Urban heat island - state of the art review

Abstract

Climate change is generally recognized as one of the main challenges of the 21st century [1]. It is significantly influencing the urban tissue, through temperature rises, abundant precipitation, strong winds and ever more frequent natural hazards, while being influenced by these factors in return. Its projection to amplify existing climate related risks [2], more acute in an urban environment, mandates immediate attention.
Therefore, be it the conservation of historical heritage or regeneration of an urban area or new insertions within the tissue or the city’s expansion, every intervention should contribute to the ecological, social, economic and cultural balance, as well as to the adaptation of the human settlements to the future climate conditions, thus enhancing their resilience [3, 4].
Among climate factors, high temperatures in urban areas are most often associated with heat-related stress and deaths [5] caused by current weather patterns. Many experts and decision makers already consider extreme hot weather a significant risk for cities worldwide. Increased urban development and the urban heat island (UHI) effect will exacerbate the frequency, magnitude and impact of hot weather events. The demographic changes, specifically the aging population, ensures the aggravated, direct impact on human health. The urban heat island, defined as the temperature difference between the urban and suburban areas and the rural areas from their vicinity, represents an active and ever-present component of the concept of urban resilience, decisive in the thermal discomfort, caused by exterior temperatures. It varies spatially and temporally, requiring complex analysis.
The current study aims at providing a state of the art review analysing both research literature and environmental strategies and reduction programs on a global perspective. United States set the State of the Art projects in this domain, by the US Environmental Protection Agency (EPA), through the Heat Island Reduction Program. In the EU, UHI are only part of two environmental strategies: Green Infrastructure (GI) — Enhancing Europe’s Natural Capital and EU Strategy on adaptation to climate change. Concerning the academia, the most developed stage of research includes studies from the scale of physical and chemical properties of materials [6,7], up to the analysis of metropolitan areas or the major urban structures and their integration in the general bioclimatic context [8-11]. In Romania, these studies only include punctual approaches [12, 13]. According to recent studies [5] the risk of heat related mortality increases in the context of climate change, continued urban development and an aging population and is expected to double in the coming years should we stagger in addressing its causes.
The UHI result from the overlapping of the urban process and climate changes. A mono-directional analysis of the phenomenon was found to be thus insufficient, since the correlation of available information and data is absolutely necessary. There are in fact different domains that influence the phenomenon, both directly and indirectly, thus justifying a multidisciplinary approach. In accordance to this situation, there are a number of measures regarding the reduction of air temperatures, implementable at a macro-territorial, global or local level.
A complex analysis of the research state of the art is of the utmost importance. Based on it, new intervention strategies can be developed while the research’s results are useful to both universities and local authorities, as they could be the base of fundamental studies for all initiated planning documents.

Full text

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URBAN HEAT ISLAND - STATE OF THE ART REVIEW
Branea Ana-Maria, Danciu Mihai Ionut, Keller Alexandra Iasmina, Bădescu Ștefana, Găman
Marius Stelian, Pașcu Gabriela
Politehnica University of Timisoara, Faculty of Architecture and Urban Planning, Traian Lalescu str, nr. 2/A,
300223, Timisoara, Romania
ana-maria.branea@upt.ro, mihai.danciu@student.upt.ro, alexandra.keller@upt.ro,
stefana.badescu@student.upt.ro, marius.gaman@upt.ro, gabriela.pascu@upt.ro
Abstract
Climate change is generally recognized as one of the main challenges of the 21st century [1]. It is
significantly influencing the urban tissue, through temperature rises, abundant precipitation, strong winds
and ever more frequent natural hazards, while being influenced by these factors in return. Its projection to
amplify existing climate related risks [2], more acute in an urban environment, mandates immediate
attention.
Therefore, be it the conservation of historical heritage or regeneration of an urban area or new insertions
within the tissue or the city’s expansion, every intervention should contribute to the ecological, social,
economic and cultural balance, as well as to the adaptation of the human settlements to the future climate
conditions, thus enhancing their resilience [3, 4].
Among climate factors, high temperatures in urban areas are most often associated with heat-related stress
and deaths [5] caused by current weather patterns. Many experts and decision makers already consider
extreme hot weather a significant risk for cities worldwide. Increased urban development and the urban
heat island (UHI) effect will exacerbate the frequency, magnitude and impact of hot weather events. The
demographic changes, specifically the aging population, ensures the aggravated, direct impact on human
health. The urban heat island, defined as the temperature difference between the urban and suburban
areas and the rural areas from their vicinity, represents an active and ever-present component of the
concept of urban resilience, decisive in the thermal discomfort, caused by exterior temperatures. It varies
spatially and temporally, requiring complex analysis.
The current study aims at providing a state of the art review analysing both research literature and
environmental strategies and reduction programs on a global perspective. United States set the State of the
Art projects in this domain, by the US Environmental Protection Agency (EPA), through the Heat Island
Reduction Program. In the EU, UHI are only part of two environmental strategies: Green Infrastructure (GI)
— Enhancing Europe’s Natural Capital and EU Strategy on adaptation to climate change. Concerning the
academia, the most developed stage of research includes studies from the scale of physical and chemical
properties of materials [6,7], up to the analysis of metropolitan areas or the major urban structures and
their integration in the general bioclimatic context [8-11]. In Romania, these studies only include punctual
approaches [12, 13]. According to recent studies [5] the risk of heat related mortality increases in the
context of climate change, continued urban development and an aging population and is expected to
double in the coming years should we stagger in addressing its causes.
The UHI result from the overlapping of the urban process and climate changes. A mono-directional analysis
of the phenomenon was found to be thus insufficient, since the correlation of available information and
data is absolutely necessary. There are in fact different domains that influence the phenomenon, both
directly and indirectly, thus justifying a multidisciplinary approach. In accordance to this situation, there are
a number of measures regarding the reduction of air temperatures, implementable at a macro-territorial,
global or local level.
A complex analysis of the research state of the art is of the utmost importance. Based on it, new
intervention strategies can be developed while the research’s results are useful to both universities and
local authorities, as they could be the base of fundamental studies for all initiated planning documents.
Keywords: Climate change, urban heat island, environmental strategies, reduction programs, Romania

