Published January 11, 2016
Journal of Environmental Quality
Special Section
The Urban Forest and Ecosystem Services
Street Orientation and Side of the Street Greatly Influence the
Microclimatic Benefits Street Trees Can Provide in Summer
Ruzana Sanusi,* Denise Johnstone, Peter May, and Stephen J. Livesley
T
he urban forest is an important element of any
town or city, representing the whole network of trees
in the urban landscape. These urban trees are expected
to provide multiple benefits, including improving microclimatic
conditions for urban residents (Georgi and Zafiriadis, 2006).
Trees do this by intercepting incoming solar radiation and preventing it from penetrating into street canyons (Matzarakis et al.,
1999), thus providing shade and cooling benefits. Urban street
trees can decrease local air temperatures and create a cooler, more
comfortable environment for urban residents, particularly pedestrians, in summer (Lohr et al., 2004). Because the number of
people living in towns and cities increases each year such that the
number of urban residents is projected to nearly double by 2050
(United Nations Population Division, 2008), these benefits are
becoming more important.
Many local government bodies aim to increase tree canopy
cover as part of their urban forest strategies. For example, the
City of Sydney aims to increase tree canopy cover from 15.5 to
23.25% by 2030 (City of Sydney, 2013). The City of Melbourne
has set a target of increasing tree canopy cover from 22 to 40%
by 2032 (City of Melbourne, 2012). These strategies aim to
mitigate the detrimental impacts of the urban heat island, heat
waves, and climate change by using trees to help improve the
environment and the health and wellbeing of urban residents.
The levels of tree canopy cover in street canyons may be small
compared with that in urban parks and reserves, but street trees
are important because they border the road network and shade
pedestrian rights of way.
The capacity of street trees to provide benefits to pedestrians,
especially at the micrometeorological level, is dependent on
percentage tree canopy cover, the tree species, and street canyon
characteristics. There are several tree characteristics or traits
that may influence the microclimatic benefits that tree canopy
cover can provide (de Abreu-Harbich et al., 2015; Lin and
Lin, 2010). Different tree species have different leaf size, leaf
orientation, transpiration rates, and canopy architecture, and
these traits can provide different microclimate modification
and shading benefits. From the perspective of the street canyon,
Abstract
Maintaining human thermal comfort (HTC) is essential for
pedestrians because people outside can be more susceptible to
heat stress and heat stroke. Modification of street microclimates
using tree canopy cover can provide important benefits to
pedestrians, but how beneficial and under what circumstances
is not clear. On sunny summer days, microclimatic measures
were made in residential streets with low and high percentages
of tree canopy cover in Melbourne, Australia. Streets with eastwest (E-W) and streets with north-south (N-S) orientation were
repeatedly measured for air temperature, relative humidity, wind
speed, solar radiation, and mean radiant temperature on both
sides of the street between early morning and midafternoon.
Physiological equivalent temperature was estimated to indicate
HTC throughout the day. In streets with high-percentage canopy
cover, air temperature, relative humidity, solar radiation, and mean
radiant temperature were significantly lower than in streets with
low-percentage canopy cover. The reductions in air temperature
under high-percentage canopy cover were greater for E-W streets
(2.1°C) than for N-S streets (0.9°C). For N-S streets, air temperature,
mean radiant temperature, and solar radiation were greater on
the east pavement in the early morning and greatest on the west
pavement in the midafternoon. The midday thermal benefits
are restricted to E-W streets, which are oriented in the same
direction as the summer sun’s zenith. High-percentage canopy
cover reduced wind speeds but not enough to offset the other
microclimate benefits. These findings can assist urban planners
in designing street tree landscapes for optimal HTC in summer,
especially in areas of high pedestrian density.
Copyright © 2015 American Society of Agronomy, Crop Science Society of America,
and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA.
All rights reserved.
