Urban Design and Urban Conservation
Term Paper
Urban Heat Island Effect Mitigation through Urban
Design
Submitted by: Submitted to:
Shriti Kishore Dr. Mayank Mathur
BP/948/2022 Department of Physical Planning
Bachelor of Physical Planning
School of Planning and Architecture
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Table of Contents
1. Introduction
2. What is Urban Heat Island?
3. Major Causes
4. Types of UHI
5. Criteria for Mitigation
6. Urban Design Guidelines to Mitigate UHI in Hot and Dry
Climates
7. Some more Urban Design strategies
8. Recommendations
9. Conclusion
10.Bibliography
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I. Introduction
The rapid urbanization observed in developing countries presents a significant environmental
challenge – the intensifying Urban Heat Island (UHI) effect. As these cities experience
densification and vertical growth, characterized by high-rise buildings constructed at high
density, the magnitude of the UHI effect is projected to worsen.
The United States Environmental Protection Agency aptly describes the phenomenon: urban
development fundamentally alters the landscape. Previously permeable and moisture-
retentive open spaces and vegetation are replaced with buildings and impermeable surfaces
like roads. These changes disrupt natural cooling mechanisms, causing urban areas to retain
heat more effectively than their surrounding rural environments. This thermal disparity
results in the formation of "islands" of significantly higher temperatures within the city.
Several key factors contribute to the exacerbation of the UHI effect. The urban structure
itself plays a critical role.
Land-use planning that prioritizes high-density development with minimal green spaces,
coupled with building morphology favoring towering structures, creates a densely packed
urban fabric. This configuration traps heat within the city, further exacerbated by the
extensive use of dark, heat-absorptive materials on building facades. Additionally,
anthropogenic heat generated by vehicles and air conditioners contributes significantly to the
overall thermal burden within the urban environment.
The consequences of a rising urban temperature are multifaceted and detrimental. Increased
heat intensifies air pollution, elevating concentrations of harmful pollutants and posing a
significant health risk to city residents. Furthermore, the demand for thermal comfort
necessitates increased energy consumption for building cooling, placing a strain on energy
resources and potentially escalating greenhouse gas emissions. The combined effects of
rising temperatures, worsening air quality, and increased energy demands ultimately lead to a
decline in the overall quality of life within the city. Addressing the UHI effect is therefore
crucial for ensuring sustainable development and the well-being of urban populations in
developing countries.
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To combat rising urban temperatures, state governments and urban planners need better tools
and guidelines. Understanding how buildings and urban design impact city climate is crucial
for developing sustainable strategies that mitigate the Urban Heat Island effect.
II. What is Urban Heat Island?
An Urban Heat Island has been best described as a dome of stagnant warm air over the
heavily built-up areas of the city. This phenomenon manifests as consistently higher
temperatures within urban areas, both during the day and at night, compared to surrounding
rural and suburban environments. The UHI effect transcends climatic zones, impacting cities
irrespective of their geographical location.
Figure: Variations of Surface and Air Temperatures in different types of urban areas compared to rural
peripheries
Several key factors contribute to the formation and intensification of urban heat islands. One
significant contributor is the widespread use of heat-absorbent materials in urban
construction. Roofs, roads, and other infrastructure elements constructed from materials like
concrete and asphalt readily absorb solar radiation during the daytime. Unlike natural
surfaces, such as vegetation, these materials retain the absorbed heat and release it even after
sunset, contributing to warmer night-time temperatures within the city.
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Urbanization also leads to a reduction in tree cover. Trees provide essential cooling
mechanisms through shade and evapotranspiration, the process by which plants release water
vapor through their leaves. The loss of natural vegetation in cities not only diminishes these
cooling effects but also exacerbates heat retention within urban environments.
The physical form of cities further contributes to the UHI effect. Densely packed buildings
create narrow "urban canyons" that trap heat and restrict wind flow. This stagnant air hinders
natural cooling processes, further amplifying thermal discomfort within the city.
Finally, human activities within a city generate significant heat. Vehicles, air conditioners,
and other anthropogenic heat sources contribute to the overall thermal burden within the
urban environment. The combined effects of these factors, as documented by Lenholzer
(2015) and earlier by Howard (1818), lead to the observed temperature disparity between
urban and surrounding rural areas. Understanding these contributing factors is crucial for
developing effective strategies to mitigate the UHI effect and create more sustainable and
thermally comfortable urban environments.
