Nighttime Urban Heat Island Research Articles

Impacts of urban configuration on urban heat island: An empirical study in China mega-cities

Urban configuration can influence the local thermal environment by altering energy balances. However, previous studies have found that either sprawling urban or compact urban development could intensify urban heat island (UHI) intensity. How urban configurations can mitigate the UHI intensity has drawn full attention. In this study, we quantified the diurnal and seasonal UHI intensities in 36 cities in China and investigated their response to urban configurations. In each city, urban land cover maps were classified from Landsat 8 and UHIs were quantified using Moderate Resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST). Results show that the average UHI intensities of 36 cities vary temporally with a sequence of summer day > summer night > winter night > winter day. Moreover, whether the daytime UHI is higher or lower than the nighttime UHI significantly depends on climatic zones and seasons. Besides, we found that UHIs significantly correlate with urban configurations in two ways. First, for the spatial structure among built-up patches, a lower UHI located in the smaller built-up area with dispersed distribution when compared to the larger built-up patches, if the total built-up area holds constant. Second, for the spatial structure of a single patch, the single patch with more complex shape would mitigate the UHI intensities. Overall, urban configuration and other control variables (e.g., urban characteristics and climatic condition) can explain 41% and 51% of the variance in UHI in summer day and night, respectively. Therefore, the design of urban configuration can serve as an essential practice to mitigate UHI intensity. Considering the difficulties of altering the urban configuration in the urbanized area, planting vegetation might be a great choice to change the urban contiguity and shape complexity with providing an extra cooling effect.

Studying Surface and Canopy Layer Urban Heat Island at Micro-Scale Using Multi-Sensor Data in Geographic Information Systems

Variations of Urban Heat Island (UHI) effect within urban areas cannot be studied in detail using traditional combination of satellite images with thermal infrared (IR) bands and local weather station data due to their limited spatio-temporal scale. In this article, a system has been built to supplement the current infrastructure and enhance the high spatio-temporal scale. The article progresses from initially traversing through the city of Greater Noida to continuous manual data collection on an academic campus and later by automating it with integrated sensors on a microcontroller while achieving the objective of the collection of continuous high spatio-temporal scale data. Geographic information systems (GISs) were used to integrate and visualize these data with land surface temperature (LST) and air temperature data. The system provided the diurnal cycle of urban materials and insights into nighttime UHI at micro-scale. Overall the low-cost sensing technology presented has the potential to monitor citywide UHI.

Is the urban heat island exacerbated during heatwaves in southern Australian cities?

The extra-tropical Australian cities of Melbourne, Adelaide, and Perth are all affected by summer heatwaves and the urban heat island (UHI) effect. While research has been undertaken on both phenomena individually, they have not been studied in tandem in Australia. This research investigates the relationship between warm season heatwaves (November to March) and the UHI from January 1995 to March 2014. Observational temperature data from six or seven Bureau of Meteorology Automatic Weather Stations in each of Melbourne, Adelaide, and Perth are used to determine the strength of the UHI during heatwave periods and these are compared to non-heatwave periods. Melbourne and Adelaide both experience an exacerbated (warmer than normal) UHI at night during heatwaves. The night-time UHI in Perth is diminished (cooler than normal) during heatwaves and often changes to an urban cool island (UCI), when compared to non-heatwave periods. Environmental factors that might affect the strength of the UHI are investigated, including wind speed and direction, and station location. Despite the proximity of all stations to the coast, coastal influences on UHI strength are minimal during heatwave conditions. Station choice is found to not affect our results, with the characteristic pattern of the UHI during heatwaves remaining consistent across all three cities in a leave-one-out sensitivity analysis.