R. Sanusi, D. Johnstone, P. May, and S.J. Livesley, School of Ecosystem and Forest
Sciences, Faculty of Science, The Univ. of Melbourne, 500 Yarra Blvd., Melbourne,
Victoria 3121, Australia; R. Sanusi, Faculty of Forestry, Universiti Putra Malaysia,
Serdang, Selangor, Malaysia. Assigned to Associate Editor Carlo Calfapietra.
J. Environ. Qual. 45:167–174 (2016)
doi:10.2134/jeq2015.01.0039
Freely available online through the author-supported open-access option.
Received 26 Jan. 2015.
Accepted 30 Apr. 2015.
*Corresponding author ([email protected]).
Abbreviations: HTC, human thermal comfort; PET, physiological equivalent
temperature; SVF, sky view factor; Tmrt, mean radiant temperature.
167
building height, street width, and street orientation can also
influence the ability of a tree canopy to provide microclimatic
benefits to pedestrians. The height of the buildings can modify
wind movement by channeling wind and changing wind
speeds (Shashua-Bar et al., 2006). The width of the street can
also influence the microclimatic conditions, such as rates of
air mixing or self-shading. For example, a wide street with low
buildings and no tree canopy cover will have lower thermal
comfort for pedestrians ( Johansson and Emmanuel, 2006). In
addition, street orientation can influence how trees can modify
street microclimate because it influences the duration and timing
of solar radiation interception according to the sun’s zenith. On
hot summer days, the potential cooling benefits from street trees
are particularly influenced by tree and street factors, and it is on
these days that improving human thermal comfort (HTC) is
most important (Ali-Toudert and Mayer, 2007).
Microclimatic modifications by street tree canopies can be
beneficial to pedestrians, but how beneficial and under what
circumstances they are most beneficial is not clear. This study
aims to investigate how street microclimate can be modified
by high-percentage tree canopy cover in summer and poses the
following research questions: (i) What are the microclimatic and
HTC differences between streets with low- and high-percentage
tree canopy cover in summer? (ii) Do the differences due to
low- and high-percentage tree canopy cover vary significantly
according to street orientation? (iii) Under high-percentage tree
canopy cover, do microclimate and HTC differ between the left
and right sides of a street on a sunny summer day, and does this
vary according to street orientation?
To minimize confounding factors common in urban
landscape studies, microclimate measurements were only made
in residential streets with and without Platanus × acerifolia
(London Plane Tree) planted at regular intervals along both
sidewalks. All streets had low height/width (H/W) ratios
and were either in a north-south (N-S) or east-west (E-W)
orientation.
Materials and Methods
This study was conducted in Richmond, Victoria, Australia
(37°49¢ S, 144°59¢ E), which is an inner suburb of Greater
Melbourne situated 3 km southeast of the Melbourne city center.
This is a typical urban residential area dominated by single- and
double-story buildings. Study streets were selected with low- and
high-percentage tree canopy covers, with street orientations of
either E-W or N-S. During summer 2012–2013, measurements
were conducted over six clear sunny days. The measurement
dates were as follows: for two E-W streets: 12 Feb. 2013, 6 Mar.
2013, and 13 Mar. 2013; and for two N-S streets: 13 Feb. 2013,
5 Mar. 2013, and 12 Mar. 2013. The mean daytime maximum
temperature in Melbourne in summer 2012–2013 was 27.4°C,
and the mean night time minimum temperature was 16.2°C. In
Melbourne, a calculated average of the maximum and minimum
temperatures measured in a 24-h period exceeding 30°C has been
shown to correlate with “excess mortality” among vulnerable
people (Nicholls et al., 2008). The 24-h air temperature, relative
humidity, and solar radiation data for the measurement dates
are presented in Fig. 1 to provide a reference to the climate
conditions experienced during the experiment.
A description of the measured streets, their orientation,
average street canyon H/W ratio, percentage of tree canopy cover
(%), and microclimate measurement points are as presented in
Table 1. Percentage of tree canopy cover was estimated using
i-Tree Canopy (i-Tree Canopy, 2013) by categorizing tree and
non-tree surface cover type from aerial photographs at 200
randomly assigned sample points within each street. Streets with
high-percentage canopy cover had 77% cover in the E-W street
and 70% cover in the N-S street. Streets with low-percentage
canopy cover had 19% cover in the E-W street and 17% cover
in the N-S street. The specific dimensions and example images
of streets with low- and high-percentage canopy covers are
presented in Fig. 2.