III. Major Causes
Rapid urbanization presents a complex challenge – it both exacerbates climate change and
offers potential for mitigation. While urbanization has driven a 14% increase in energy
consumption over the past decade, projections estimate that by 2030, 61% of the global
population will reside in cities. Furthermore, a staggering 95% of population growth will
occur in developing countries, with these urban centres expected to house nearly four billion
people, constituting 80% of the world's urban population. However, this urbanization trend
also presents an opportunity for "climate-friendly renewal" of urban design.
• Surface Materials: The materials used in urban construction significantly impact UHI
formation. During the day, solar energy is absorbed and stored within these materials,
then released back into the environment at night. The greater the difference in
thermal admittance (heat absorption and release) between urban and rural areas, the
larger the heat island effect becomes. Conversely, factors like high sky emissivity
(ability to release heat) and strong wind speeds can help reduce the size of a UHI.
Calm, cloudless nights often see the greatest urban-rural temperature discrepancies
because these conditions limit natural heat dissipation. The high thermal capacity of
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urban building materials further exacerbates the problem, as they readily absorb solar
energy. Materials with high short-wave radiation absorption are particularly
detrimental.
• Lack of Vegetation: Numerous studies have documented the positive impact of green
spaces and vegetation on air temperature. Larger parks can be 1-2°C cooler than
built-up areas, with this difference potentially reaching as high as 5°C. In urban
environments, a lack of vegetation reduces evapotranspiration (the release of water
vapor from plants), shade, and overall cooling effects, all of which contribute to
higher temperatures and exacerbate UHIs.
• High-Rise Buildings: Tall buildings with continuous slab structures can impede fresh
air circulation and wind movement. Additionally, these structures provide extensive
surfaces that reflect and absorb sunlight, further contributing to urban heat gain.
Careful consideration should be given to the placement of such buildings to minimize
their negative thermal impact.
• Human Activity: Activities like air conditioning use, vehicle traffic, and industrial
production all contribute directly and indirectly to urban heat. These activities release
heat and moisture into the environment, while also polluting the air, which in turn
affects incoming and outgoing radiation
Figure: Causes of UHI.
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The design and construction of buildings within urban environments significantly influence
factors like solar radiation absorption and reflection, heat storage capacity, wind flow, and
evapotranspiration. Airborne aerosols, generated by vehicles and industrial activities, can
reduce incoming solar radiation while increasing its diffusion. Highly polluted cities may
experience a reduction in global solar radiation exceeding 20%, compared to a typical
reduction of less than 10% in most cities. Pollution can also contribute to increased UHI
intensity by causing the atmosphere to absorb more outgoing long-wave radiation and re-emit
it back towards the ground. Furthermore, various pollutants can alter the atmospheric
radiative properties, further intensifying the UHI effect in highly polluted urban areas.
The Cascading Effects of Urban Heat Islands
• The negative impacts of Urban Heat Islands (UHIs) extend far beyond just elevated
air temperatures. Here's a breakdown of some key consequences:
• Water Quality Deterioration: As stormwater flows over heated pavements and urban
surfaces, its own temperature rises. This hot runoff then disrupts the delicate balance
of aquatic ecosystems when it enters rivers, streams, and lakes.
• Altered Weather Patterns: A study by NASA involving Atlanta, Dallas, San Antonio,
and Nashville revealed that cities with a high concentration of impervious surfaces
like buildings and roads retain heat, leading to warmer surrounding temperatures [6].
During summers, this rising heat can create wind circulation patterns that enhance
cloud formation and potentially increase rainfall.
• Health Risks in Tropical Climates: In regions with tropical climates, the intensified
heat associated with UHIs can lead to a range of health problems including heatstroke,
exhaustion, and respiratory issues. Furthermore, the combination of high
temperatures and suspended particulate matter (air pollution) in densely populated
urban areas creates ideal conditions for fires. Dry vegetation and trees are particularly
susceptible during summer months.
Summary of UHI's Detrimental Effects:
• Increased Indoor Heat Transfer: Rising outdoor temperatures lead to increased heat
transfer into buildings, necessitating greater energy consumption for cooling,
especially in tropical countries.
