Maximum Urban Heat Island Research Articles

Dynamics of land use land cover change and its effect on urban heat island in Halaba Kulito Town.

Halaba Kulito Town in Ethiopia has experienced significant urbanization over the past three decades, leading to the conversion of natural land into built-up environments, causing environmental deterioration and impacting the local climate. This study set out to look at the dynamics of land use and cover change and how they have affected Halaba Kulito Town's Urban Heat Island during the previous thirty years. The analysis used the Landsat imagery (30m×30m) of 1991, 2001, 2011, and 2021. The supervised classification approach was used to classify the images. The evaluation of classification accuracy was checked using a confusion error matrix. For the years 1991, 2001, 2011, and 2021, the LULC change classification accuracy evaluation showed results of 70.81%, 81.3%, 81%, 91%, and 0.71, 0.81, 0.81, and 0.89, respectively, for overall classification accuracy and kappa coefficient. In 2001, 2011, and 2021, the built-up increased by 23.6%, 31.6%, and 43.13%, respectively. Over the first two decades, from 1991 to 2001 and 2001 to 2011, the quantity of land under cultivation increased by 15.7% and 14.3%, respectively, but dropped by 59.18% from 2011 to 2021. Between 1991 and 2001, range land dropped by 33.3%; nevertheless, in the next two decades, it expanded by 3.3% and 83.2%. The amount of bare land declined by 67.1% between 2001 and 2011, although it increased by 38.8% between 1991 and 2001 and 3.36% between 2011 and 2021. In addition, wetland increased by 47.1% and 191.95% between 2011 and 2021 and between 1991 and 2001, respectively, while declining by 62% between 2001 and 2011. Compared to the baseline 1991-year aerial coverage, built-up, range land, and wetland expanded by 132.9%, 26.2%, and 63.3% over the previous thirty years, whereas bare land and cultivated land dropped by 46% and 52.8%, respectively. The mean urban heat island (UHI) for 1991, 2001, 2011, and 2021 incorporating seasonal influence was 0.003, 0.002, 0.005, and 0.00, respectively. This is because the intensity of the UHI is correlated with changes in land use and land cover (LULC). In contrast, the lowest UHI dropped by 35.3% in 2021 compared to the value in 1991, while the maximum UHI increased by 18.6% from its 1991 level. In comparison to 1991, the mean intensity of UHI increased by 106.21% and 120.4%, respectively, in 2011 and 2021, whereas it declined by 20.67% in 2001. The findings of this study offer decision makers with scientifically proven knowledge about land use and its implications for the urban heat island effect. It will serve as a guideline for developing and implementing integrated land use management to produce climate resilient cities and create livable urban environments for citizens.

Multi-Temporal Analysis of the Impact of Summer Forest Dynamics on Urban Heat Island Effect in Yan’an City

In this study, MODIS land products and China land cover datasets were used to extract normalized difference vegetation index, land surface temperature, and vegetation cover type in Yan’an City during the summers of 2017–2022. On this basis, analysis of spatial change and correlation were carried out as a way to study the mitigation effect on urban heat islands in Yan’an City with forest. The study showed that: (1) The coverage of normalized difference vegetation index over 0.4 in summer in Yan’an City increased from 59.38% to 69.12%, and the vegetation showed good growth conditions. It has a spatial distribution pattern of more in the south and less in the north. (2) The proportion of the urban heat island in Yan’an City increased from 15.51% to 16.86%. Urban heat island intensity fluctuated year by year, with the maximum urban heat island intensity of 6.26 °C appearing in 2019. It has a spatial distribution pattern of less in the south and less in the north. The transition rate of temperature field grade from low to high is 73.32%, and the transition rate to low is only 0.31%. (3) There is a negative correlation between land surface temperature and normalized difference vegetation index in Yan’an City. Vegetation has a mitigating effect on the UHI and the best cooling effect among the vegetation is shown by forest. The cooling effect of forest in Yan’an City is attenuated by an increase in distance, and the effective range is greater than 1000 m. In this study, the regulation effect of forest on the urban heat island was obtained by digging deeper into the intrinsic connection between spatial change in vegetation cover and land surface temperature change in Yan’an City. It provides an important reference for the formulation of meteorological protection policy as well as the promotion of sustainable development of the urban ecological environment and is of guiding significance for future urban planning and ecological construction.