AN ASSESSMENT OF SPATIAL VARIATION OF LAND SURFACE CHARACTERISTICS OF MINNA, NIGER STATE NIGERIA FOR SUSTAINABLE URBANIZATION USING GEOSPATIAL TECHNIQUES

AN ASSESSMENT OF SPATIAL VARIATION OF LAND SURFACE CHARACTERISTICS OF MINNA, NIGER STATE NIGERIA FOR SUSTAINABLE URBANIZATION USING GEOSPATIAL TECHNIQUES

Investigating the nighttime urban heat island (Canopy Layer) using mobile transverse method: A case study of colon street in Cebu City, Philippines

Rapid urbanization has resulted in temperature differences between the urban area and its surrounding areas. Academics have called this as the urban heat island phenomenon. Among the places that have seen rapid urbanization is the City of Cebu. The Philippine’s oldest street, Colon, was chosen as the study area due to the near absence of vegetation and closely spaced buildings. Buildings that are spaced more closely as well as multiple absorptions and reflections produce higher and more viable street temperatures. This study tries to systematically understand the urban heat island effect between Colon and Lawaan, the rural area defined in this study. In order to quantify the urban heat island between two given locations, the mobile traverse method during the summer time, for a 10-day period in May 2016. A digital thermometer measuring platform was mounted on top of a vehicle to measure the different temperatures of Colon Street. Urban temperatures were also gathered in the Lawaan area using the same device. Preliminary results showed the presence of the urban heat island phenomenon between the two areas (ΔT =1.17 °C). The provision of green spaces and proper urban planning are essential in mitigating future urban heat stress due to anthropogenic changes of existing cities.

Effects of urban planning indicators on urban heat island: a case study of pocket parks in high-rise high-density environment

Due to urbanization, the urban heat island (UHI) effect has become a modern endemic threat to human health and energy consumption, especially in compact urban areas with high population density. Pocket parks, as the major component of urban green space in high-rise high-density built area of Hong Kong, play important roles in shaping the city. By conducting in situ climate measurements and morphological analysis of 12 sites in Hong Kong, this study investigates the impact of pocket parks on UHI intensity, and the relationships between five urban planning indicators and UHI. Results show that pocket parks and their surrounding areas in Hong Kong have a moderate UHI intensity (0.1°C) during the daytime but a very high nighttime UHI intensity (2.39°C). Ten parks are cooler than their surrounding urban streets for both daytime and nighttime, which means that pocket parks in Hong Kong can help to alleviate UHI intensity at the micro scale. Floor Area Ratio (FAR), building density and Tree Cover Ratio (TCR) are found to be significantly and negatively related with daytime UHI. It indicates that higher FAR, building density and TCR in high-rise high-density urban environment may help to reduce daytime UHI intensity without increasing the early nighttime intensity. Planting trees is an effective measure to reduce the UHI intensity inside the parks for high-rise high-density urban environment. But a threshold for TCR (around 42%) shall be achieved to maximize its cooling effect.

Urban heat island intensity and spatial variability by synoptic weather type in the northeast U.S.

Understanding regional air temperature (Ta) variability and urban heat island (UHI) magnitude has posed challenges given the minimal observational stations in urban areas. This study uses a dense network to evaluate the degree to which synoptic weather influences the UHI intensity and fine scale Ta variability on days of extreme heat. We examine daytime and nighttime temperatures in four large northeastern cities (urban-suburban-rural) using a dataset with relatively high spatiotemporal resolutions (100–200 stations per area, 5min or 60min intervals) for May–Sept, 2006–2013. Results show stronger UHI intensity and enhanced Ta variability under hot, dry weather types, with the most intense UHIs overnight in dry conditions. Absolute Ta magnitudes within both urban and rural areas remain heightened under moist weather type conditions. New York City presents the highest average nighttime UHI intensity (3.51°C). Minimal, and at times negative, UHIs are often present in the daytime, appearing on 38% and 28% of days in New York City and Boston, respectively. An exploratory analysis demonstrates a significant ability to predict average Ta at each station using common environmental predictors. Findings emphasize the importance in distinguishing between absolute maximum Ta versus UHI intensity by weather type for communication and translation of heat mitigation practices and health protection by public health and urban planning agencies.

Correlation Analysis of the Urban Heat Island Effect and Its Impact Factors in China