Fig. 1. Air temperature, relative humidity, and solar radiation data measured on 12 Feb. 2013, 6 Mar. 2013, and 13 Mar. 2013 for east-west streets
and on 13 Feb. 2013, 5 Mar. 2013, and 12 Mar. 2013 for north-south streets. These data were obtained from a meteorological station in Burnley
Campus, The University of Melbourne, Victoria, Australia (2 km from the measurement location).
168
Journal of Environmental Quality
Table 1. Characteristics of the streets used for data collection.
Street
orientation
Tree canopy
cover
Charlotte St.
east-west
low
Elm Grove
east-west
Burnley St.
Bendigo St.
Street
Average H/W†
ratio
Sky view
factor
%
19
0.25
0.73
high
77
0.29
0.26
north-south
low
17
0.21
0.72
north-south
high
70
0.23
0.27
Canopy cover
Measurement points
measured where there was no shading
by trees at any time of the day
measured at random points under the
near continuous canopy
measured where there was no shading
by trees at any time of the day
measured at random points under the
near continuous canopy
† Height/width.
Estimation of Sky View Factor
Hemispheric (fish-eye) photography was used to determine
sky view factor (SVF) of each street (Osmond, 2010). Sky view
factor indicates the degree of open sky seen from
a point 1.1 m above street ground level and is
influenced by tree canopy cover, topography,
street furniture, and building heights. Sky
view factor was measured at the same points as
microclimate measurements early in the morning
or at sunset once during the measurement period.
Sky view factor was highest with low canopy
cover, with mean values of 0.72 and 0.73 in the
N-S and E-W street orientation, respectively
(Table 1). Streets with high canopy cover had
lower SVFs of 0.26 and 0.27 in both street
orientations than low canopy cover streets
(Table 1).
coefficient correction. The measured air temperature, wind speed,
and globe temperature and a convective coefficient were used in the
calculation of Tmrt.
Measurement of Street Microclimate
Street microclimate was measured using three
portable weather stations (Table 2) recording air
temperature, relative humidity, wind speed, solar
radiation, and mean radiant temperature (Tmrt),
which is the mean temperature from all sources
of radiation, including shortwave and longwave
radiation, direct from the sun and reflected in all
directions from all entities, objects, surfaces, and the
atmosphere surrounding the human body (Fanger,
1970). For this study, Tmrt was measured through
globe temperature by using a black hollowed
copper sphere with a temperature sensor inside
(Fanger, 1970; Humphreys, 1977; Nikolopoulou et
al., 1999). For measurement of Tmrt, the black globe
was allowed 15 min to reach equilibrium with the
measurement location conditions. Wind speed
was logged every 10 s but averaged to be recorded
every minute (CR1000 data logger, Campbell
Scientific), and then wind speed data were averaged
to represent each 15-min period. Coutts et al.
(2015) established an additional calibration of the
black globe Tmrt measurement through comparison
to direct measures of radiation, which corrected
the convective coefficient value to 0.65 × 108v0.53.
Our study used the same black globes, loggers, and
component parts, so we used the same convective
Fig. 2. The dimensions and image of streets with low- and high-percentage canopy
covers. Top left: Street with high-percentage canopy cover (70%). Top right: Street with
low-percentage canopy cover (17%). Bottom: Visual description of the four sites for northsouth and east-west orientations showing the position of the weather stations on both
sides of the street for the microclimate measurement. The weather stations in the street
with a low-percentage canopy cover were placed away from the tree canopy.
Journal of Environmental Quality169
Table 2. Sensor specification for the portable weather station used in
this study. Sensors used for this study were to measure air temperature,
relative humidity, mean radiant temperature, wind speed, and solar
radiation.