Characteristics of the Urban Heat Island in Dhaka, Bangladesh, and Its Interaction with Heat Waves

This study examines the characteristics of the urban heat island (UHI) in Dhaka, the densely populated capital city of Bangladesh under the influence of the South Asian monsoon, and its interaction with heat waves. For this, meteorological data at Dhaka (urban) and Madaripur (rural) stations and reanalysis data for the period of 1995–2019 are used for analysis. Here, the UHI intensity is defined as the urban-rural difference in 2-m temperature, and a heat wave is defined as the phenomenon which persists for two or more consecutive days with the daily maximum 2-m temperature exceeding its 90th percentile. The UHI intensity in Dhaka is in an increasing trend over the past 25 years (0.21 °C per decade). The average UHI intensity in Dhaka is 0.48 °C. The UHI is strongest in winter (0.95 °C) and weakest in the monsoon season (0.23 °C). In all seasons, the UHI is strongest at 2100 LST. The average daily maximum UHI intensity in Dhaka is 2.15 °C. Through the multiple linear regression analysis, the relative importance of previous-day daily maximum UHI intensity (PER), wind speed, relative humidity (RH), and cloud fraction which affect the daily maximum UHI intensity is examined. In the pre-monsoon season, RH is the most important variable followed by PER. In the monsoon season, RH is the predominantly important variable. In the post-monsoon season and winter, PER is the most important variable followed by RH. The occurrence frequency of heat waves in Dhaka shows a statistically significant increasing trend in the monsoon season (5.8 days per decade). It is found that heat waves in Bangladesh are associated with mid-to-upper tropospheric anticyclonic-flow and high-pressure anomalies in the pre-monsoon season and low-to-mid tropospheric anticyclonic-flow and high-pressure anomalies in the monsoon season. Under heat waves, the UHI intensity is synergistically intensified in both daytime and nighttime (nighttime only) in the pre-monsoon (monsoon) season. The decreases in relative humidity and cloud fraction are favorable for the synergistic UHI-heat wave interaction.

Efficiency of urban greening systems with maximized latent heat effect in urban heat island and climate change mitigation

Street, wall, and rooftop greening systems are essential for urban heat reduction and carbon neutrality. In this study, we compared the temperature-reducing effect of current and developed technologies that maximize the latent heat of evaporation through such greening systems. A research site with the maximum urban heat island effect was selected by analyzing the vulnerability of Suwon City, Korea. The latent heat of evaporation for each method was determined by conducting actual measurements and verified by performing computational fluid dynamics simulations. Based on the results of statistical techniques, the validated model was highly reliable. When developed technologies were applied, the temperature of the entire city was reduced by approximately 2 °C. Compared with the existing street greening system, the developed technology achieved a temperature reduction effect even at a distance of 5 m. Current wall greening systems only have a temperature reduction effect at 1 m, but that of the developed technology was approximately 1 °C even at a distance of 2 m. The existing rooftop greening system had a temperature reduction effect only at the height of 1.2 m, whereas that of the developed technology was effective even at 6 m, contributing to a reduction in the temperature of the entire city.

Application of a Semi-Empirical Approach to Map Maximum Urban Heat Island Intensity in Singapore

Differences in land surface characteristics across a city produce great spatial and temporal variability in air temperature. This fact is particularly pronounced between urban and surrounding rural areas giving rise to the canopy-layer urban heat island (CL-UHI) phenomenon. In the present study, we apply the dimensional analysis technique to develop a simple semi-empirical equation to map daily maximum CL-UHI (UHImax) intensities during nighttime over the city of Singapore for specific weather conditions. By adopting the methodology proposed by Theeuwes et al., but selecting meteorological and morphological parameters that affect UHImax intensity most for Singapore, evaluation of the developed equation shows good agreement with observations (RMSE = 1.13 K and IOA = 0.76). Model performance depends strongly on wind conditions and is best during weak winds when ‘ideal’ conditions for UHI development are approached (RMSE = 0.65 K and IOA = 0.85). Results using the simple equation developed to map UHImax intensities in Singapore under dry weather conditions are comparable to those obtained from more sophisticated numerical models, which demand significant computational resources, and the complex parameterizations involved require expertise to carry out the simulations. The resulting maps of the present study can be used to investigate less favorable thermal conditions and assess population vulnerability to a certain temperature excess, as well as provide insights for urban planning strategies of mitigation measures according to the land cover and morphology of a location.

