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Urban overheating is the most documented phenomenon of climate change impacting humans. This article presents the most recent developments on the magnitude and characteristics of urban overheating and the potential synergies with global climatic change. It analyses the latest qualitative and quantitative data on the impact of higher urban temperatures on buildings' energy supply and demand, heat-related mortality, morbidity and wellbeing, human productivity, survivability of low-income populations, and environmental quality of cities. It describes the state of the art on the development of innovative mitigation materials, advanced urban greenery, heat dissipation, and evaporative techniques as the main mitigation and adaptation technologies to offset the impact of urban overheating. It also analyses the current knowledge on the impact of each mitigation technology on energy, health, environmental quality, urban economy, and survivability. Finally, this article presents the main future challenges related to urban overheating and proposes a specific research agenda to alleviate and counterbalance its impact on human life. [ABSTRACT FROM AUTHOR]

The urban heat island is increasing the temperature of cities up to 10 °C and has a very important impact on energy, environmental quality and health. Materials used in the building and urban fabric affect the urban thermal balance and contribute highly to urban overheating. The article presents the progress achieved on the design, development and implementation of mitigation materials presenting a low and very low surface temperature. The recent technological progress and developments concerning natural, light colour, IR reflective, PCM doped, thermochromic, fluorescent, photonic and plasmonic materials is presented. Experimental results on the cooling capacity and the thermal performance of conventional and advanced materials are described in a comparative way. It is demonstrated that innovative materials can exhibit sub-ambient surface temperatures and contribute highly to mitigate urban overheating. • New innovative mitigation materials are recently developed. • Innovative advanced materials present a superior performance. • New materials contribute to counterbalance urban overheating. • Super cool materials operate under sub ambient surface temperatures. [ABSTRACT FROM AUTHOR]

Local and global climate change increases the ambient temperature of cities by several degrees with important consequences on energy consumption, health and the economy. Advanced urban mitigation technologies contribute to decrease the ambient temperature and counterbalance the impact of urban heat islands. The present paper analyses and presents in a comparative way the mitigation potential of the known mitigation technologies using performance data from about 220 real scale urban rehabilitation projects. The average and peak temperature drop of reflective technologies, greenery, evaporative systems, earth to air heat exchangers and their combinations is calculated and presented. The mitigation potential of the main systems like cool roofs, cool pavements, green roofs, urban trees, pools and ponds, sprinklers, fountains, and evaporative towers, is analysed. It is found that the potential of the main mitigation technologies is considerable and can counterbalance UHI effects partly or fully. The average peak temperature drop calculated for all projects is close to 2 K, while the corresponding decrease of the average ambient temperature is close to 0.74 K. Almost 31% of the analysed projects resulted in a peak temperature drop below 1 K, 62% below 2 K, 82% below 3 K and 90% below 4 K. [ABSTRACT FROM AUTHOR]

Urban heat island and global warming increase significantly the ambient temperature. Higher temperatures have a serious impact on the electricity consumption of the building sector increasing considerably the peak and the total electricity demand. The present paper aims to collect, analyze and present in a comparative way existing studies investigating the impact of ambient temperature increase on electricity consumption. Analysis of eleven studies dealing with the impact of the ambient temperature on the peak electricity demand showed that for each degree of temperature increase, the increase of the peak electricity load varies between 0.45% and 4.6%. This corresponds to an additional electricity penalty of about 21 (±10.4) W per degree of temperature increase and per person. In parallel, analysis of fifteen studies examining the impact of ambient temperature on the total electricity consumption, showed that the actual increase of the electricity demand per degree of temperature increase varies between 0.5% and 8.5%. [ABSTRACT FROM AUTHOR]

Urban heat island is the more documented phenomenon of climate change. Information on the magnitude and the characteristics of the canopy layer urban heat island measured in 101 cities and regions of Asia and Australia and collected through 88 scientific articles, are compiled, evaluated and presented. Data are classified in several clusters according to the experimental protocol used and the type of statistical information reported regarding the magnitude of the urban heat island. Results and detailed analysis are given for each defined cluster. Very significant differences on the UHI intensity are found between the clusters and analyzed in detail. The detailed impact of the main weather parameters and conditions on the magnitude of the UHI is also investigated. The specific influence of anthropogenic thermal fluxes as well as of the urban morphological and construction characteristics to UHI is thoroughly examined. The relation between the UHI intensity and the city size is assessed and global relationships of UHI as a function of the urban population are proposed. The seasonal and diurnal variability of the UHI is analyzed and discussed while specific features and conditions like the urban heat island characteristics in coastal cities and the existence of daytime cool islands are explored. Finally, the impact of the selected reference station and its characteristics is considered. [ABSTRACT FROM AUTHOR]

