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ImprovedPedestrian
ThermalComfortThroughUrban
DesignJennifer Love
Master of Urban and
Environmental Planning
Professional Project
May 2009
INTRODUCTION
Open space in urban environments, be it a formal park, an informal
gathering place or inviting streetscape, contributes to a sense of
community and can encourage pedestrian activity. The challenge to
creating inviting open spaces in hot, arid climates such as Phoenix is
mitigating extreme summer temperatures to allow year-round pedestrian
and outdoor activity. In this study, a microscale climate analysis was
conducted using the software ENVI-met to examine the effects of
various urban design elements on thermal comfort with the goal of
creating open spaces that are usable year round. Specically, this
project develops a set of site scenarios with various physical, design,
and materials differences, and applies the ENVI-met program to each
scenario to analyze ambient site temperature.
TECHNICAL CONTEXT
There has been a rise in concern regarding Urban Heat Island
(UHI), which results in the increase in nighttime temperatures due
to the absorption of heat in materials such as asphalt. Urban design,
architecture techniques, and heat reducing materials can be used toreduce both the UHI effect and increase daytime thermal comfort,
allowing for pedestrian spaces to be comfortable year round, day and
night.
Recent research has examined the impact of UHI in Phoenix, which has
become a particular concern as the urbanized area grows. The number
of hours per day with temperatures exceeding 100F during July and
August (the peak summer months) have increased signicantly, from
3.6 hours to 6.4 hours in the last 50 years. Additionally, nighttime
temperatures have increased by 12 F over the last 20 years in the
Phoenix area (Gober, 2006). This exacerbates the need for urban designstandards which can improve the outdoor environment, especially since
the nighttime temperature is projected to increase approximately 15F by
2015 and 25F by 2100 (Brazel, 2003). Fortunately, both the UHI effect
and thermal comfort can be improved through urban design.
STUDY APPROACH
This study looked to build upon the open space concepts of the
Urban Form Project, but also to adapt these concepts at a microscale
level, integrating practical urban form to the existing structure of the
downtown and retrotting existing public spaces. Various urban designelements which impact thermal comfort were examined in this study,
applied to a specic test site in downtown Phoenix, and subsequently
analyzed using the software ENVI-met.
Urban Design Elements Inuencing Thermal Comfort
Urban design can be used to create a pleasant outdoor space while
reducing outdoor ambient air temperature. As building placement and
design have previously been studied extensively, this project focused
on landscaping type and placement, air circulation, and material type,
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Improved Pedestrian Thermal Comfort Through Urban Design
which can all contribute to creating acceptable ambient temperatures for
outdoor urban site activities during periods of extreme heat.
The following are key design elements which may be used to improve
thermal comfort:
Shaded Pedestrianways Clustered/Broad Canopy Trees
Grass
Water
Material Type
Shaded Pedestrianways
Building shade, covered walkways and awnings, depicted in Figure 1,
are important urban design elements that should be utilized to provide
shaded open space and pedestrianways. These design elements reduce
the amount of direct sun, thus improving thermal comfort and reducing
sun exposure, which can improve the ambient air temperature by up to
15% (Urban Form Project Working Paper #8, 2007).
At the Phoenix latitude, buildings can be used to provide shade on the
south side of the street in the summer months during the hottest part of
the day by casting a shadow to the north of the building. To balance this,
shade trees should be planted on the north side of the street, which is the
south side of a building.
Covered walkways and canopies extending from buildings can also
be used to shade and cool pedestrians. These can be built along east-west streets for extended lengths or for individualized locations. These
amenities can provide pedestrian refuge and shade along the north side
of the street, while the south side requires less intensive pedestrian
oriented shading due to the building shadow.
Clustered/Broad Canopy Trees
Increasing the cover of trees in an urban environment is a simple and
effective way to improve thermal comfort. Street trees will be most
effective in providing true shade, commanding visual contrast between
dense landscaping and open space when planted in groups. Trees should
be located to maximize shade for pedestrians, such as along walkwaysand sidewalks, depicted in Figure 2.
Planting trees to provide building shade is important to reduce
the radiant temperature of buildings and objects such as benches.
Additionally, evapotranspiration from a trees leaves in hot weather
cools the air around the tree with 65% of the heat from full sunlight on a
tree being dissipated by the trees canopy (Coder, 1996).
