green canyon forms have not been explored. This

study will look into this aspect of work with the aim of pro-

viding a better understanding of the microclimatic environ-

ment in these canyons. This article involves two aspects of

study: (i) investigation of the microclimatic condition of

two different pedestrian canyons and (ii) the cooling effect

inside the canyons, including canyon ability to provide

thermal comfort to pedestrians.

CLIMATE OF SINGAPORE1

Located between latitudes 18090N and 18290N and longitudes

1038360E and 1048250E, Singapore can be classified as

having a hot humid climate. Uniform high air temperatures,

humidity and rainfall throughout the year characterize the

climate. The diurnal temperature variations are small with

the range for minimum and maximum air temperatures of

23–268C and 31–348C, respectively. The mean annual air

temperature was 27.48C between the period 1982 and 2001.

Relative humidity (RH) is generally high and although it

invariably exceeds 90% in the early hours of the morning just

before sunrise, it frequently falls to 60% during the after-

noons when there is no rain. During the prolonged heavy

rains, RH often reaches 100%. Between the period 1982

and 2001, the mean annual RH was 83.5%.

There are two main seasons in Singapore: northeast (NE)

monsoon and southwest (SW) monsoon seasons. The NE

monsoon occurs between November and early March, with

the prevailing wind blowing from north to northeast. Mean-

while, the SW monsoon occurs between June and Septem-

ber, with the prevailing wind blowing from south to

southwest. Two short inter-monsoon periods with a duration

of two months separate the main seasons.

There is no clear distinct wet or dry season as rainfall

occurs throughout the year. However, the NE monsoon

season is considered as wet weather, since the wind is gener-

ally cool and brings frequent spells of wet weather at about

48% of total annual rainfall. On the other hand, the SW

monsoon wind brings about 36% of total annual rainfall.

METHODOLOGY

Object of studyThe National University of Singapore (NUS) campus

complex can be considered as a ‘city’ on a smaller scale

(Figure 1). From the estate-wide air temperature measure-

ment (Wong et al., 2007a, b), daytime heat island intensity

(4K) was found to be higher than night-time heat island

intensity (3K). Prince George Park Residence (PGP) is one

of the hottest areas in the NUS campus due to its high-

density building arrangement and less greenery.

Study on the microclimate condition along a green pedestrian canyon in Singapore 197

ARCHITECTURAL SCIENCE REVIEW

Two different characteristics of pedestrian canyons, in the

Faculty of Engineering (ENG) and the PGP of NUS, were

selected for the measurements. The first canyon was situated

in ENG, between the E4 and E5 buildings. It was 14m wide,

enclosed between two blocks of four-storey-high buildings

(Figure 1). The average H/W was 1.3. The site was predomin-

antly mature trees and they covered the canyon extensively.

The second canyon was in PGP . It was on average 13m wide

Figure 1 | Measurement points at the ENG (see Table 1 for the detailed equipment)

198 Wong and Jusuf


with 7–13-storey-high residential buildings adjoining it

(Figure 2). The calculated average H/W was 1.7. The

canyon had a relatively young and moderate or low

canopy height of trees and shrubs as compared to the

canyon sidewalk at ENG. Both locations had the same

NW–SE orientation.

Figure 2 | Measurement points at the PGP (see Table 1 for the detailed equipment)



Field measurementsField measurements were carried out from 17 July to 20

October 2007. Table 1 and Figure 3 show the complete

measured parameters and equipment. All the meteorological

equipment was calibrated for a few days in a controlled

environment.

Microclimate condition inside the canyonFigures 1 and 2 show the various measured parameters and

the locations of measurement points in both locations. At

ENG, there were eight measurement points stretching over

an approximate length of 135m. Points 1–5 were placed

within the canyon, while Points 6–8 were placed outside

it. At PGP Residences, there were seven measurement

points installed over a span of 125m.