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Main Text
1 INTRODUCTION
Cities, as well as buildings, are developed taking past and present climatic conditions into consideration. But
climate change and all its implications, temperature growth and precipitation pattern change, have a
significant effect on the wellbeing of the inhabitants and the microclimate of the cities. According to Peter
Brimblecombe [14], is is not enough to just analyse the current state of the regional climate and
microclimate of the city. One must be aware of the changes and take future predictions into consideration
in order to fully understand the implications on urban areas and develop well-adjusted strategies.
According to recent studies [5] the risk of heat related mortality increases in the context of climate change,
continued urban development and an aging population and is expected to double in the coming years
should we stagger in addressing its causes.
2 URBAN HEAT ISLANDS
The academic literature on urban heat islands was review in order to identify how various studies are
defining the phenomenon of heat islands, how the theoretical view in the field developed in time and which
the main causes of heat island appearance are. The most influential published paper on the topic of urban
heat islands is “City size and the urban heat island” published in 1973, by Timothy Richard Oke [15] and
cited 621 times, according to the Scopus database. The paper is the first one that presents the connection
between the intensity of UHI and the urban structure. The definition of UHI and their intensity presented in
the paper (“difference between background rural and highest urban temperatures”), is a definition that is
used in most of the recent reviewed papers, even today, with a slight change of used terms (Table 1).
Table 1. Recent definitions of UHI
Year
Author
Definition
2015
Paulina W., Poh-Chin L., Melissa H.
[16]
„This impact on local climate in an urban city, in particular with
reference to the increase in outdoor temperature”
2015
Sismanidis P., Keramitsoglou I.,
Kiranoudis C.T. [17]
„Relative warmth of the dense urban areas with respect to their
suburban/rural surroundings”
2016
Hsieh C.M., Huang H.C. [18]
„A phenomenon in which the temperature in a city is higher than
the temperature in the neighboring rural areas”
2016
Chen A., Zhao X., Yao L., Chen L. [19]
„that more urbanized areas have a higher air temperature than
their surrounding suburban areas”
2016
Gülten A., Aksoy U.T., Öztop H.F. [20]
„The heat island effect is defined as higher air temperatures in the
urban area than in the rural area in the same region”
2016
Wang Y., Akbari H. [21]
„a phenomenon whereby a metropolis is usually significantly
warmer than its rural surroundings”
2016
Lanza K., Stone B. [22]
„the phenomenon through which cities are warmer than nearby
rural areas”
2016
Aflaki A., Mirnezhad M.,
Ghaffarianhoseini A.,
Ghaffarianhoseini A., Omrany H.,
Wang Z.H., Akbari H. [23]
„’heat island’ generally describes the urbanized areas with higher
temperatures compared to their neighboring non-urbanized
areas”
2016
Buchin O., Hoelscher M.T., Meier F.,
Nehls T., Ziegler F. [24]
„The urban heat island (UHI) effect is a localized anthropogenic
climate modification in the canopy layer of the urban atmosphere
where almost all daily human activities take place”
Over 3700 published papers in the field were identified in the Web of Science Database, starting with 1989
until 2016. Most of them were written in the field of Meteorology, 26% of the identified papers,
Environmental sciences (19%) and Engineering (11%). They are followed with papers written in the field of
building technology, energy, geology, photographic technology and physical geography. Although all the
studies are related to urban structures only 5% of all the written papers were published in urban studies
related journals, which brings this topic only on the 10th place of the list (Fig 1.).