Analysis of a City's Heat Island Effect on the Micro-Climate Parameters within Cities

The Urban Heat Island (UHI) is a micro-climatic phenomenon that influences the urban areas by elevating its temperature. UHI not only causes the thermal discomfort but also exert serious health issues along with aggravation of urban microclimate. Although a lot of research has been done on this phenomenon but UHI effect on micro scale is still less explored. This paper attempts to make a contribution in UHI studies of micro-climate. It consists of examination of UHI impact on microclimate of Aligarh city areas using mobile traverse method. This study determined the presence and extent of UHI’s microclimate variation within urban communities of different environmental layout and functional uses. The UHI effect started to appear from early afternoon and continue to rise with maximum UHI intensity recorded at early night. The highest recorded UHI intensity was 3.1 °C (at 21:00 hrs.), and the lowest was 0.6 °C (at 09:00 hrs.). A comparison of two districts of the same city located at a distance of 3 km and differing in population density, the number of buildings and landscaping showed that in the L1 area with more dense population and low landscaping, the temperature was consistently higher during the daily period; also in the L1 region had less humidity, which, combined with the already high temperature, makes it difficult to breathe and control the microclimate. These findings can be used for consideration for the future sustainable development of the affected area in regard of thermal comfort, environmental health and urban planning.

Climate Change Trends for the Urban Heat Island Intensities in Two Major Portuguese Cities

Urban Heat Island (UHI) intensities are analyzed for the metropolitan areas of the two major Portuguese cities, Lisbon and Porto, in the period 2008–2017. Projections for the UHI intensity averaged over 2008–2017 and a future period 2021–2050 are calculated under the Representative Concentration Pathway (RCP) 8.5. The spatiotemporal characteristics of the UHI intensity are assessed for daytime, nighttime, and average daily conditions. This analysis is carried out for the winter (Dec-Jan-Feb, DJF) and summer (Jun-Jul-Aug, JJA) meteorological seasons. Maximum UHI intensities of about 3.5 °C were reached in 2008–2017 in both metropolitan areas, but over a wider region during winter nighttime than during summer nighttime. Contrariwise, the most intense urban cool island effect reached −1.5 °C/−1 °C in Lisbon/Porto. These UHI intensities were depicted during summer daytime and in less urbanized areas. Overall, the UHI intensities were stronger during the winter than in the summer for both cities. Results show that the UHI intensity is closely related to underlying surfaces, as the strongest intensities are confined around the most urbanized areas in both cities. Until 2050, under RCP8.5, the highest statistically significant trends are projected for summer daytime, of about 0.25 °C (per year) for Lisbon and 0.3 °C (per year) for the UHI 99th percentile intensities in both metropolitan areas. Conversely, the lowest positive statistically significant trends (0.03 °C/0.02 °C per year) are found for the winter daytime UHI intensities in Lisbon and the winter nighttime and average UHI intensities in Porto, respectively. These statistically significant patterns (at a 5% significance level) are in line with the also statistically significant trends of summer mean and maximum temperatures in Portugal, under RCP8.5 until 2050. Scientists, urban planners, and policymakers face a significant challenge, as the contribution of urbanization and the forcing promoted by global warming should be duly understood to project more sustainable, go-green, carbon-neutral, and heat-resilient cities.