Meanwhile, HOBO weather stations (WS 1) were put on a

nearby (within 100m) open place with minimal effect from the

canyon and greenery. The WSs recorded the weather data,

which included air temperature, RH, solar radiation and wind

speed/direction. These WSs served as reference points for the

respective canyons and were configured to measure at 1-min

intervals. Other HOBO weather stations (WS 2) were put in

the middle of the canyon, mainly to measure wind speed/direc-

tion and solar radiation at pedestrian level inside the canyon.

NUS weather station (NUS WS)Meteorological data from the NUS WS were also collected to

obtain the climatic data above the canyons. The WS is

located on the rooftop of a building in the ENG and main-

tained by the Department of Geography (2008), NUS.

Calculation of mean radiant temperature (MRT)and PETTo study the performance of two different characteristics of

canyons in providing thermal comfort to the pedestrian, a

thermal comfort index, PET, was calculated. PET is

defined to be equivalent to the air temperature that is required

to reproduce, in a standardized indoor setting and for a stan-

dardized person, the core and skin temperatures that are

observed under the conditions being assessed (Hoppe,

1999). Hoppe developed PET based on the Munich energy-

balance model for individuals (MEMI), which models the

thermal conditions of the human body in a physiologically

relevant way. This model is a modification of Fanger’s

indoor thermal comfort indices ‘predicted mean vote’ and

‘predicted percentage dissatisfied’, so that it is applicable

to the outdoor conditions by assigning appropriate par-

ameters to adjust the model with a much more complex

outdoor radiation condition. The MEMI model is based on

the energy balance Equation (1) for the human body,

where the unit of all heat flows is in Watt:

M þW þ Rþ C þ ED þ ERe þ ESw þ S ¼ 0 ð1Þ

where M is the metabolic rate (internal energy production);

W is the physical work output; R is the net radiation of the

body; C is the convective heat flow; ED is the latent heat

flow to evaporate water diffusing through the skin (imper-

ceptible perspiration); ERe is the sum of heat flows for

Table 1 | Measurement equipment and parameters

Symbol Parameter Equipment Accuracy Sensorheight

Logginginterval

Air temperature and RH(Continuous)

HOBO H08-003-02 +0.28C at218C

1.8m 1min, averagedto 1h data

Ground surface temperature(Continuous)

T-type thermocouplewire and HOBO U12-014

+1.58C Onsurface

1min, averagedto 1h data

Wind speed/direction insidecanyon and solar radiationbelow tree (Continuous)

S-WCA-M003 wind speedand direction sensorsSilicon pyranometer #S-LIB-M003

+0.5m/s+10W/m2

1.8m1.8m

1min, averagedto 1h data1min, averagedto 1h data

Complete weather stations(Continuous)

HOBO weather station 1.8m 1min, averagedto 1h data

Figure 3 | Weather station at reference point (left) and HOBOon the lamppost for temperature/RH measurement inside thecanyon (right)

200 Wong and Jusuf


heating and humidifying the inspired air; ESw is the heat flow

due to evaporation of sweat and S is the storage heat flow for

heating or cooling the body mass.

The individual heat flows in Equation (1) are controlled

by the following meteorological parameters (Hoppe, 1999):

† air temperature: C, ERe

† humidity: ED, ERe, ESw

† wind velocity: C, ESw

† mean radiant temperature: R.

PET has been used in many outdoor thermal comfort studies

(Matzarakis et al., 1999, Spagnolo and de Dear, 2003) and is

sufficient for use in a comparison study of thermal comfort in

tropical climate canyons (Johansson and Emmanuel, 2006).

Another important parameter to determine thermal

comfort is MRT, especially in outdoor conditions, and

solar radiation intensity is the main climatic parameter that

influences the MRT value. RayMan 1.2 software (Matzara-

kis et al., 2000) was used to calculate the MRT and PET

values of the ENG and PGP canyons.