Parameter
Air temperature
Relative humidity
Mean radiant
temperature
Wind speed
Solar radiation
Instrument/sensor
Specification
(accuracy)
iCelcius
iCelcius
±0.3°C
±3%
iCelcius Pro
±0.2°C at 25°C
RM Young Wind Speed Sensor
Apogee Pyranometer MP-200
±0.5 m s−1
±5%
Microclimatic measurements were taken on the sidewalk on
both sides of the street at three points within an approximately
60-m section of the street, giving a total of six measurement
points for each street (Fig. 2). Measurements were made in the
early morning (6:00–8:00), midmorning (8:00–11:00), midday
(12:00–14:00), and midafternoon (14:00–17:00). Within a 2-h
period, three point measurements were performed on one side of
the street simultaneously, followed by measures on the other side
of the street, before moving to the second paired street to repeat
these measurements. The measurements were made at 1.1 m above
ground level. Sensors for air temperature and relative humidity
were calibrated against Campbell Scientific air temperature and
relative humidity sensors to verify their accuracy.
Physiological Equivalent Temperature Index for Human
Thermal Comfort
The thermal index used to estimate HTC in this study was
physiologically equivalent temperature (PET). This index
standardizes human factors with a metabolic rate at 80 W
(equivalent to light activity) and clothing at 0.9 clo (equivalent
to a light business suit). The microclimatic parameters of air
temperature, relative humidity, wind speed, solar radiation, and
Tmrt were used as an input for PET estimation. RayMan software
was used in this study to estimate PET (Matzarakis et al., 2007).
Physiologically equivalent temperature gives a value in °C, which
is easy for professionals, such as urban planners, to use and relate
to (Deb and Ramachandraiah, 2010).
Data Analysis
Data were analyzed using ANOVA and t test (GENSTAT
16). A LSD test was used to determine significant (p ≤ 0.05)
difference among groups.
Results
Street Microclimate and Human Thermal Comfort
between Streets with Low- and High-Percentage
Canopy Cover
There were significant (p ≤ 0.05) microclimate benefits in
high-percentage canopy cover streets as compared with the lowpercentage canopy cover streets for both street orientations (Fig.
3). However, this is not apparent in the early morning because
similar mean air temperatures were measured in both N-S
(~20°C) and E-W (~21°C) streets under both high- and lowpercentage canopy covers (Fig. 3). For E-W streets, midday and
midafternoon measures of air temperature were significantly
different (p ≤ 0.001) between high- and low-percentage canopy
170
covers, whereas in N-S streets only midafternoon measures
of air temperature were significantly different (p ≤ 0.001). On
the other hand, solar radiation, Tmrt, and PET were found to be
significantly different between high- and low-percentage canopy
streets at most measurement times for both street orientations
(p ≤ 0.05).
The reduction of solar radiation in high-percentage canopy
cover streets assists in the reduction of Tmrt in both street
orientations (Fig. 3). For example, at midday in N-S streets, the
mean solar radiation differed significantly (p ≤ 0.001) from 908
W m-2 under the low-canopy cover to 379 W m-2 under the
high-percentage canopy cover, thus significantly influencing Tmrt,
which was 44.8 and 36.8°C, respectively. These large reductions
in solar radiation and Tmrt in turn significantly reduce midday
PET estimates (p ≤ 0.01) from “strong heat stress” (37.6°C) to
“moderate heat stress” (33.6°C). Although the high-percentage
canopy cover significantly reduced the wind speed (p ≤ 0.001)
in N-S streets, wind speeds were small (range, 0–1.0 m s-1) (Fig.
3). In contrast, E-W streets did not show a consistent reduction
in wind speed with higher-percentage tree canopy cover (Fig. 3).
Street Microclimate and Human Thermal Comfort
Differences between Two Street Orientations
Air temperature, Tmrt, and PET were significantly greater
(p ≤ 0.05) in E-W streets than in N-S streets. For example, at
midday, the mean air temperature difference between low- and
high-percentage canopy covers in E-W streets could reach 2.1°C,
whereas in the N-S street it was restricted to 0.9°C (Table 3).