Urban heat islands (UHIs), which are urban areas with higher surface or air temperatures than surrounding rural areas, can further enhance the heat stress already exacerbated by global warming. This poses great challenges to human health and sustainable development. China has been burdened with heavy air pollution in recent years, and the effect of haze pollution on UHIs is still far less well understood. This study investigated the spatial variations of daytime, nighttime, and seasonal surface UHI effects in China during 2003-2013, based on MODIS land surface temperature data, and analyzed the correlations between the UHI effect and its impact factors from both biophysical and biochemical perspectives. Our results show that MODIS-derived annual nighttime UHI effect (3.4 K±0.2 K, mean±1 s.e.) is higher than the annual mean daytime UHI effect (2.1 K±0.3 K). The daytime UHI effect is strongest in summer and weakest in winter but contrasts with the seasonal variation characteristics of nighttime UHI effect. During daytime, UHI effects in humid and semi-humid regions are much more obvious than those in semi-arid/arid regions; during nighttime, the UHI effect in semi-humid and semi-arid/arid regions is much stronger than that in humid regions. A Daytime Urban Cool Island effect exists in semi-arid/arid regions during spring, autumn, and winter seasons. Population, vegetation activity, and irrigation are three factors controlling annual mean daytime UHI effect. The nighttime UHI is significantly influenced by latitude, albedo, precipitation, and aerosol concentrations. We provide evidence for a long-held hypothesis that the biogeochemical effect of urban aerosols is an important contributor to the UHI effect. The important role played by rural background environment in calculating the UHI effect is further discussed in terms of surface aerodynamic roughness and the contrast in albedo between urban and rural areas. Mitigation of haze pollution has a co-benefit of reducing the UHI effect and heat stress for urban dwellers. It is also conducive to reducing negative impacts of regional and global climate change.

Impact of urban morphology on microclimate and thermal comfort in northern China

Abstract This work is an experimental study focusing on the impact of urban morphology on the urban heat island (UHI) intensity, microclimate conditions and thermal comfort in a newly-developed urban area in Tianjin city, China. According to the Koppen–Geiger climate classification system, the studied area is classified as hot summer continental climate, characterized by hot and humid summers as well as cold and dry winters. Air temperature, relative humidity (RH) and wind speed at 46 points within an 8-km2 area are measured during both winter and summer seasons. Based on measured results and climatic mapping, the impacts of urban constituents such as building, pavement, greenery and water area on UHI intensity and microclimate conditions are analyzed. Results show that UHI intensity reaches up to 4.5 °C during daytime and 5.3 °C at night in summer, and 2.6 °C during daytime and 5.0 °C at night in winter. Thermal comfort level at the measurement points is predicted using the Thermal Sensation Vote (TSV) model developed for the local climate. The cooling effect of trees is evident during both daytime and nighttime in summer, but negligible in winter due to the reduction of leaf area and evaporation. The presence of both greenery and water body result in an increase in RH in air. Trees tend to reduce wind speed and improve thermal comfort in winter. Radiant heat dissipated from buildings and roads is the main contributor to nighttime UHI in both summer and winter seasons. Based on research results, urban design recommendations are proposed so as to improve outdoor thermal comfort in urban areas located in temperate climate zone during summer and winter.

Optimizing green space locations to reduce daytime and nighttime urban heat island effects in Phoenix, Arizona

The urban heat island effect is especially significant in semi-arid climates, generating a myriad of problems for large urban areas. Green space can mitigate warming, providing cooling benefits important to reducing energy consumption and improving human health. The arrangement of green space to reap the full potential of cooling benefits is a challenge, especially considering the diurnal variations of urban heat island effects. Surprisingly, methods that support the strategic placement of green space in the context of urban heat island are lacking. Integrating geographic information systems, remote sensing, spatial statistics and spatial optimization, we developed a framework to identify the best locations and configuration of new green space with respect to cooling benefits. The developed multi-objective model is applied to evaluate the diurnal cooling trade-offs in Phoenix, Arizona. As a result of optimal green space placement, significant cooling potentials can be achieved. A reduction of land surface temperature of approximately 1–2°C locally and 0.5°C regionally can be achieved by the addition of new green space. 96% of potential day and night cooling benefits can be achieved through simultaneous consideration. The results also demonstrate that clustered green space enhances local cooling because of the agglomeration effect; whereas, dispersed patterns lead to greater overall regional cooling. The optimization based framework can effectively inform planning decisions with regard to green space allocation to best ameliorate excessive heat.

Thermodynamic characterisation of urban nocturnal cooling.