To calculate the MRT and PET values inside the canyon,

solar radiation below tree data was used rather than solar

radiation at the reference points from the respective

canyons. The tree in the ‘obstacle’ menu of RayMan 1.2 soft-

ware serves as a parameter to calculate the sky view factor

(SVF) value. It is important to note that using the reference

point’s solar radiation data will present a misleading MRT

and PET value for the ENG canyon that has mature trees

fully covering its canyon. It will provide high MRT and

PET values, which to the contrary is not the case.

The PET threshold value of 338C was used as the upper

limit value to achieve outdoor thermal comfort, borrowed

from the Dhaka outdoor thermal comfort zone (Ahmed,

2003). Johansson and Emmanuel (2006) used this method,

since there is no established outdoor thermal comfort zone

for tropical climate. In the Singapore context, Dhaka’s

PET threshold value is probably too high, since Singapore

has a greener environment and less pollution, which may

give a higher tolerance for Singapore’s inhabitants towards

hot and humid climate conditions.

For the subject parameters, light trousers and short-

sleeved clothing of 0.5clo were used and activity of 115W

was set as walking at a speed of 0.89m/s (ASHRAE,

1989). The other climatic parameters (air temperature, RH,

wind speed, solar radiation) were structured as the average

of 24-hour data on selected hot days.

FINDINGS AND DISCUSSIONThe data analysis focuses on fairly clear and hot weather

conditions, selected by analysing the solar radiation and air

temperature data. Table 2 shows the selected dates for the

analysis.

Microclimatic condition inside the canyonsAir temperatureBefore discussing the air temperature profile inside the

canyon, the background air temperature of the respective

canyons was studied (see Figure 4). On the typical hot

days 5–6 August 2008, from morning until noontime, the

ENG site has slightly higher air temperature than the PGP

site and it remains higher until around 19.00 hours,

whereas the PGP site is warmer until around 08.00 hours.

The heat storage flux in the ground during this period is

negative and it gives heat to the surface after 16.00 hours,

which is greater in the PGP site than in the ENG site since

the PGP site is characterized by high-density building

arrangement and less greenery (Hamdi and Schayes, 2005).

The air temperature difference between NUS WS and both

canyons’ reference points (WS 1) is larger during daytime

than at night-time. As mentioned, NUS WS was located on

the engineering building rooftop, where the wind speed is

higher than at the ground level and the prevailing wind direc-

tion was relatively perpendicular to canyon orientation (see

the Wind section below). Hence, during daytime, the wind

shear provides a more cooling effect at the rooftop level as

compared to the ground level. Meanwhile, during night-time

when there is no solar radiation, the thermal properties of the

building fabrics and the pavement generate the temperature

difference (Arnfield, 2003).

The average air temperatures of all the measurement

points inside the ENG (Points 1–5) and PGP (Points 1–7)

canyons are shown in Figure 5 together with their respective

reference point to show the average air temperature condition

along the canyon. The air temperature inside the ENG

canyon is lower around 0.7–1.1K as compared to the PGP

site during daytime and maintains its coolness at about

0.4–0.5K during night-time. The mature trees planted

along the ENG canyon provide very good shading to the ped-

estrian walkway (see Figure 1 as compared to Figure 2).

Shashua-Bar and Hoffman (2000) also found that higher

tree canopies reduce the heating effect from the surrounding

environment through the shading effect.

It is very clear from Figure 5 that the air temperature

inside the ENG canyon is lower than its reference point.

Meanwhile, the condition is reversed in the PGP site where

young palm trees dominate its canyon. These palm trees

are not able to provide sufficient shading and cooling to

the environment due to the condition of their leaves. The

LAI value is only 2.2 as compared to 5.3, the mature trees

in the ENG site. The time shift of peak air temperature is

Table 2 | Selected days of fairly clear and hot weather condition (total 20 days)

July August September October

Selected dates - 2, 5, 6, 7, 11, 25 5, 9, 13, 19, 20, 21, 22, 25, 28, 30 3, 4, 5, 6



Figure 4 | Temperature condition at ENG, PGP reference point (WS 1) and NUS WS

Figure 5 | Average air temperature (20 hot days) of all the measurement points inside the ENG and PGP canyons as comparedto ENG, PGP reference point (WS 1) and NUS WS during the period between 17 July and 20 October 2007

202 Wong and Jusuf


probably due to differentiation of solar reception inside the

canyon. The air temperature in the ENG site is at the

maximum at 15.00 hours, while at the PGP site

the maximum is at 14.00 hours. It can be concluded that

the maximum air temperature at different sites may not

occur at the same time. There is a strong influence from

the site’s specific characteristics, such as canyon geometry

and the existence and density of greenery.