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• Thermal Discomfort and Health Concerns: Both indoor and outdoor thermal
discomfort can occur due to UHIs, potentially leading to health hazards.
• Intensified Rainfall: UHIs may contribute to increased rainfall intensity within urban
areas.
• Enhanced Greenhouse Gas Emissions: Increased energy use for cooling can
contribute to higher greenhouse gas emissions, further exacerbating climate change.
• Elevated Fire Risk: The combination of high temperatures and air pollution creates a
heightened risk of fires, especially for dry vegetation within cities.
IV. Types of UHI
On the basis of its impacts, the urban heat island effect can be of two types: Surface UHI and
Atmospheric UHI.
1. Surface-Urban Heat Islands
These are caused when the heat from solar radiation is absorbed by dry and exposed
surfaces of the urban set-up. Its magnitude is thus dependent on the intensity of solar
radiation, which changes seasonally and diurnally. This is why Surface Urban Heat Islands
are highest during summers, especially during the day-time. Another reason why summers
characterize high Surface UHI is that: in summers, due to prevalent clear-sky conditions, the
solar radiation remains undispersed. Also, the days are calm, with low wind speeds, because
of which the mixing of air is minimized.
2. Atmospheric Urban Heat Islands
These are formed where there is a difference between the air temperatures of urban and
rural areas. These are further sub-dived into two types:
• Canopy Layer UHI
They occur close to the ground surface, where people and built environment exists, that is
from the ground surface to the topmost level of trees and roofs.
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• Boundary Layer UHI
They occur at a level starting from the rooftops and tree tops, until the point where urban
landscapes no longer affect the atmosphere.
V. Criteria for Mitigation
There are several criteria to control UHI in hot and dry cities which should be
investigated:
• Height-to-Width Ratio (H/W): The ratio between building height (H) and street width
(W) is recognized in the literature as a significant factor influencing thermal comfort,
especially in tropical regions
• Orientation: The orientation of a street network relative to solar movement and
prevailing winds is crucial for optimizing thermal comfort
• Reflectivity: The selection of building roof materials, street surfaces, and wall
finishes with varying reflectivity can significantly impact environmental heating
• Conductivity: The thermal properties of materials used in urban environments
demonstrably affect the local microclimate. Understanding the relationship between
thermal conductivity and outdoor human thermal comfort is essential
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• Plot Coverage: As previously discussed, limitations exist concerning building
placement within a plot and the percentage of land it occupies
• Balconies: Provision of shade is a primary strategy for enhancing outdoor thermal
comfort in hot and arid cities. Balconies, colonnades, pergolas, and similar
installations can be employed to create shade in public areas, particularly pedestrian
walkways
• Vegetation: Urban vegetation and green infrastructure are recognized as effective
tools for promoting outdoor thermal comfort and mitigating UHIs
Merging Strategies for Optimal Impact:
✓ Plot Coverage and Vegetation Fusion: To ensure adequate ventilation throughout a
city, even with weak air currents, smaller surface areas and lower building densities
are preferred. However, trade-offs often exist between reserving urban areas and
achieving climate-friendly development goals. Open space development can lead to
more compact settlements, promoting sustainability in terms of transportation and
energy use. Conversely, building compaction can exacerbate the heat island effect.
Therefore, it is crucial to seek development limitations as a compromise, allowing
remaining urban open spaces to counteract the negative effects of compaction. When
sufficient surface area is available, green open spaces can serve a dual purpose:
providing climate regulation and functioning as structural elements within the urban
context. Green belts separating residential areas from industrial or commercial zones
with high traffic are a prime example.
Furthermore, green spaces act as air filters, diluting and filtering airborne contaminants while
reducing the heat island effect. Street greening with trees and shrubs can also help mitigate
UHIs. The shade cast by trees, along with the process of evapotranspiration (plant water
release), helps moderate temperatures. Since streets often serve as air channels, these plants
can cool the air circulating throughout the city. Selecting appropriate foliage for street
greening is crucial. While certain tree types with large canopies offer more shade, they may
also lead to the accumulation of air pollutants at street level. However, unless a significant
source of pollution exists under the canopy, this effect can be disregarded. Plant species
chosen should be adaptable to future climate conditions. Preserving urban green spaces and
unsealed open areas (e.g., agricultural land) is of central importance for climate adaptation.