Investigating the Impact of Weather Conditions on Urban Heat Island Development in the Subtropical City of Hong Kong

Subtropical monsoon climates, high-density and heterogeneous urban built environments, as well as coastal–mountainous geographical environments influence the development of urban heat island (UHI) effects in Hong Kong. For better weather control of in situ observations and spatial analysis of UHI effects, it is necessary to quantitatively understand the influence of weather conditions on UHI development in Hong Kong and establish weather-based UHI estimation models. Meteorological records of four urban stations, one rural reference station, and one wind reference station at an hourly interval during the period of 2002–2012 were collected from Hong Kong observatory. A frequency analysis of the mean values of multiple meteorological elements and UHI parameters in urban stations was conducted to examine the prevailing and critical weather conditions, as well as the associated UHI conditions in Hong Kong. Multiple Linear Regression (MLR) was used to estimate the daily maximum UHI intensity (UHImax) based on a set of meteorological elements including cloud amount, wind speed, wind direction, relative humidity, and air temperature, as well as a UHI parameter of the daily maximum UHI intensity of the previous day (UHIpre-max). The results showed that MLR-based models can explain 33% and 56% variations of the UHImax in the summer and the whole year, respectively. The relative importance of each meteorological element on UHI development differed in the summer and annual periods, and the UHImax tended to be intensified under high temperature conditions in the summer.

Spatio-temporal analysis of urban heat island (UHI) and its effect on urban ecology: The case of Mekelle city, Northern Ethiopia

The bio-geophysical effects of land cover classes are considered to be an important factor in land surface temperature variations between urban and suburban areas. This means that major cities are significantly warmer than surrounding suburban or rural areas, which is known as the Urban Heat Island (UHI) effect. The aim of this study was to assess and analyze the spatiotemporal variation, correlation and impact of UHIs on Mekelle city (1990–2020) using remote sensing techniques. The study's primary objective was accomplished using the a number of techniques, including the extraction of LULC classes, estimation of the seasonal LSTs, assessment of UHI and UTFVI, and showing the relationship between LULC and LST as well as the interactions between UHI, UTFVI, and urban LULC classes. By analyzing TIRs/OLI thermal band data after calibrating uncertainty in the images and validating it using the concept of theoretical relationships and least squares fitting method, estimates of local LST, UHI (both in Mekelle and periphery rural areas), and UTFVI were obtained. The result of standard multiple regression models showed that impervious urban land surface, built-up areas, and dry bare soil highly contribute and influence variation at LST intensity caused for the formation of UHI in the study area. The result showed that the maximum UHI value in Mekelle city was 2.73 °C during the dry season in 1990. It decreased slightly to 2.53 °C in 2000 and then increased regularly to 2.83 °C and 2.98 °C in 2010 and 2020, respectively. To determine the city's UHI status in comparison to eight selected peripheral suburban areas, a trend analysis has been done. The UHI intensity of Mekelle city was higher relatively to that of most of the periphery suburban districts, particularly in 2020 (both dry and rainy seasons); this could be due to the city's explosive growth. It's worth noting that the research area affected by the urban heat island effect has grown over time, and as a result, the study area also has severe microclimate conditions that primarily damage the quality of urban life and create the worst conditions for thermal discomfort. The results of this study provide major conceptual understandings of how improper distribution and use of urban land affects the urban environment and fuels the creation of the UHI and thermal discomfort phenomena. Hence policy makers and urban planners should consider the effects of LST and UHI and integrate UHI comprehensive mitigation strategies with urban development patterns, and current and projected local climate changes in order to create sustainable urban environments, cities, and communities. In conclusion, compared to the conventional method, satellite remote sensing provides a faster and more efficient method for researching LST and UHI.

Applying a diagnostic equation for maximum urban heat island intensity based on local climate zones for Guangzhou, China

The Urban Heat Island (UHI) is a widely studied phenomenon, characterized by unique spatiotemporal variations depending on urban structure. Seeking a simple and rapid method to monitor the Urban Heat Island Intensity (UHII) for local urban area with multiple inner morphology is essential for energy saving, citizen health, and urban planning. Hence, this study proposed a diagnostic equation for daily maximum urban heat island intensity (UHIImax) based on routine meteorological data and basic urban properties. The applicability of equation, previously proposed by European scholars, was evaluated based on Local Climate Zone (LCZ) scheme by a long-term temperature observation experiment conducted in Guangzhou. Overall, the underestimation of UHIImax was caused by LCZ1,2,3,4, in which the morphological parameters were outside the application range of the original equation. Then, a revised equation was proposed by adding the impervious surface fraction (ISF) in morphological parameters based on the spatiotemporal variance of UHII for different LCZ. The revised equation was evaluated against a year dataset and revealed a higher accuracy than the original one with a decrease of RMSE and MEAE at 0.4K, 0.15K, an increase of dr at 0.1. Moreover, the efficiency of the equations for all the seasons and LCZs were elucidated. In summary, the results can be used as a tool for monitoring the development of UHI in outdoor temperature studies.