Relative humidityIn the early morning, the RH on both sites is relatively

similar to the average of around 82% (see Figure 6). The

RH difference is then becoming larger when the sun starts

rising and reaching its maximum around 15.00 hours when

the ENG site is 5% higher. This can be explained by the

fact that PGP, on average, has a higher air temperature and

less greenery.

Solar radiation below tree and surface temperatureThe solar radiation inside the PGP canyon is higher as com-

pared to the ENG canyon (see Figure 7). Although both the

solar sensors were placed below the tree, it shows that palm

trees, which dominate the PGP canyon, are not able to

provide sufficient shading. The tree’s leaves density has a

strong correlation with the tree’s ability to intercept solar

radiation that is described in Beer’s law (Jones, 1992). The

sudden drop in the solar radiation intensity at the PGP site

and the increase at the ENG site at 14.00 hours are mainly

due to the positioning of the solar radiation sensors. At

14.00 hours, at the PGP site, multiple layers of palm

leaves coincidentally blocked the solar radiation. Similarly,

at the ENG site, the solar radiation was able to penetrate

through the gap between mature trees.

The ground surface temperature at the PGP site is higher,

on average, by 2K as compared to the ground surface temp-

erature at the ENG site (Figure 8). This is because the solar

radiation received by the PGP canyon is higher. However,

this surface temperature is not as high as compared to the

bare asphalt surface, which can reach a maximum surface

temperature up to 618C (Santamouris, 2001). The pavement

of the PGP site is made from yellow sandstone, which is con-

sidered as a light colour. A light colour pavement has been

proved to have a lower surface temperature, since it helps

to reflect the heat (Doulos et al., 2004). The measured

average solar reflectivity from the ENG pavement is 0.25

as compared to 0.3 in the PGP site. However, the high reflec-

tivity of a sandstone pavement reduces the visual comfort of

pedestrians.

The ground surface temperature in the ENG site is similar

to the ambient air temperature, although the pavement is

Figure 6 | Average RH of all the measurement points inside the ENG and PGP canyons on clear hot days between 17 July and20 October 2007



Figure 7 | Average solar radiation below tree (20 hot days) measured at point WS 2 in the ENG and PGP canyons during theperiod between 17 July and 20 October 2007

Figure 8 | Average ground surface temperature inside the ENG and PGP canyons (20 hot days) during the period between 17July and 20 October 2007

204 Wong and Jusuf


made of black pebble stone. This is because the mature trees

can provide very good shading. Meanwhile, in the PGP site,

the ground surface temperature can be up to 2K higher as

compared to the ambient temperature. With a higher

surface temperature, the thermal comfort of pedestrians can

be affected due to a higher mean radiant temperature

(Johansson and Emmanuel, 2006).

WindIn Figure 9, the average wind speed over the 20 hot days is

between 1.0 and 4.0m/s measured at the NUS WS (rooftop).

Meanwhile, the wind speed at the pedestrian level is low, less

than 1.5m/s during daytime and less than 0.5m/s during

night-time. The graph also shows a pattern in which the

wind speed starts to increase at 09.00 hours and reaches its

maximum speed at 15.00 hours. The ENG reference point

(WS 1) has the lowest average wind speed due to its location,

surrounded by some mature trees and building blocks, which

prevents it from prevailing wind blow most of the time,

although the location was relatively open to the sky. On

average, the wind speed inside the ENG canyon is lower

than inside the PGP canyon. The dense greenery inside the

ENG canyon is believed to reduce the wind speed that

passed through inside the canyon. Trees are able not only

to provide shading, but also to block and redirect wind direc-

tion (Olgyay, 1992).