Similarly, mean Tmrt exhibited a greater difference between lowand high-percentage canopy cover at midday (10.7°C) in E-W
streets compared with N-S streets (8.0°C); PET showed a similar
trend, with differences of 4.6 and 4.0°C, respectively (Table 3).
Street Microclimate and Human Thermal Comfort
Differences on Different Sides of the Street
The microclimate of N-S streets with high-percentage canopy
cover was significantly influenced by the time of day and on
which side of the street measurements were made (i.e., east or
west). Solar radiation was significantly higher (p < 0.05) on the
west side than on the east side of the street at midday (Fig. 4). For
example, midday solar radiation on the west pavement (681 W
m-2) was almost double the value for the east pavement (390 W
m-2) (Fig. 4). A similar pattern was observed for Tmrt and therefore
for PET. In midmorning, Tmrt and PET were significantly greater
on the east side than the west side (p ≤ 0.05). By midafternoon,
the reverse was true such that PET was equivalent to “strong heat
stress” on the west side (37.3°C) and equivalent to “moderate
heat stress” on the east side (32.0°C) (Fig. 4).
In contrast, there was little difference between opposing sides
of the street (north or south) in high-percentage canopy cover
in E-W streets. Most microclimatic parameters, for a given time
of the day, showed no significant difference between north and
south sidewalks of these E-W streets (Fig. 4).
Discussion
This study measured significant microclimate benefits under
high-percentage tree canopy cover for both N-S and E-W street
orientations. Solar radiation, Tmrt, and PET were significantly
Journal of Environmental Quality
Microclimate and Human Thermal Comfort
Effects of Street Tree Canopy in Summer
The microclimate benefits from higherpercentage tree canopy cover are due greatly to
the lower SVF, which reduced solar radiation
access, ground surface heat accumulation, and
thus localized air temperature. The tree canopies
reduce SVF and increase the reflection and
absorption of solar radiation, thus allowing
only a small amount of solar radiation below
the canopy (Brown and Gillespie, 1995). The
contrast in local microclimate between streets
with high- and low-percentage tree canopy cover
was greatest in the midafternoon, after solar
radiation input in streets with few trees had
accumulated throughout the middle of the day.
We observed that solar radiation, Tmrt, and PET
were significantly reduced for streets with highpercentage tree canopy in both orientations, but
air temperature was only significantly reduced
at midday and midafternoon for E-W streets
and only at midafternoon for N-S streets.
In agreement with our findings, Souch and
Souch (1993) noted that the effect of trees on
air temperature was small in early morning
(07:00–09:00), but the cooling effect became
apparent by midday (12:00–14:00), reducing
air temperatures by 0.7 to 1.3°C. Less incoming
solar radiation in the midmorning observed in
our study meant there was less solar radiation
within the street canyon due to the shading
effect of the trees and possibly the shading effect
from buildings along the street canyon as well.
Less solar radiation penetrated into the street
canyon, resulting in the insignificant cooling
effect by the high-percentage canopy covered
street in midmorning.
Reduced solar radiation transmittance
in streets with high-percentage tree canopy
cover decreases the radiant heat intercepted by
pedestrians (Shashua-Bar et al., 2011). In our
study, Tmrt and PET was greater in streets with
Fig. 3. Average air temperature, relative humidity, wind speed, solar radiation, mean
radiant temperature (Tmrt), and physiologically equivalent temperature (PET) in streets
low-percentage tree canopy cover than in streets
with high Platanus × acerifolia canopy cover and low canopy cover. Data collected for three
with high-percentage canopy cover, suggesting
summer days using spot measurements in north-south (left column, A) and east-west
that the re-radiated heat from the surfaces
(right column, B) orientations in Richmond, Victoria, Australia. Levels of heat stress were
presented at the right bottom corner of the graph.