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Figure 1. Percentage of written papers about UHI in different research areas
A clear rise of the interest of researchers concerning the topic of Urban heat islands was observed after
analysing the number of articles written every year since 1990 (Fig. 2). Moreover, higher interest in this
topic was noticed in countries with a high urbanisation degree, USA and China, where this effect is far more
present (Fig. 3). Even in Romania, this topic is of great interest, 33 papers being written since 2009.
Figure 2. Number of written papers about UHI

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Figure 3. The distribution off written papers about UHI at a global level
The causes and the effects of UHI are analysed from different points of view according to the topic of the
research. Papers in the field of environmental sciences assess the relationship between the appearance of
UHI and meteorological factors, environmental problems, topography, landscape features, anthropogenic
heat sources and urban canyons and extensive urban developments [25,26]. According to the studies, this
combination of factors favours the appearance of UHI [27-29].
Engineering journals and building technology studies connect the appearance of UHI with various used
building materials, their capacity to absorb and release solar radiation and their reflective properties [30,
31].
Remote sensing, geology and photographic technology related studies and closely related. Remote sensing
and photographic technology research papers evaluate the appearance of UHI in relation to land cover and
global climatic parameters and atmospheric conditions which are evaluated mainly using satellite sensors
[32]. UHI are identified by evaluating the air temperature, surface energy, carbon cycles and soil moisture
for different areas and comparing them with earlier measured parameters [33-35]. Studies in this area are
highly statistical. Studies in the area of geology, interpret the obtained satellite data and develop HUI and
micro-UHI maps [36-37].
Energy related studies evaluate the impact of urban heat island on the interior microclimate of buildings
and present various methods in order to reduce these effects. Studies in this field are rather related to the
prediction and mitigation of the UHI effect on the interior microclimate than to the analysis of the
appearance of UHI [38-41].
Physical geography studies are closely linked with those in the urban research area. Physical geography
related papers evaluate the impact of the spread on urban areas and of landscape related parameters on
the appearance of UHI [42, 43]. Urban studies related papers on the other hand associate the appearance
of UHI with the typology of streets, colour and materials of urban surfaces and urban green areas [44-46].
The main purpose of urban studies is to identify what urban structures are most likely to favour the
appearance of UHI and the develop and evaluate various UHI mitigation strategies [47, 48].
Table 2 presents some of the most complex descriptions of the causes that lead to the appearance of UHI.
While analyzed causes are different according to the research direction of every topic, some similarities and
a slight overlap between the descriptions can be observed.
Table 2. Recent definitions of UHI
Author
Research area
Analysed causes of UHI
Ward, K., Lauf, S.,
Environmental
„UHI is induced by a combination of factors, including street canyon

5
Kleinschmit, B.,
Endlicher, W. [26]
sciences
geometry, the amount of artificial surfaces with increased emissivity, and
also anthropogenic heat production”
Li, X., Li, W.,
Middel, A., Harlan,
S. L., Brazel, A. J.,
Turner, B. L. [35]
Remote sensing/
Photographic
technology
„ land architecture of neighborhoods [...] affect LST and the SUHI, and that
this architecture is critical to neighborhood LST. [...] development or
rearrangement in the composition and configuration of the land-cover of
neighborhoods, including at the parcel level, can be used to ameliorate the
UHI effect.”
Sailor, D. J. [31]
Engineering/
Building
technology
„urban heat island (UHI) phenomenon is a result of a number of factors
including the prevalence of thermally massive and low reflectivity surfaces,
the general lack of surface moisture, and waste heat emissions from
energy-consuming activities”
Akbari, H., &
Kolokotsa, D [39]
Energy
„increased absorption of solar radiation, the corresponding increase of
sensible heat released by urban structures, higher anthropogenic heat,
reduced urban vegetation, and higher emission of infrared radiation”
Aniello, C., Morgan,
K., Busbey, A., &
Newland, L. [36]
Geology
„[..] micro-urban heat islands are resulting from the lack of tree cover
related to newly developed residential neighborhoods, parking lots,
business districts, apartment complexes, and shopping centers. All micro-
urban heat islands were radiative in nature [...].”
Ma, Q., Wu, J., &
He, C. [43]
Physical
Geography
„Urban expansion plays a dominant role in the urban heat island (UHI)
formation and is thus the essence and fundamental characteristic of the
urban fabric”
Sismanidis, P.,
Keramitsoglou, I.,
Kiranoudis, C. T.
[17]
Urban studies
„the rapid expansion of the urban areas and the subsequent conversion of
pervious surfaces [...] alongside (1) the decrease in evapotranspiration; (2)
the reduction in turbulent heat transport due to the geometry of the street
canyons; (3) the anthropogenic heat fluxes; and (4) the air pollution, causes
the urban heat island”
3 MITIGATION OF URBAN HEAT ISLANDS
From all the reviewed published papers concerning UHI, 87 papers were identified as being written about
UHI mitigation strategies. Out of these papers, only 4% are in regard to the urban studies field; this
illustrates a profound lack of information regarding the issue of UHI mitigation strategies from the point of
view of urban planning, while at the same time justifies the research efforts in this field (Fig. 4).
Figure 4. Percentage of written papers about mitigation strategies of UHI in different research areas
The number of papers regarding mitigation strategies of UHI was higher during 2009 or 2012 (Fig. 5), when
heavy heat waves were also recorded and produced many victims; this illustrates a rather reactive attitude
towards the issue of UHI. In order to better understand and prevent the severe effects of this phenomenon,
the scientific community must adopt a proactive approach and invest more time and effort in the research
process.