Figure 10 shows that the prevailing wind direction during

the period of measurement came from the southern and

southeastern directions, relatively perpendicular to the

canyon’s orientation, where around 22% was calm wind

(0.5–2.1m/s). The wind inside the ENG canyon followed

the canyon orientation. The wind blew in a direction from

Point 1 to Point 5. This is understandable because the area

outside the canyon edge in Point 1 is an open space, which

has a higher altitude than the other side of the canyon

Meanwhile, the observation found inside the ENG canyon

is different from that inside the PGP canyon. In the PGP

canyon, the wind direction follows the southern prevailing

wind direction. The position of WS 2 for wind measurement

was located in a small open space in the middle of the canyon

(see Figure 2). Hence, the wind direction at this location was

not influenced by the canyon orientation as in the ENG

canyon.

Calculated MRT and PET valuesFigures 11 and 12 show the calculated MRT and PET values

inside the ENG and PGP canyons and their respective refer-

ence points. The representative point for the ENG canyon is

Point 3 (ENG 3) located in the middle of the canyon and

Figure 9 | Average wind speed (20 hot days) in ENG, PGP and NUS WS (20 hot days) during the period between 17 July and20 October 2007



Figure 10 | Wind speed and prevailing wind (‘blowing from’) direction in ENG, PGP and NUS WS

206 Wong and Jusuf


Figure 11 | Calculated MRT inside the ENG and PGP canyons and its respective reference point on selected clear hot days

Figure 12 | Calculated PET value inside the ENG and PGP canyons and its respective reference point on selected clear hotdays



fully covered below by mature trees and that for the PGP

caynon is Point 3 located in the middle of the canyon

under a young palm tree. As mentioned in the methodology,

the calculation made use of the solar radiation data below

the trees. Consequently, for the PGP canyon, the calculated

MRT and PET values for a subject who walks along the

PGP canyon were expected to be higher, close to the value

of the PGP reference point value, since young palm trees

are not able to fully cover the canyons and high solar radi-

ation intensity inside the canyon can be expected.

The calculated MRT-ENG 3 value is lower than the

MRT-PGP 3 one (see Figure 11). The young palm tree is

not able to provide shading to the subject as compared to

the mature tree. On the other hand, the MRT of the

ENG reference point is slightly higher than the PGP refer-

ence point due to a higher SVF value (0.91 compared to

0.86).

The calculated PET value inside the ENG and PGP

canyons is shown in Figure 12. At the ENG canyon, the

mature trees are able to provide thermal comfort at almost

all daytime hours, except at 14.00 hours, when the solar radi-

ation penetrated in between the trees. On the other hand,

young palm trees in the PGP canyon are not able to

provide comfort to pedestrians due to their less dense leave

characteristics.

Cooling effect of greenery along the canyonIn this section, the air temperature behaviour in each

measurement point along the canyon and the cooling effect

of greenery are discussed. The cooling effect of each

measurement point was calculated by the subtraction of the

air temperature at respective points with the reference point.

At the ENG siteTo determine the time of maximum cooling effect occur-

rence inside the canyon, the cooling effect profiles of

each measurement point were plotted against the 24h time-

line (see Figure 13). This shows that at most of the

points, especially points inside the canyon (Points 1–5),

the maximum cooling effect occurs twice a day, in the

morning at 11.00 hours and in the afternoon at 15.00

hours. This phenomenon is different from the findings of

Shashua-Bar and Hoffman (2000), who describe in the

methodology that the maximum cooling effect occurred

only at 15.00 hours, which coincided with the maximum

daily air temperature in Tel-Aviv, Israel. For Points 5

and 6, the maximum cooling effect occurred only once in

the morning at around 10.00 hours, while in the afternoon

the cooling effect was negligible. These Points 5 and 6

were located at the edge of the canyon and outside the

canyon, respectively, where there is less shading from

Figure 13 | Averaged cooling effect (20 hot days) measured at points inside the canyon in the ENG site

208 Wong and Jusuf


the greenery, and it was observed that anthropogenic heat

from the opposite building’s air-conditioning cooling

tower further dismisses the cooling effect from the sur-

rounding trees. Meanwhile, although located outside the

canyon, the maximum cooling effect occurred twice at

Points 7 and 8. This is because they were located near

mature trees.