Figure 1
Shaded Pedestrianways in Phoenix
Figure 2
Shade Trees in Phoenix
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Clusters of trees are ideal for providing pedestrian and open space shade
to facilitate comfortable pedestrian circulation and outdoor seating
areas. For example, palm trees cast long shadows and are very useful
for providing pedestrian shade when grouped together, but they must be
used thoughtfully due to their long shadow.
GrassGrass can be used to cool an area and can be appropriately used in a
desert climate if designed in conjunction with summer shade. Creating
open spaces, with or without grass, improves the air circulation
throughout the core area of a downtown, which can help to mitigate hot
air trapped within dense developments. Open space areas with grass
will achieve more pleasant conditions given the cooling effect of grass
through evapotranspiration. Temperatures over grass areas are cooler
than over bordering areas, as illustrated in Figure 3.
Water Features
The use of water features, such as fountains or waterfalls, can have
a signicant impact on thermal comfort, potentially improving
temperatures up to 7.5% (UFP, WP#8, 2007). In an arid climate, water
promotes cooling through evaporation as well as absorbing heat from
surrounding materials. Flowing water promotes air movement which
can cool a space. Water also has a psychological cooling effect, making
a space seem cooler even if there is no measurable temperature change
(UFP WP#8, 2007).
Material Type
Materials used for pavement and amenities such as benches should beselected based on their color, permeability, conductivity and emissivity
(ability of objects to shed heat). Lighter color materials with low
emissivity retain less heat and contribute to lowering the temperature in
a space. Suggested material types and benets are:
Concrete lighter color reects energy from sun
Recycle crumb rubber added to material mix lower heat retention
Porous pavement less dense, able to transmit air and moisture to the
environment encouraging healthy growth of trees (UFP, 2008).
The heat retention of different materials is visible in Figure 4. Areas
paved with asphalt appear red indicating higher temperatures fromretained heat. By contrast, the areas paved with lighter colored concrete
appear green indicating lower temperatures.
Existing Conditions
Prior to conducting the scenario analysis as part of this study, detailed
temperature measurements were obtained for two sites and material
surfaces throughout downtown Phoenix. Key temperature ndings are
Figure 3
Cooling Effect of Grass
Figure 4
Temperature Difference in Paving
Materials
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Improved Pedestrian Thermal Comfort Through Urban Design
presented in Figure 5 and Table 1. These temperatures were collected
using a Raytek infrared heat sensor on July 28, 2008, between 12:00
p.m. and 2:00 p.m. All of the listed materials in Table 1 are commonly
used throughout the Phoenix area.
Figure 5 Existing Site Conditions Field Measurements
Te
mpera
ture
(Fa
hren
he
it)
Material Temperature F
Textured Pavement (medium gray), Filtered Shade 108.9
Sandstone Bench (pink), Filtered Shade 104.0Concrete (light gray), Filtered Shade 103.1
Wooden Bench, Filtered Shade 97.6
Dirt and Gravel walkway, Filtered Shade 95.5
Concrete (light gray), Solid Shade 88.2
Grass, Solid Shade 85.4
Table 1 Existing Surface Conditions Field Measurements
The rst site measured was the Arizona Center, which has
successfully implemented many urban design techniques which cool
the public space. This site served as the study control, evaluating
the success of the proposed techniques on the test site. The second
site, in downtown Phoenix, was selected to test the microclimate
conditions of various urban design scenarios. The site, shown in
Figure 6, is a 5,260 square foot underutilized open space at the
northeast corner of 3rd Avenue and Jefferson Street adjacent to
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Improved Pedestrian Thermal Comfort Through Urban Design
several municipal buildings.
Methodology
Figure 6
Northeast Corner of 3rd Avenue and Jefferson
The test site layout established for the ENVI-met model was extended
to include each of the structures at the corners of the intersection to
incorporate the air ow through the space, as well as the heat retention
of the street pavement. This site was selected to test various scenarios,
applying urban design principles and propose site changes in order to
improve pedestrian thermal comfort.
The ENVI-met modeling software can be used for macro- or
microscale analysis to evaluate daytime climate. Material type,
building conguration, air ow, climate conditions, and landscape areinput into the software with the output being a climate analysis of a
site. The software allows for the evaluation of various scenarios and
congurations by performing multiple analyses, as outlined in Figure 7.