increased heat load to the surroundings, thus
reducing HTC (increasing the PET). Mean
reduced for streets in both orientations, but air temperature was
radiant
temperature
is the most important meteorological
only significantly reduced in N-S streets in the midafternoon.
parameter
that
affects
HTC and the human energy balance of
Microclimate benefits were greater for the high-percentage
pedestrians
during
summer
(Matzarakis et al., 2007). As such,
canopy cover in E-W streets because the differences in air
T
has
been
shown
to
have
a stronger correlation to PET than
mrt
temperature and Tmrt between low- and high-percentage tree
air
temperature
(Gulyás
et
al.,
2006). Therefore, the presence of
cover were several degrees Celsius greater than that measured
trees
in
urban
residential
areas
is important not only because it
for N-S streets. This may partly be explained by the significant
reduces
air
temperatures
but
also
because it significantly reduces
differences between microclimate measures made on opposing
T
and
as
such
pedestrian
thermal
stress for most of the day. The
mrt
sidewalks in N-S streets; in other words, the microclimatic
high-percentage
tree
canopy
covers
in this study (70–77%) were
benefits were “time of day” and “side of street” dependent for
able
to
improve
HTC
by
reducing
thermal stress. For example,
N-S streets.
from midday onward, in N-S streets high-percentage tree canopy
Journal of Environmental Quality171
Table 3. Average air temperature, solar radiation, mean radiant temperature, and physiologically equivalent temperature differences comparing low
canopy cover to high canopy cover.
Microclimate parameters†
Early morning
Air temperature, °C
Solar radiation, W m−2
Tmrt, °C
PET, °C
0.49 ± 0.08‡
−10.39 ± 5.14
−0.68 ± 0.10
1.12 ± 0.33***
Air temperature, °C
Solar radiation, W m−2
Tmrt, °C
PET, °C
−0.06 ± 0.01
7.17 ± 2.82
−0.51 ± 0.02
−0.47 ± 0.04**
Midmorning
Midday
North-south orientation
0.30 ± 0.01
0.85 ± 0.05
309.62 ± 13.77***
529.13 ± 84.70***
5.23 ± 0.21***
6.44 ± 0.78**
1.68 ± 0.03
3.29 ± 0.30***
East-west orientation
0.91 ± 0.46
2.07 ± 0.20***
188.87 ± 11.89***
708.90 ± 25.44***
3.95 ± 0.17***
8.99 ± 0.25***
2.53 ± 0.30***
4.57 ± 0.30***
Midafternoon
1.53 ± 0.06***
417.33 ± 85.46***
7.35 ± 0.71***
4.44 ± 0.34***
1.39 ± 0.09
365.38 ± 64.10***
4.76 ± 0.50***
3.86 ± 0.25***
** Significant at the 0.01 probability level.
*** Significant at the 0.001 probability level.
† PET, physiologically equivalent temperature; Tmrt, mean radiant temperature.
‡ A positive result represents higher value in the low canopy cover street.
cover reduced PET from “strong heat stress” to “moderate heat
stress.”
Microclimate benefits from trees are also provided through
evapotranspiration. In the microclimate assessment, significantly
higher relative humidity was found in the high-percentage canopy
covered streets, suggesting that the water vapor released to the
atmosphere from transpiration increases the humidity of the air
in that street canyon. As such, it is also assumed that the reduction
in air temperature under high-percentage tree canopy cover in our
study was partly due to evapotranspiration by those trees (ShashuaBar and Hoffman, 2000). The potential for evapotranspirative
cooling by a tree is, however, reliant on the water balance of that
tree (Oke et al., 1989). Therefore, we investigated tree water status
during our study. Predawn leaf water potential was measured
and was -0.4 Mpa on average (data not shown), indicating good
availability of water to the trees. However, water stress level will
be different depending on the species (Ranney et al., 1990). In
hot, sunny summer conditions, the transpirative cooling may
be important to pedestrians. However, from the pedestrians’
perspective, higher humidity from transpiration will not be
perceived as a benefit because this will lead to less efficient sweat
evaporation from the human body, especially at high levels of
physical activity (i.e., brisk walking) (McNall et al., 1967).