Comparative Analysis and Mitigation Strategy for the Urban Heat Island Intensity in Bari (Italy) and in Other Six European Cities

The presence of higher air temperatures in the city in comparison with the surrounding rural areas is an alarming phenomenon named the urban heat island (UHI). In the last decade, the scientific community demonstrated the severity of the phenomenon amplified by the combination of heat waves. In southern Italy, the UHI is becoming increasingly serious due to the presence of a warming climate, extensive urbanization and an aging population. In order to extensively investigate such phenomenon in several cities, recent research calibrated quantitative indexes to forecast the maximum UHI intensity in urban districts by exploiting multicriteria approaches and open-source data. This paper proposes different mitigation strategy to mitigate the Urban Heat Island Intensity in Bari. Firstly, the research evaluates the absolute max UHI intensity of the 17 urban districts of Bari (a city in southern Italy, Puglia) by exploiting the recent index-based approach IUHII. Secondly, a comparative evaluation of seven European cities (Bari, Alicante, Madrid, Paris, Berlin, Milan and London) is achieved to point out the positives and negative aspects of the different urban districts. In total, the comparison required the analysis of 344 districts of 7 European cities: 17 districts in Bari (Italia); 9 districts in Alicante (Spain); 21 in Madrid (Spain); 80 in Paris (France); 96 in Berlin (Germany); 88 in Milan (Italy) and 33 in London (UK). Finally, the results emphasize some virtuous examples of UHII mitigation in the major European cities useful to draw inspiration for effective mitigation strategies suitable for the urban context of Bari.

A novel risk-based design framework for urban heat island: A case study of Kempten, Germany

Studies on the urban heat island (UHI) effect have increased over the past decades, mainly focusing on topics such as thermal comfort, building energy, and sensitivity studies. However, there is currently a research gap in the development of risk-based design criteria for UHI. This paper proposes a novel risk-based UHI design criteria using heat-related mortality risk. The design criteria was developed based on the annual hottest seven-day mean temperatures. Frequency-consequence curves (or F–N curves) were used to quantify tolerable levels of societal risk. Exposure-response relationships for temperature and mortality for southern Germany were also used to determine the heat-related mortality rate. The heat-related mortality rate was used with the F–N curves to obtain a maximum allowable UHI intensity (UHII) for several risk scenarios. This allowable UHII was compared to the actual 50-year UHII for a residential district in Kempten, Germany. The Urban Weather Generator was used to model the UHII. If the actual UHII did not exceed the allowable UHII, the analyzed area was considered adequate. The 50-year UHII of the study area was 0.17 °C. The calculated allowable UHII ranged from 0.3 °C–13.2 °C. Therefore, the study area was adequate for all risk scenarios that were considered. This research lays the foundation for decision-makers and urban planners to incorporate more precise design targets in a number of applications from planning new developments to mitigation of UHI in existing urban areas.

Variabilidad temporal de la isla de calor urbana de la ciudad de Zaragoza (España)

En este artículo se analiza la intensidad y la variabilidad temporal de la isla de calor urbana (ICU) de la ciudad de Zaragoza (España) y se evalúa la acción del viento como importante factor atmosférico condicionante de la misma. A partir de los datos horarios proporcionados por la red meteorológica urbana de mesoescala de la ciudad, se calculó la diferencia de temperatura entre dos observatorios, uno urbano (Plaza Santa Marta) y otro en las afueras del área urbana (Ciudad Deportiva), en el periodo 2015-2020. Los resultados indican que la temperatura en el centro de la ciudad es, con mucha frecuencia, 1º o 2ºC más elevada que en el entorno, y en ocasiones ha llegado a superar los 8ºC. La ICU es más intensa en verano (promedios horarios de 2,5ºC) que en invierno (promedio de 2,2ºC) y es más intensa durante la noche que durante el día. El valor máximo de la ICU se alcanza en situaciones de calma atmosférica; en cambio, se debilita claramente con vientos de más de 10 km/h y llega prácticamente a desaparecer con velocidades superiores a 50 km/h.