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Vegetation structures contribute to thermal regulation and improve overall climate and air
quality. By considering terrain morphology, the type of green and open space, and the course
of ventilation lanes, targeted interventions can be implemented to compensate for the effects
of heat islands in stressed areas. Given the potential consequences of climate change, this
urban climate compensation function is becoming increasingly important. As climatic
conditions change, with an anticipated increase in heat stress events, expanding peri-urban
and urban green areas, along with unsealed open spaces, is vital. Additionally, peri-urban
green areas are subject to increasing stress from drought, heat, and heavy rain. These
challenges necessitate situation-adapted maintenance efforts, such as more frequent irrigation
or pest control. The use of more heat- and drought-resistant plant species is also
recommended.
✓ Conductivity and Orientation Fusion: The generation of cool and fresh air through
natural surfaces is determined by the thermal properties of the materials used in those
surfaces. Materials with higher density absorb more solar energy and consequently
produce less cool air compared to those with lower density (Figure 4). The size of
these open spaces is also important. Connecting fresh-air fields with inner-city
districts through fresh-air corridors can help reduce the heat island effect (Figure 4).
Special attention should be given to strategic locations such as pedestrian zones and
city entry points. From a climatic perspective, water elements are particularly well-
suited as design elements due to their positive impact on urban climate, specifically in
mitigating the heat island effect.
VI. Urban design Guidelines to mitigate UHI in Hot and Dry Climates
Key Considerations:
❖ Shading for Summer Comfort: The findings emphasize the critical role of shade
provision in achieving thermal comfort during summer months. Streets designed with
high H/W ratios (deep canyons) promote a more comfortable environment. However,
as these canyons can be less thermally favorable in winter, a strategic mix of H/W
ratios across different streets is recommended.
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❖ Preservation of Urban Identity: The importance of protecting and developing
significant urban and street spaces is acknowledged due to their impact on a city's
overall character, image, and citizen connection. Existing pathways, visual
connections, and historical references are all valuable considerations.
❖ Redevelopment and Fresh Air Access: A trade-off is identified in the redevelopment
of former industrial areas. While such projects can create high-density residential
districts, they also risk eliminating crucial fresh-air surfaces. Buildings with
continuous slab structures can impede fresh air movement, highlighting the need to
prioritize summer comfort in most street designs.
VII. Some more Urban design strategies
1. Green infrastructure
One of the main causes of UHI is the lack of vegetation and natural
surfaces in urban areas, which reduce evapotranspiration and
increase heat absorption. Green infrastructure, such as parks, green
roofs, green walls, street trees, and rain gardens, can help restore
the natural cooling functions of the urban ecosystem, by providing
shade, moisture, and albedo. Green infrastructure can also improve
stormwater management, air quality, biodiversity, and urban
aesthetics.
2. Cool Materials
Another cause of UHI is the high thermal conductivity and
emissivity of conventional urban materials, such as asphalt,
concrete, and metal, which store and radiate heat throughout the
day and night. Cool materials, such as reflective coatings, porous
pavements, and light-coloured surfaces, can help reduce the heat
absorption and emission of urban structures, by increasing their
solar reflectance and thermal emittance. Cool materials can also
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lower the energy demand for cooling buildings and vehicles, and
reduce greenhouse gas emissions
3. Urban Geometry
The shape and orientation of urban forms, such as buildings,
streets, and blocks, can also influence the UHI effect, by affecting
the solar exposure, wind circulation, and thermal mass of the urban
fabric. Urban geometry can be optimized to enhance natural
ventilation, daylighting, and shading, by considering factors such
as street width, building height, density, and alignment. Urban
geometry can also create microclimates that can moderate the local
temperature and humidity.
4. Water features
Water is a vital element of urban design, not only for its functional
and aesthetic purposes, but also for its thermal and psychological
benefits. Water features, such as fountains, ponds, streams, and
wetlands, can help reduce UHI by evaporating water into the air,
creating a cooling effect. Water features can also provide a sense of
relief and comfort for urban dwellers, especially in hot and dry
climates.