Figure 7 ENVI-met Model Process
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Scenarios
Ten scenarios were modeled using ENVI-met in addition to the existing
conditions. The scenarios were iterative and cumulative, measuring the
impacts of incremental changes in material type, landscape, and shading.
The overall approach to the scenario modeling was to examine the
impact of minor changes to the existing site, adding additional shading
and changing the paving materials, and modifying site layout. Thesescenarios, described in Table 2, examined:
Impact of maximizing shade within the space,
Improving only the area between the street and open space by
creating a buffer from the heat radiating from the street,
Creating a shade buffer from the sun angles of the east and west,
Shade circumventing the open space while not actually providing
any improvements to the core of the space.
Scenario
1 Existing infrastucture Add significant shade trees
2
Existing infrastucture Add significant shade trees Change hardscape surfaces to light coloredpermeable pavers
3
Existing infrastucture Add significant shade trees and a Add permeable paver pathway through turf area
ScenarioDescription Description
7
Fill the entire space with turf and shade trees Leave only minimal walkways around theperimeter utilizing light colored permeable
pavers
8
Retain existing core space Add shade trees and use light coloredpermeable pavement along the southernand western edges
9
Retain existing core space Add shade trees along the eastern andwestern edges
4
Existing infrastucture Add significant shade trees Add light colored permeable paver pathwaythrough turf area
Change hardscape surfaces to light coloredpermeable pavers
5
Modify layout to increase turf and shade treearea adjacent to roadway
Add significant shade trees Add permeable paver pathway through turf area Change hardscape surface
6
Modify layout to increase turf and shade treearea adjacent to roadway
Add significant shade trees Add permeable paver pathway through turfarea
Change hardscape surface Add a water feature
10 Retain existing core space Add shade trees around the perimeter
Table 2 Scenario Descriptions
RESULTS
The characteristics and results for each scenario are detailed in Figure
8. This summarizes the improvements made, estimated investment
required, and temperature improvement.
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Landscaping/Building Inp ut
GrassBroad Canopy TreePalm TreeBuildingNo Vegetation
ConcretePermeable PaverBuildingSoil
104.3 F105.4 F106.4 F107.5 F108.6 F109.7 F110.8 F111.8 F112.9 F114.3 FT
emperature(F)
Mater ial Temperature Temperature Range Output
Key
Existin
g
Scenario1
Scenario2
Scenario
3
Scenario4
Scenario5
Scenario6
Scenario7
Scenario8
Scenario9
Scenario10
% Low (103-106F)% Medium (106-109F)
% High (109-112F)
% Very High (112-115F)
Total CostAverage Site Temper atur e (F)
Peak Site Temperature (F)
Temperatur e Change (F)
0
070
30
-
112.9114
-
0
08614
$4,400
113.1113.8-2.1
3
23704
$65,400
108.6113-6.6
0
48511
$23,400
109.4113.5-5.8
2466
100
$180,400105.8
109.9-9.4
086
140
$144,100106.5
109.9-8.7
685
90
$157,100106
110-9.2
07624
0
$154,200108.4110.2
-6.8
05347
0
$118,600109.1111.9
-6.1
00
93
7
$3,300110.7113
-4.5
0
7930
$24,400
110.3111.8-4.9
Figure 8 Temperature Difference in Paving Materials
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Figure 9 outlines the peak, average and low temperatures by scenario.
The largest temperature improvement is Scenario 4, with a nearly 10
F reduction in average site temperatures and a 4F reduction in the
peak site temperature. This scenario utilized a combination of elements
consisting of cool pavement types, shade, and buffering the heat
owing into the site from the street. An important nding is that the
peak temperature can be signicantly reduced through urban designimprovements.
Scenario
Ex
isting
Scenario
2
Scenario
1
Scenario
3
Scenario
4
Scenario
5
Scenario
6
Scenario
7
Scenario
8
Scenario
9
Scenario
10
Figure 9 Peak, Average and Lowest Temperature, by Scenario
Figure 10 outlines the intensity of heat by scenario. The results indicate
that Scenario 4 is the most effective conguration to reduce the overall
intensity of temperatures in the site. This scenario has the highest
percentage oflow temperatures and the lowest percentage of the space in
the high temperature range.
The maximum average site temperature reduction in the scenario
analysis is comparable with the measurements taken at the Arizona
Center in downtown Phoenix. The Arizona Center is an example of a
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Improved Pedestrian Thermal Comfort Through Urban Design
site which utilizes design techniques that improve thermal comfort.