Local climate effects of urban wind corridors in Beijing

Based on a summer observational experiment and the meteorological data from 2009 to 2018, this paper investigated the local climate effects of representative 1 Level One and 2 Level Two wind corridors (LOC and LTCs, respectively) in the central urban area (CUA) of Beijing by introducing the wind speed ratio (WsR) and urban heat island (UHI) intensity indices. In addition, the impacts on the ventilation of wind corridors from the 8 spatial landscape parameters were analyzed. The results indicated that LOC and LTCs have significant ventilation benefit and certain UHI mitigation benefit varying with season, day and night and weather，which were able to accelerate winds when wind speeds exceed 0.57 m/s and 0.75 m/s, respectively. The most important influencing parameters on the ventilation capability were frontal area index and roughness length. The multiyear average WsRs of the LOC and LTCs were 57% and 31% higher than that of the CUA, respectively. The UHI reduction did not occur along the whole corridor or throughout the whole day. The maximum seasonal mean UHI reduction of the LOC during daytime and nighttime were 0.89 °C and 0.75 °C, respectively. The LTCs mainly reduced UHI during daytime with the maximum reduction of 0.30 °C.

Evaluation of Absolute Maximum Urban Heat Island Intensity Based on a Simplified Remote Sensing Approach

The Urban Heat Island (UHI) phenomenon can be harmful during the summer season jeopardizing the safety of some vulnerable population classes. Typically, the study of the UHI requires long data acqu...

A Method for Selecting the Typical Days with Full Urban Heat Island Development in Hot and Humid Area, Case Study in Guangzhou, China

The urban heat island (UHI) poses a significant threat to urban ecosystems, human health, and urban energy systems. Hence, days with a relatively higher UHI intensity should be selected for UHI observation and analysis. However, there is still a lack in the method and criteria for selecting the typical meteorological days for UHI survey and simulation. In this study, field measurements were conducted based on Local Climate Zone (LCZ) schemes over a one-year period to assess the UHI behavior in Guangzhou, China. The relationship between the diurnal temperature range (DTR) and UHI intensity was evaluated and analyzed quantitatively under different meteorological conditions classified by precipitation. The average daily maximum UHI intensity (UHIImax) during precipitation days was approximately 1.8 °C lower than that during non-precipitation days, confirming that precipitation has a negative effect on UHI development. The monthly DTR distribution was similar to the daily UHIImax distribution, which was higher in autumn and winter, but lower in spring and summer. DTR has a significant linear correlation with the daily UHIImax, with a Pearson’s correlation coefficient of >0.7 and statistical significance of <0.001. Based on a quantitative evaluation of our results, we determined that 10 °C could be regarded as the appropriate DTR threshold to identify the meteorological conditions conducive to UHI development; the meteorological conditions exhibited a high daily UHIImax in Guangzhou. This study provides a simple method to select typical meteorological days for UHI measurement and simulation, and a method to early-warning of intense UHI events based on weather forecasts.

Is Urban Heat Island intensity higher during hot spells and heat waves (Dijon, France, 2014–2019)?

The Urban Heat Island (UHI) phenomenon is frequently associated with heat waves, both in scientific literature and in the media. Health problems and other issues often result from the co-occurrence of these two phenomena. Previous studies have not yet provided sufficient evidence to validate potential links between UHIs and heat waves. The MUSTARDijon network was set up in 2014, in Dijon, France. Its spatial density (60 sensors) and temporal depth (6 summers) provide information about several Hot Spells and Heat Waves (HS&HW), as well as the daily intensity of UHIs. For the period from June to August (the warmer months), no statistical relationship could be established between temperature and UHI intensity. The UHI phenomenon is strong when days are sunny and not very windy; these two conditions are frequently met during HS&HW, but not systematically. By contrast, these conditions can be fulfilled even without high temperatures. Strong UHIs can thus develop outside Hot Spell/Heat Wave (HS/HW) events. These HS&HW are not necessarily in phase with UHIs: maximum UHI intensity tends to occur before the onset or during the first few days of HS&HW. The intensity of the UHI decreases during HS&HW because nocturnal urban temperature generally remains stationary, whereas rural temperature tends to increase at night. The hypothesis proposed is that this increase in rural night temperature during HS&HW could be linked to the decrease in Actual Evapotranspiration (ETa) and soil wetness.