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Figure 5. Number of written papers about mitigation strategies of UHI
In accordance to the Arup report [49], the UHI are influenced by three main categories of factors, which
form a „triple risk index”; its contents are described in Table 3.
Table 3. „Triple risk index” regarding the UHI, adapted from Arup report [49]
CATEGORY OF
FACTORS
DESCRIPTION
Location
This category of factors discusses two issues that influence the formation of a UHI:
- a „hot spot”, which refers to a densely built area or a major road junction;
- a „cool spot”, which can be a large park or a water body.
These need to be analysed in regard to the levels of air quality, noise, crime and socio-
economic deprivation: „The closer to a ‘hot spot’, the lower the amount of green or blue
space, the lower the levels of air quality, the higher the levels of noise, crime and socio-
economic deprivation, the higher the location risk”.
Characteristics of
building
This category of factors includes the age of a construction, the building materials from
which it is made, its orientation, layout and height, the number of storeys it comprises, its
deep plan and dual aspect, whether or not it has a balcony or a garden, its thermal mass,
shading levels and ventilation.
„The higher the number of characteristics which contribute to overheating and do not
easily enable cooling, ventilation and respite from the heat, the higher the building risk”.
Characteristics of
people
The factors which need to be taken into consideration are the age and sex of the subjects,
their health, and socio-economic status, the possibilities of mobility and levels of social
connection, their culture and spoken languages, their awareness and perception of heat
risk.
„The higher the levels of people over 65 (particularly over 75), under 5s, people with
respiratory, cardiovascular or mental health conditions, pregnant women, and the less
mobile, informed and socially connected, the higher the people risk”.
The Arup Report [49] further establishes four main approaches to addressing the UHI, further presented in
Table 4.
Table 4. The four main approaches to address the UHI, adapted from the Arup Report [49].
APPROACHES TO ADDRESS
UHI
EXPLANATIONS (what the approaches cover)
Physical
This approach should involve an alteration of the existing situation, such as „an
intervention or improvement to the urban environment, or a particular
neighbourhood or building”.
Social
This approach should be based on creating policies and examples of good practice,

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referring to „awareness raising, communication and behaviour change”.
Strategic - before a hot
weather event
This approach needs to be conceived as a long term strategy, based on „larger
investments of time and resources with less immediate results”.
Operational - during a hot
weather event
This approach should involve „shorter term reactive efforts with more immediate
results”.
In order to properly respond to the issue of UHI, it is of utmost importance to first properly understand the
factors that influence this subject, as well as the possible approaches towards addressing the UHI.
Also, we must take into consideration the fact that no matter how proper the operational interventions are,
they are not effective unless they are conceived as part of a larger strategy. Finally, one should not ignore
the fact that all the physical measures should be in accordance with the social realities, responding to all the
needs of the community [49].
4 CONCLUSION
The review of state of the art papers regarding the issue of UHI reveals the fact that not only is this
phenomenon of great importance within the urban environment, but also that this matter will become
even more problematic in the context of present-day realities, namely:
- Climate changes, a phenomenon which already affects a number of countries and is projected to
amplify in future years;
- Urban densification, which favour an increase in temperature and thus the formation of UHI;
- Demographic changes, such as urbanisation, ageing population and under 5s.
It is therefore essential for the scientific community and practitioners alike to fully understand both the
factors that influence the formation of UHI, as well as the possible responses and approaches regarding this
issue.
These approaches fall into four categories - physical, social, strategic and operational. Of course, many of
our actions are difficult to categorize and are interchangeable, but all of these should be conceived,
according to the Arup Report [49] in regard to four different spatial scales: city scale, neighbourhood scale,
building / block scale and community / individual scale.
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