Increase of the urban ambient temperature caused by local and global phenomena is probably the more incontestable incident of climate change. Urban warming has a very significant impact on human life increasing the energy consumption, deteriorating the comfort levels, increasing the pollution concentration, threatening the human health and affecting the urban economy. Mitigation technologies aiming to countermeasure the impact of the phenomenon are rapidly developed and applied in real scale projects. The present paper aims to present in a critical and comprehensive way the recent scientific knowledge on the causes of urban warming and also stress the main problems and inconsistencies concerning the experimental and the theoretical findings and analysis. In parallel, it classifies and summarizes the main knowledge on the energy, environmental, health and economic impacts of urban warming. Quantitative information on the energy penalties, mortality rates and environmental degradation induced by urban warming is presented. Finally, it presents the main developments regarding urban mitigation techniques and technologies and in particular, urban greenery and planted roofs, cool pavements, cool roofs and increase of the urban albedo, decrease of the anthropogenic heat and use of the ground as a sink for heat dissipation. [ABSTRACT FROM AUTHOR]

Highlights: [•] Green and cool roofs are studied versus their urban heat mitigation potential. [•] Insulation level and thermal mass of cool roofs are examined. [•] Irrigation rate and vegetation type of green roofs are analyzed. [•] Free floating conditions for various climatic regions are used. [Copyright &y& Elsevier]

Abstract: In this study the performance of organic PCMs used as latent heat storage materials, when incorporated in coatings for buildings and urban fabric, is investigated. Thirty six coatings of six colors containing different quantities of PCMs in different melting points were produced. Accordingly, infrared reflective (cool) and common coatings with the same binder system and of the same color were prepared for a comparative thermal evaluation. The samples were divided in six groups of different color and eight samples each: three PCM coatings of different melting temperatures (18 °C, 24 °C, 28 °C) each one of two different PCM concentrations (20% w/w, 30% w/w), an infrared reflective and a common coating of matching color. Surface temperature of the samples was recorded at a 24 h basis during August 2008. The results demonstrate that all PCM coatings present lower surface temperatures than infrared reflective and common coatings. Analysis of the daily temperature differences showed that peak temperature differences occur between PCM and common or cool coatings from 7 am to 10 am. Investigating the temperature gradient revealed that for this time period the values for PCM coatings are lower compared to infrared reflective and common. From 10 am to 12 pm, temperature gradients for all coatings have similar values. Thus coatings containing PCMs store heat in a latent form maintaining constant surface temperatures and discharge with time delay. PCM doped cool colored coatings have the potential to enhance thermal inertia and achieve important energy savings in buildings maintaining a thermally comfortable indoor environment, while fighting urban heat island when applied on external surfaces. [ABSTRACT FROM AUTHOR]

Abstract: The present study reports the development and comparative testing of thermochromic coating to be used in buildings and urban structures. Experimental results from an extensive comparative analysis of the thermal and physical behaviour of thermochromic, highly reflective (cool), and common coatings are reported and analyzed. The surface temperature was monitored on 24h basis from August to mid-September 2007. It was revealing that the temperature of thermochromic coatings was lower than cool and common coatings. Measurements of spectral reflectance indicated that the thermochromic coatings at the colored phase (below the transition temperature of 30°C) are energy-absorbing while at the colorless phase (above the transition temperature of 30°C) are energy-reflecting. The data obtained was used for the calculation of solar reflectance. The results showed that the solar reflectance of the thermochromic samples was significally higher compared to the cool and common ones. A 10-day period test was also performed showing the impact of solar radiation on thermochromism. The comparative results demonstrate that the use of thermochromic coatings can both contribute to energy savings in buildings, providing a thermally comfortable indoor environment, while can contribute highly to improve the urban microclimate. [Copyright &y& Elsevier]

Cities face important overheating problems caused by the local and global climate change. Detailed mitigation and adaptation plans and strategies, are designed and applied to counterbalance the impact of the overheating challenges. Albedo management seems to be among the more considered mitigation strategies. A barrier to the wider use of reflecting technologies is the lack of quantitative experimental data and concrete knowledge about their mitigation contribution. Up to now, the performance of the reflective mitigation technologies is assessed based on simulation results considering the initial optical properties of the materials and neglecting ageing and weatherization phenomena. The article present information on the global performance of reflecting mitigation technologies, as measured in a large scale urban mitigation project employing several types of reflecting technologies. It is shown, that because of the weatherization effects, the mitigation potential of the materials is reduced, during the first year after their installation, by at least, 25%. Despite the ageing effects, reflective pavements have found to contribute in reducing the peak summer ambient temperature up to 1,7 K, while their surface temperature was up to 12,3 K lower than that of conventional pavement reducing considerably the sensible heat released to the atmosphere. [ABSTRACT FROM AUTHOR]