As shown previously, the difference between the existing baseline
temperatures at the Arizona Center and the 3rd Avenue and Jefferson
Street site is approximately 12F, which is similar to that reected in
the scenario analysis. This conrms that the ENVI-met model can
approximate temperature reductions due to urban design improvements
and validates the scenario modeling for this study.
Tempera
ture
Range
%
Ex
isting
Scenario
2
Scenario
1
Scenario
3
Scenario
4
Scenario
5
Scenario
6
Scenario
7
Scenario
8
Scenario
9
Scenario
10
Scenario
Figure 10 Intensity of Heat, by Scenario
Cost estimates for each scenario were developed based on past bid
tabulations from the City of Phoenix and are included in Figure 8. While
the costs of the scenarios vary, several cost effective recommendations
can be implemented to provide an improvement to thermal comfort.
The most expensive scenario, #4 at $185,000, was the scenario with the
greatest temperature improvement. Several other scenarios provided an
improvement in thermal comfort at a lower cost. Scenario #8, which
buffered the open space from the street provided a 6F reduction in
average temperature at a cost of $122,000. Providing additional shade
and lighter, porous material in the interior of the space, as modeled in
Scenario #3, cooled the space approximately 6F at the cost of $71,000.
The least expensive option, Scenario #9, examined the sun angle impact
to the space and showed a temperature reduction of 4.5F, although the
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Improved Pedestrian Thermal Comfort Through Urban Design
intensity of the heat was not reduced as much as the other scenarios with
93% of the space in the high temperature range.
Key ndings of the study, grouped by individual and cumulative effects,
were:
Individual:1. The use of lighter colored and porous material can reduce the
temperatures radiating from the paving materials by nearly 7F
2. Increasing shade through vegetation in a space can reduce the
temperature by 2F.
3. Simply buffering the air ow from hotter street pavement can
decrease the temperature 6 F.
Cumulative:
1. Site conguration which maximizes shade, use of turf and lighter,
porous paving materials can decrease the site temperatures between
8-9F.
2. Shade used to provide protection from sun angles can cool the space
approximately 4F, although it does not signicantly reduce the
intensity of the heat in the space.
3. Both peak temperature and intensity or percentage of high
temperature areas in the space can be reduced.
CONCLUSIONS
Overall, the analysis results show that retrotting existing spaces can
provide substantial improvements and that costly, major infrastructure
investment is not required to improve thermal comfort in open spaces.Inexpensive improvements, such as protecting a site from sun angles,
can have a cooling impact to a space at a relatively low cost. Improving
or protecting a site from hotter air owing in can also reduce the overall
intensity of the heat in a space. The use of signicant tree coverage
results in improved thermal comfort in urban spaces to encourage
day time use. Pavement material type is signicant, and the use light
colored, porous materials has been demonstrated to reduce heat retention
and site temperature.
The implications of this study for hot, urban communities are that
material types are key factors in thermal comfort in urban environments
and that adjacent spaces, such as intersections or streets, can impact the
temperature of a space due to airow. The model results can be used to
project potential temperature reductions in other spaces such as plazas
and courtyards. The cooling concepts can also be applied to linear
spaces such as pedestrianways, as the radiative material qualities and
shade impacts are much the same, with a greater impact from airow
into a space.
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This project contributes to the creation of a more inviting pedestrian
network in the interest of establishing a sense of identity for downtown
Phoenix as well as other urban areas. The results from this project,
specically the resulting temperature analysis, can be applied to
understand the impacts urban design and site planning have on
microclimates in order to create more climate appropriate spaces.
The results are intended for use by the City of Phoenix to understand
the impact of the built environment on daytime pedestrian activity
in the urban core of the city. Although this study examined a specic
site, ndings can be transferred to other sites to allow communities
to implement measures to reduce the overall daytime temperatures of
spaces in the urban core and thereby improving thermal comfort. The
City will potentially expand upon this research, requiring developers
and municipal infrastructure projects to meet a minimum temperature
threshold.
This study shows that the ENVI-met software is a useful tool for
planners to analyze and predict the thermal comfort impacts of various
developments. This research also translates to other communities, both
in arid climates similar to Phoenix and seemingly dissimilar climates
which can adapt the modeling process for their needs. Additional study
would be required to determine urban design impacts in other types of
climates.
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