Nocturnal cooling of urban areas governs the evolution of thermal state and many thermal-driven environmental issues in cities, especially those suffer strong urban heat island (UHI) effect. Advances in the fundamental understanding of the underlying physics of nighttime UHI involve disentangling complex contributing effects and remains an open challenge. In this study, we develop new numerical algorithms to characterize the thermodynamics of urban nocturnal cooling based on solving the energy balance equations for both the landscape surface and the overlying atmosphere. Further, a scaling law is proposed to relate the UHI intensity to a range of governing mechanisms, including the vertical and horizontal transport of heat in the surface layer, the urban-rural breeze, and the possible urban expansion. The accuracy of proposed methods is evaluated against in-situ urban measurements collected in cities with different geographic and climatic conditions. It is found that the vertical and horizontal contributors modulate the nocturnal UHI at distinct elevation in the atmospheric boundary layer.

Comparing the diurnal and seasonal variabilities of atmospheric and surface urban heat islands based on the Beijing urban meteorological network

AbstractThis study compared the diurnal and seasonal cycles of atmospheric and surface urban heat islands (UHIs) based on hourly air temperatures (Ta) collected at 65 out of 262 stations in Beijing and land surface temperature (Ts) derived from Moderate Resolution Imaging Spectroradiometer in the years 2013–2014. We found that the nighttime atmospheric and surface UHIs referenced to rural cropland stations exhibited significant seasonal cycles, with the highest in winter. However, the seasonal variations in the nighttime UHIs referenced to mountainous forest stations were negligible, because mountainous forests have a higher nighttime Ts in winter and a lower nighttime Ta in summer than rural croplands. Daytime surface UHIs showed strong seasonal cycles, with the highest in summer. The daytime atmospheric UHIs exhibited a similar but less seasonal cycle under clear‐sky conditions, which was not apparent under cloudy‐sky conditions. Atmospheric UHIs in urban parks were higher in daytime. Nighttime atmospheric UHIs are influenced by energy stored in urban materials during daytime and released during nighttime. The stronger anthropogenic heat release in winter causes atmospheric UHIs to increase with time during winter nights, but decrease with time during summer nights. The percentage of impervious surfaces is responsible for 49%–54% of the nighttime atmospheric UHI variability and 31%–38% of the daytime surface UHI variability. However, the nighttime surface UHI was nearly uncorrelated with the percentage of impervious surfaces around the urban stations.

The Urban Heat Island Effect in the City of Valencia: A Case Study for Hot Summer Days

Extreme heat poses significant risks to the world’s growing urban population, and the heat stress to human health is likely to escalate with the anthropogenically increased temperatures projected by climate models. Thus, the additional heat from the urban heat island (UHI) effect needs to be quantified, including the spatial pattern. This study focuses on the city of Valencia (Spain), investigating the intensity and spatial pattern of UHI during three consecutive hot summer days accompanying a heat record. For the analysis, long-term in situ measurements and remote sensing data were combined. The UHI effect was evaluated using two approaches: (a) based on air temperature (AT) time-series from two meteorological stations and (b) using land surface temperature (LST) images from MODIS products by NASA with 1 km resolution. The strongest nighttime UHI estimated from AT was 2.3 °C, while the most intense surface UHI calculated as the difference between the LST of urban and rural regions (defined by NDVI) was 2.6 °C—both measured during the night after the record hot day. To assess the human thermal comfort in the city the Discomfort Index was applied. With the increasing number of tropical nights, the mitigation of nighttime UHI is a pressing issue that should be taken into consideration in climate-resilient urban planning.

Spatially Explicit Mapping of Heat Health Risk Utilizing Environmental and Socioeconomic Data.

Extreme heat events, a leading cause of weather-related fatality worldwide, are expected to intensify, last longer, and occur more frequently in the near future. In heat health risk assessments, a spatiotemporal mismatch usually exists between hazard (heat stress) data and exposure (population distribution) data. Such mismatch is present because demographic data are generally updated every couple of years and unavailable at the subcensus unit level, which hinders the ability to diagnose human risks. In the present work, a human settlement index based on multisensor remote sensing data, including nighttime light, vegetation index, and digital elevation model data, was used for heat exposure assessment on a per-pixel basis. Moreover, the nighttime urban heat island effect was considered in heat hazard assessment. The heat-related health risk was spatially explicitly assessed and mapped at the 250 m × 250 m pixel level across Zhejiang Province in eastern China. The results showed that the accumulated heat risk estimates and the heat-related deaths were significantly correlated at the county level (Spearman's correlation coefficient = 0.76, P ≤ 0.01). Our analysis introduced a spatially specific methodology for the risk mapping of heat-related health outcomes, which is useful for decision support in preparation and mitigation of heat-related risk and potential adaptation.