This twice-maximum cooling effect phenomenon can be

explained as the effectiveness of mature trees to provide

shading to the canyon. In the morning (09.00–11.00 hours),

the sun radiation has not radiated the canyon directly due to

the shadowing effect of the building and upper tree canopies.

The air temperature inside the canyon increases gradually

along with the increase of background temperature. At

around 11.00 hours, the shading effect of trees gives its first

maximum cooling effect. When the sun moves gradually to

exactly above the head at around 13.00 hours in Singapore,

the canyon receives the maximum solar heat gain. Then, as

the sun sets (lower solar elevation), the trees provide shading

to the canyon and the second maximum cooling effect occurs.

The maximum cooling effects in the morning (11.00

hours) and in the afternoon (15.00 hours) are shown in

Figure 14. In general, the cooling effect was observed

inside the canyon sidewalk. It ranges from 0.9 to 1.5K.

The cooling effect pattern of both times is similar to the

cooling effect inside the canyon (Points 2–4). When

the air temperature reaches the maximum at 15.00 hours,

the trees stabilize the air temperature inside the canyon,

with the result of a further cooling effect at Point 4.

At 15.00 hours, Points 2–4 are found to display a higher

cooling effect than the other points at the edge of the canyon

and outside the canyon. This is because the points are situated

at the mid-section of the canyon, well shaded by dense cano-

pies and buildings. The cooling effect at Point 1 is slightly

lower as it is located at the edge of the canyon, with increased

exposure to direct sunshine. It can be realized that leaving the

canyon can result in an immediate air temperature increase of

about 2.1K. There is no cooling effect from the trees at

Points 5 and 6. The probable reasons are that they are

exposed to the open space, a reduction of tree shade at their

immediate environment and, as mentioned, there may be

anthropogenic heat from the opposite building’s cooling tower.

At Point 8, the cooling effect is found to be around 1.2K.

This is higher than the cooling effect at Points 5–7, even

though they are all located outside the canyon. The dense

tree cluster planted at the measurement point causes the

lower ambient temperature. It has provided shade to its sur-

roundings and has lowered the air temperature beneath.

At the PGP siteFigure 15 shows the 24h cooling effect pattern of each point at

the PGP site. On average, no significant cooling was observed

along the canyon sidewalk at the PGP site as compared to the

Figure 14 | Averaged cooling effect (20 hot days) measured at different points in the ENG site



Figure 15 | Averaged cooling effect (20 hot days) measured at points inside the canyon in the PGP site

Figure 16 | Averaged cooling effect (20 hot days) measured at different points in the PGP site

210 Wong and Jusuf


ENG site, with only at around 0.5K. The cooling effects are

noticeable at some points with the maximum of 0.8K, that is,

Points 2, 3 and 5. At these points, the maximum cooling

effect pattern follows the phenomenon as in the ENG site. It

occurs twice a day, but at a different time, at 10.00 hours in

the morning and at 17.00 hours in the afternoon. This difference

is believed to be due to the difference of site characteristics, such

as the building density, H/W ratio and greenery condition.

The cooling effects at Points 2 and 3 were at the same inten-

sity for both times (see Figure 16). Point 3 is situated at the

centre of a semi-open space, planted with groups of shrubs,

small plants and palms, and received no shading from adjoining

buildings. The plants brought about the maximum cooling

effect for the location. At Point 2, young palm trees were at

the left and right sides of the walkways. The collective

impacts from the cooling effect of young palm trees near the

open space and the shading from the buildings provide

the same cooling effect as the air temperature measured at

the centre of the semi-open space.