Wind speed, on the other hand, was also expected to be
greater in streets with low-percentage tree canopy cover because
trees are able to alter wind movement within a street canyon by
blocking the wind from channeling into the street canyon. In a
study by Heisler (1990), the wind speed measured at 2 m height
in a residential area also showed that areas with trees reduced
the wind speed to 70% in summer due to the obstruction by
the trees and buildings. Wania et al. (2012) also suggested that
dense tree cover reduced the wind speed and air flow in the street
canyons especially in the street with a greater H/W ratio (H/W
> 0.5). There was only an apparent reduction in wind speed in
N-S streets with high-percentage tree canopy cover, and these
wind speeds were all small such that they could not outweigh
the beneficial influence of the tree canopy on other street
microclimate parameters (i.e., solar radiation, Tmrt). Similarly,
in a street PET study by Cohen et al. (2012), wind contributed
little to pedestrian PET because wind speed values remained
small. Wind can be beneficial for pedestrian thermal comfort
172
because the efficiency of sweat evaporation is greater with wind
speed; however, with the presence of trees, lower wind speed
will restrict the cooling breeze, thereby lowering the pedestrian
thermal comfort (Akbari et al., 1992).
Effect of Street Orientation: Microclimate and Human
Thermal Comfort Benefits
In our study, microclimate benefits from high-percentage tree
canopy cover were greater for streets in an E-W orientation. This
demonstrates the dominant effect of the sun’s zenith in summer.
Because the E-W streets are oriented in the same direction as the
sun’s zenith, the high-percentage tree canopy cover reduced the
exposure of the street canyon to solar radiation regardless of time
of day, whereas in a N-S street solar radiation was able to transmit
under the high-percentage canopy cover in the early morning and
midafternoon. Interestingly, the opposite is true for the streets
with low-percentage canopy cover, as simulated by Ali-Toudert
and Mayer (2006) in terms of human thermal stress duration and
intensity. In another study, Oliveira et al. (2011) found greater
differences in air temperature, Tmrt, and PET between open and
tree canopy–shaded areas in an E-W street as compared with a
N-S street, surmising that this was due to the higher duration of
solar radiation exposure in E-W streets in summer.
Pearlmutter et al. (2007) suggested that the improvement
of thermal comfort through H/W is small for E-W–oriented
streets. A street in an E-W orientation has a greater potential
for high human thermal stress, but likewise the presence of
high-percentage tree canopy cover will give a greater cooling
benefit because of the higher risk potential from solar radiation
exposure. Hence, planting trees in E-W streets is strategically very
important because PET can be greatly reduced, thus improving
pedestrian thermal comfort. Therefore, even though increasing
tree canopy cover in N-S streets will provide microclimate and
PET benefits, these benefits are not as great because some solar
radiation will still penetrate the street canyon. This finding
highlights the dominant role of solar radiation in different street
orientations. Therefore, the prioritized planting of street trees
also needs to consider street orientation because this influences
the level of solar radiation risk exposure and therefore street
microclimate and HTC benefit. For example, Norton et al.
Journal of Environmental Quality
human thermal stress on the east sidewalk in the
morning and higher thermal stress on the west
sidewalk in the afternoon. This result confirms
the simulation study by Ali-Toudert and Mayer
(2007) in Northern Hemisphere street scenarios.
This demonstrates that tree shade microclimate
benefits may not necessarily be experienced
directly beneath the tree canopy, especially not
in the early morning and midafternoon in N-S
streets. This has considerable implications for
the strategic placement of street trees for shade
in N-S streets, especially regarding the times of
day that pedestrians may be making greatest use
of a sidewalk.
The gap between the building and the tree
canopy can be another factor that can greatly
influence the microclimate (i.e., solar radiation
interception) and HTC in N-S streets. The use of
external shading facilities on the buildings, such
as awnings and retractable canopies, can reduce
this gap and improve the street microclimate
condition as well as that inside the building
(Berry et al., 2013; Porritt et al., 2012).