5. Social Behaviour
Urban design is not only about physical interventions but also
about influencing the social behaviour and culture of urban
residents. Urban design can encourage more sustainable and
adaptive lifestyles, such as using public transportation, cycling,
walking, and sharing spaces and resources. Urban design can also
promote social cohesion, participation, and awareness, by creating
inclusive, accessible, and diverse public spaces and facilities.
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VIII. Recommendations:
❖ H/W Ratio Variations: To achieve broader thermal comfort, a combination of streets
with diverse H/W ratios is advised. North-south oriented canyons, generally more
thermally favorable, should ideally have H/W ratios of 2 or higher. For east-west
streets, even deeper canyons with H/W ratios exceeding 4 are desirable. While
increasing building heights and narrowing streets are effective ways to achieve higher
H/W ratios, each approach has limitations. Narrower streets can create traffic
congestion, and tall buildings can raise privacy concerns. Careful consideration
should be given to these potential drawbacks during the design process.
❖ Shade for Pedestrians: To enhance pedestrian thermal comfort, front setbacks should
be discouraged. In streets with lower-than-recommended H/W ratios, shading devices
like balconies, colonnades, and trees should be incorporated to provide shade during
peak sun hours. Deciduous trees are well-suited for these environments, offering
summer shade while allowing solar access in winter.
❖ Building Material Reflectivity: While pedestrian thermal comfort is not solely
determined by radiation, building material reflectivity significantly impacts thermal
comfort indicators. Higher reflectivity reduces surface temperatures and increases re-
radiation into the environment, leading to more favorable thermal conditions.
Therefore, building materials with lower reflectivity are recommended, particularly
near pedestrian walkways. Replacing currently popular materials like aluminum
cladding, glazed facades, and glossy stones with more suitable options in these areas
is necessary.
❖ Urban Code Modifications: Changes to urban codes to permit higher H/W ratios for
streets are essential. Allowing and encouraging taller buildings with projecting upper
floors would facilitate a more compact urban design, promoting efficient land use
(28).
❖ Passive Design Limitations: While vernacular architecture often incorporates
climate-regulating mechanisms, these passive design strategies may be inadequate to
ensure thermal comfort in particularly harsh arid climates. However, they can extend
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the duration of comfortable outdoor conditions, encouraging more frequent use of
public open spaces.
❖ Balancing Seasonal Needs: The significant influence of environmental redesign on
thermal bioclimate is well-established (55). In the cases studied, reduced shading had
the most significant impact on thermal comfort. While this translates to more pleasant
conditions during cooler periods, it also exacerbates thermal stress during hot
summers. Finding a compromise that addresses the thermal needs of both seasons is
crucial.
By carefully considering these factors and recommendations, urban planners and designers
can create thermally comfortable street environments that are also sustainable and contribute
to a city's unique identity.
IX. Conclusion
The Urban Heat Island (UHI) effect is a significant environmental challenge in developing
countries. The UHI effect leads to rising urban temperatures, worsening air quality, increased
energy consumption, and a decline in the overall quality of life within cities. To address these
issues, state governments and urban planners need better tools and guidelines for mitigating
the UHI effect. Several strategies can be employed to combat the UHI effect, including the
provision of shade through the use of balconies, colonnades, and pergolas, as well as the
promotion of urban vegetation and green infrastructure. Additionally, factors such as building
height-to-width ratio, orientation, reflectivity, and thermal conductivity of materials used in
urban environments should be considered in urban design and planning. It is crucial to
prioritize sustainable development and the well-being of urban populations in developing
countries by addressing the UHI effect. By implementing the recommended strategies and
considering the factors mentioned, urban planners and designers can create thermally
comfortable and sustainable street environments that contribute to a city's unique identity.
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X. Bibliography
❖ https://journals.utm.my/jurnalteknologi/article/view/4619/3212
❖ https://issuu.com/coltersonneville/docs/22.0903_final_mrp_report
❖ https://www.researchgate.net/publication/266390217_Mitigating_Urban_Heat_Island
_Effect_by_Urban_Design_Forms_and_Materials
❖ https://www.teriin.org/sites/default/files/2018-03/urba-heat-island-effect-report.pdf
❖ https://www.linkedin.com/advice/0/what-most-effective-urban-design-strategies-
reducing
❖ https://www.linkedin.com/advice/0/what-most-effective-urban-design-strategies-
reducing
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