[Display omitted] • A total of 95 papers systematically reviewed cool material as a mitigation strategy for urban heat island (UHI). • Based on the type of reflection and the spectral range, the analysis found 7 material technologies (UV–Vis-NIR). • Optical properties such as reflectance (ρ), absorptance (α), and emittance (ε) are key factors in cool materials design. • Technological setbacks constitute a key limitation to scale cool material technologies and low-cost alternatives. • DRC and super cool materials represent the most efficient innovations amongst current technologies. The challenging urban expansion represents a pressing need to implement climate mitigation strategies for the urban heat island (UHI) effects and significant threats to the environmental quality, energy consumption, and human health. Thus, this manuscript systematically evaluates 95 relevant studies through a robust evidence-based analysis centered on the latest advances in cool materials, their potential, technological innovation, and global trends. The results showed studies in 21 countries, 81 cities, and 16 climate zones, of which the majority belong to Humid subtropical climates (Csa) including 29 % of studies, Oceanic climates (Cfb) with 15 %, and Mediterranean climates (Cfa) with 13 %. Spectrophotometry analysis is used in 48 % of the studies, while the most studied technologies are the Daytime Radiative Cooling (DRC); 22 %, followed by Thermochromics (TC); 19 %, Cool-Colored (CC); 17 %, Phase Change Materials (PCM); 17 %, Retroreflectives (RR); 11 %, Light-colored (LC); 5 % and Fluorescents (FL); 5 %. Finally, DRC reported the highest cooling power of up to 136 W/m2, however, the latest developments advocate for climate-dependent dynamic modulation techniques of Super Cool Materials (SCM) with Passive Daytime Radiative Cooling (PDRC) attributes and engineered spectral properties to dissipate urban heat to outer space in different meteorological conditions and wavelengths levels of the atmospheric window (8–13 μm). [ABSTRACT FROM AUTHOR]

Buildings are responsible for about the 40% of the global annual energy consumption, therefore, innovative strategies for buildings’ energy efficiency are under development. Strategies of re-roofing with “cool” materials have a non-negligible cooling energy saving potential, as they contribute to the reduction of the peak ambient temperatures during summer and, moreover, they contribute to the improvement of the urban microclimate by decreasing the intensity of heat island phenomena. In this paper, the experimental characterization and optimization of a new membrane for buildings’ roof is carried out. To this aim, laboratory measurements were performed to determine its optic-energy properties and, therefore, to optimize its “cool roof” behavior. A full scale field test was also setup in order to measure the global solar radiation reflected by each membrane, before and after optimization, with varying boundary conditions, e.g. time during the day, seasonal period, and weather conditions. The in-field experimental campaign allowed to characterize the optic-energy behavior of the cool membranes in real boundary conditions, showing non-negligible variation of measured solar reflection capability with varying environmental constraints in winter conditions. The research showed interesting results from the in-lab optimization campaign, and non-negligible unreliability due to environmental agents affecting in-field albedo measurement. [ABSTRACT FROM AUTHOR]

[Display omitted] • A repository of 171 UHI papers were systematically reviewed to address relevant questions. • An endogenous research geospatial distribution, disadvantaging UHI most vulnerable regions was observed. • UHI strategies, tools, techniques, indexes and data source trends were identified through through data curation. • Seven major urban fields of impact were identified and multidimensionally analyzed. • Future research opportunities on UHI regulatory framework development were proposed. Scientific research on Urban heat islands (UHI) and urban. overheating has set new challenges for societies regarding risk and vulnerabilities, energy demands and mitigation strategies to favor urban environmental quality conditions against urban pollution. This review manuscript addresses systematically 171 relevant studies to identify and characterize gaps in the field of knowledge, through a robust evidence-based analysis focused on identifying taxonomic recurrences, trends in the study of urban fields of impact, global leaderships, mitigation strategies, methodological divergences, remote sensing indexes, data sources, legal frame, among other significant aspects on the context of environmental degradation and energy. The results showed that studies were carried out in 37 countries and 92 cities, mainly in North America (33%), Europe (31%), and Asia (27%) although mainly concentrated in five countries (59%) with an endogenous bias towards domestic microscale purposes. Prevailing topics, multidisciplinary interactions and regulations are focused on the study of Urban Environmental Quality degradation (95%) and on issues related to Energy (25%) demand and efficiency with applications on land cover with high interest on material science, and urban design and planning. The need to implement multiscale studies, motivate policies oriented to prevent health vulnerability and promote energy safety for urban resilience is imperative. [ABSTRACT FROM AUTHOR]