Buildings themselves cast shade, and therefore
the aspect ratio of the street (H/W ratio) can
influence the street microclimate (Coutts et al.,
2015). However, Ali-Toudert and Mayer (2006)
point out that street orientation plays a role
because even in deep canyon (H/W = 4) streets
in an E-W orientation shading from buildings
was limited, whereas in N-S orientation such a
deep street canyon would significantly lower the
PET and periods of human thermal discomfort.
As such, if there is tree canopy cover in a deep
canyon in a N-S orientation, it will provide less
cooling benefit because of self-shading from
the buildings themselves (Coutts et al., 2015).
Shashua-Bar et al. (2006) modeled that the
cooling effect in a street with 70% of tree canopy
cover was almost halved when the street canyon
H/W was ≥2.0 as compared with a shallow street
canyon (H/W = 0.25). In our current study, low
H/W ratios for both street orientations (ranging
Fig. 4. Air temperature, relative humidity, wind speed, solar radiation, mean radiant
temperature (Tmrt), and physiologically equivalent temperature (PET) of high Platanus ×
between 0.21 and 0.29) were expected to provide
acerifolia canopy cover streets comparing both sides of the street. Data collected using
minimal building shade; therefore, the greater
spot measurements in north-south (13 Feb. 2013; left column, A) and east-west (6 Mar.
cooling effects found on both street orientations
2013; right column, B) orientations are comprised of two different sides of the street in
Richmond, Victoria, Australia. Levels of heat stress were presented at the right bottom
were largely determined by the high-percentage
corner of the graph.
tree canopy cover. This further indicates that
high-percentage tree canopy cover would be
(2015) suggest that planting of street trees in E-W streets should
more beneficial when located in canyons with low H/W ratios.
be prioritized, especially for wide streets with a small H/W ratio.
The Side of the Street You Walk On: Microclimate and
Human Thermal Comfort Benefits
Street orientation also determines the HTC experienced by
a pedestrian walking on one or the other side of the street at a
given time of the day. For N-S streets, the microclimatic and
pedestrian thermal comfort benefits were “time of day” and “side
of street” dependent. For these N-S streets, there was higher
Conclusions
By examining two different street orientations (E-W and N-S)
with two different percentage tree canopy covers (high and low),
this study leads to the following conclusions: (i) Greater cooling
of street microclimate occurs in residential streets with dense, welldeveloped canopy cover than in streets with a low canopy cover.
(ii) In Melbourne’s warm temperate climate, high-percentage tree
canopy cover in an E-W street can reduce midday air temperatures
Journal of Environmental Quality173
by up to 2.1°C and PET by up to 4.6°C as compared with a
similar street with little tree canopy cover. The microclimate
benefits of high-percentage tree canopy cover were less in N-S
streets as compared with E-W streets because solar radiation was
transmitted under the canopy in the morning and midafternoon.
(iii) Greater street tree canopy cover lowered wind speeds but not
significantly and not enough to offset the other microclimatic
benefits the canopy shade and transpiration provided.
Our study provides an evidence base to urban planners and
landscape professionals as to the magnitude of the microclimate
benefits they can gain from increasing tree canopy cover in
suburban residential streets. This will help to improve land use
planning strategies that consider heat-related health problems
during the summer months. These planning strategies should also
consider where to plant street trees in high pedestrian areas, such
as commercial shopping streets, primary schools, and transport
hubs. These planning strategies should consider street orientation
and which side of the street is used most by pedestrians and at
what time(s) of the day there is greatest pedestrian activity so as
to place trees to achieve the greatest microclimatic and HTC
benefits.
Acknowledgments
This research was part of the City of Melbourne Research Project Fund.
The authors thank the City of Yarra for giving us permission to conduct
this research in Richmond, the Statistical Consulting Centre of The
University of Melbourne for their help in the data analysis, and the
other individuals involved indirectly in this project for their technical
assistance and thoughtful suggestions.
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