Synergies between urban heat island and heat waves in Seoul: The role of wind speed and land use characteristics.

The effects of heat waves (HW) are more pronounced in urban areas than in rural areas due to the additive effect of the urban heat island (UHI) phenomenon. However, the synergies between UHI and HW are still an open scientific question and have only been quantified for a few metropolitan cities. In the current study, we explore the synergies between UHI and HW in Seoul city. We consider summertime data from two non-consecutive years (i.e., 2012 and 2016) and ten automatic weather stations. Our results show that UHI is more intense during HW periods than non-heat wave (NHW) periods (i.e., normal summer background conditions), with a maximum UHI difference of 3.30°C and 4.50°C, between HW and NHW periods, in 2012 and 2016 respectively. Our results also show substantial variations in the synergies between UHI and HW due to land use characteristics and synoptic weather conditions; the synergies were relatively more intense in densely built areas and under low wind speed conditions. Our results contribute to our understanding of thermal risks posed by HW in urban areas and, subsequently, the health risks on urban populations. Moreover, they are of significant importance to emergency relief providers as a resource allocation guideline, for instance, regarding which areas and time of the day to prioritize during HW periods in Seoul.

Urban Overheating and Cooling Potential in Australia: An Evidence-Based Review

Cities in Australia are experiencing unprecedented levels of urban overheating, which has caused a significant impact on the country’s socioeconomic environment. This article provides a comprehensive review on urban overheating, its impact on health, energy, economy, and the heat mitigation potential of a series of strategies in Australia. Existing studies show that the average urban heat island (UHI) intensity ranges from 1.0 °C to 13.0 °C. The magnitude of urban overheating phenomenon in Australia is determined by a combination of UHI effects and dualistic atmospheric circulation systems (cool sea breeze and hot desert winds). The strong relation between multiple characteristics contribute to dramatic fluctuations and high spatiotemporal variabilities in urban overheating. In addition, urban overheating contributes to serious impacts on human health, energy costs, thermal comfort, labour productivity, and social behaviour. Evidence suggest that cool materials, green roofs, vertical gardens, urban greenery, and water-based technologies can significantly alleviate the UHI effect, cool the ambient air, and create thermally balanced cities. Urban greenery, especially trees, has a high potential for mitigation. Trees and hedges can reduce the average maximum UHI by 1.0 °C. The average maximum mitigation performance values of green roofs and green walls are 0.2 °C and 0.1 °C, respectively. Reflective roofs and pavements can reduce the average maximum UHI by 0.3 °C. In dry areas, water has a high cooling potential. The average maximum cooling potential using only one technology is 0.4 °C. When two or more technologies are used at the same time, the average maximum UHI drop is 1.5 °C. The mitigation strategies identified in this article can help the governments and other stakeholders manage urban heating in the natural and built environment, and save health, energy, and economic costs.

Development of a holistic urban heat island evaluation methodology

Urban Heat Island (UHI) phenomenon concerns the development of higher ambient temperatures in urban districts compared to the surrounding rural areas. Several studies investigated the influence of individual parameters in the UHI phenomenon, on the other hand, an exhaustive study that quantifies the influence of each parameter in the resulting UHI is missing in the related literature. This paper proposes a new index aimed at quantifying the hazard of the absolute maximum UHI intensity in urban districts during the Summer season by taking all the parameters influencing the phenomenon into account. In addition, for the first time, the influence of each parameter has been quantified. City albedo and the presence of greenery represent the most important characteristics with an influence of 29% and 21%. Population density, width of streets, canyon orientation and building height has a medium influence of 12%, 10%, 9% and 8% respectively. The remaining parameters have an overall influence of 11%. These results are achieved by exploiting three synergistically related techniques: the Analytic Hierarchy Processes to analyse the parameters involved in the UHI phenomenon; a state-of-the-art technique to acquire a large set of data; and an optimization procedure involving a involving a Jackknife resampling approach to calibrate the index by exploiting the effective UHI intensity measured in a total of 41 urban districts and 35 European Cities.