The ‘heating’ effect was identified at Points 6 and 7 and

a very minimal cooling effect at Point 5 in the morning. At

these points, the cooling effect intensity increases at almost

the same rate as in the afternoon at 17.00 hours. The effect of

solar radiation is the main reason for this air temperature behav-

iour. The curved form of the PGP canyon makes these points

more exposed to the eastern solar radiation than the other

points. Thus, although not intensively planted with greenery,

these points receive some shade from the surrounding buildings.

It is believed that the shade provided by these buildings is the

main factor contributing to the cooling effect in the vicinity.

Both Points 1 and 7 are situated at the ends of the canyon,

where they are constantly exposed to direct solar radiation. In

addition, there are only small trees and shrubs planted around

the points, which are inadequate to cool the environment

through their shading. The high building densities and con-

crete pavement at the site have also resulted in more heat

being radiated back to the built environment, thus contributing

to the heating effect. Comparable to Points 1 and 7, Point 4 is

situated at the edge of a semi-open space, exposed to similar

conditions as Points 1 and 7: hence, a ‘heating effect’ is

found. The ‘heating effect’ identified at this point is 0.6K.

SUMMARY AND CONCLUSIONSA comprehensive field measurement has been conducted to

study the microclimatic condition in two different canyons,

ENG and PGP. ENG has the characteristic of mature trees

covering its canyon, while PGP has less greenery along the

canyon, dominated by young palm trees. The objective of

this study is to investigate the microclimatic condition of

these two different canyons. The summaries from this

study are as follows:

† The mature trees inside the ENG canyon have the ability

to lower the air temperature rather than the young palm

trees in the PGP. The average air temperature inside the

ENG canyon is lower by around 0.7–1.18C as compared

to the PGP canyon during daytime and maintains its cool-

ness at about 0.4–0.58C during night-time. However,

higher RH can be expected by having dense greenery,

measured up to 5% on average as compared to the PGP

canyon.

† The lower air temperature in the ENG canyon is the result

of good shading provided by the dense foliage, which is

able to intercept much more incoming solar radiation.

The mature tree is able to reduce the solar radiation inten-

sity to less than 150W/m2 as compared to the young palm

tree to less than 300W/m2. The lower ground surface

temperatures at the ENG canyon will in turn result in

less heating of the air.

† The wind speeds inside both canyons are calm (,0.5m/s)

with more than 80% of the total occurrence. The wind

speed inside the canyon is reduced quite substantially at

the maximum of around 0.5–1.5m/s as compared to

above the canyon, which can be up to 4m/s.

† The calculated MRT and PET show that the mature tree is

able to provide thermal comfort to the pedestrian walking

along the ENG canyon. On the other hand, assuming that

the young palm trees cover the whole stretch of the PGP

canyon, they are still not able to provide thermal

comfort to its pedestrian. This is mainly due to the

inability of young palm trees to provide sufficient shading.

† Mature trees and young palm trees are able to generate a

cooling effect up to 1.5 and 0.5K, respectively. The

maximum cooling effect inside the canyon, especially

the location below the trees, occurs twice a day, in the

morning (around 10.00–11.00 hours) and in the after-

noon. The afternoon maximum cooling effect may not

happen at the same time for every canyon. It depends on

the characteristics of the canyons.

This study concludes that two main contributory factors influ-

ence the thermal effect within the canyons. They are vege-

tation covers and shading from buildings and trees. This

study has provided an indication that planting of vegetation

can be an effective passive measure to improve the microcli-

mate inside a canyon. However, in general, it can be noted

that increasing the greenery density will result in a higher RH.

ACKNOWLEDGEMENTSThis research was supported by the Department Building and

Office of Estate and Development (OED), NUS. The authors

express their sincere thanks to Ms Lina Goh for providing the

necessary information.

NOTE

1 Based on meteorological data on 1982–2001, National

Environmental Agency, Singapore.



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Study on the Micro Climate Condition Along Green Pedestrian Canyon in Singapore

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