Microscale Heat Island Characterization of Rigid Pavements

Rigid pavements have an impact on the urban heat island (UHI) and hence the surrounding environment and human comfort. Currently, most studies use a mesoscale approach in UHI characterization of pavements. This study proposes a microscale approach that can be incorporated into a pavement life-cycle assessment (LCA). The heat flux of various concrete pavements containing layers of varying thermal diffusivity and inertia was simulated. The surface pavement radiative forcing (RFp) was developed as a metric for use in a pavement LCA. Additionally, the heat conducted and stored in each concrete pavement system was analyzed using an average seasonal day metric to understand the temporal pavement energetics. Of the various thermal cases, only a higher albedo surface significantly changed the RFp for a fixed climate. However, a time lag was induced by the thermal inertia of the base course, which decreased the amount of heat conducted out of the pavement at night by storing heat in the base course for a longer time, effectively reducing nighttime UHI. Diurnal variations in thermal behavior can be controlled by changing the thermal properties of subsurface layers, which can be used to partially mitigate UHI.

Urban Heat Island Intensification during Hot Spells—The Case of Paris during the Summer of 2003

Heat waves are projected to become more frequent, longer-lasting, and intense. At the same time, urban areas are confronted with the urban heat island (UHI) phenomenon, which adds to the thermal stress experienced during hot spells. Focusing on the Paris area during the hot summer of 2003, we investigated the influence of heat waves on UHI intensity, i.e., the urban-rural temperature contrast. In a first step, this was done based on observed temperatures from an urban and a rural site, showing that per C increase in the daytime temperature, the nighttime UHI intensity increased by 0.086 C. Recognizing the limited spatial representativeness of the urban experimental site, located in a park, we then performed simulations with an urban climate model, covering the wider Paris area for the summer of 2003. First, a validation was done using the aforementioned temperature measurements to do so. Subsequently, we estimated the sensitivity of the nighttime UHI intensity with respect to the daytime temperature, this time using simulated temperatures of the densely built-up areas in the center of Paris, yielding an increase of UHI intensity of 0.19 C per C increase in the daytime temperature. While these results only apply to the domain and period studied, they do confirm recent reports that the UHI intensity increases during heat waves. The results also show that for the cooler parts of the urban fabric (e.g., parks), the UHI intensification during heat waves is around half of that of the dense urban fabric, thus providing some insights into possible mitigation strategies for the future.

Predicting air temperature simultaneously for multiple locations in an urban environment: A bottom up approach

Cities are characterized by high heterogeneity that results in varied microclimate effects. The current study introduces a new bottom–up approach linking the urban Canyon Air Temperature (CAT) model with spatially distributed inputs extracted from a GIS data-base and remote sensing products to predict intra-urban temperature variability simultaneously for multiple locations in an urban environment. To provide proof of concept, the model was applied for the city of Bat-Yam, Israel. Simulation shows a maximum nighttime urban heat island (UHI) intensity of 2–2.25 °C, relative to a rural reference point, during both summer and winter, with significant spatial variability related to the height-to-width ratio of urban street canyons and to the surface land cover. The CAT simulation also highlighted the important influence of the local wind regime on the development and persistence of the nocturnal UHI. We conclude that linking CAT to a GIS data-base supports simulations at the city scale that reflect the local intra-urban variability. The model can be used to investigate both macro and micro scale spatio-temporal characteristics of the UHI in various urban development scenarios, which may be applied to generate appropriate geographically-explicit mitigation and adaptation measures.

Evolution of urban heat wave intensity for the Brussels Capital Region in the ARPEGE-Climat A1B scenario

Urban areas are particularly vulnerable to heat wave (HW). This paper will examine the important question on urban heat island (UHI) during HW and their interactions under present and future conditions. Therefore, a 1km-resolution urban dynamical downscaling technique has been applied over the Brussels Capital Region. Results from observations, ERA-Interim driven simulations (1981–2010), present (1990–1999) and future (2046–2055) climate simulations under the A1B scenario of the global climate model ARPEGE-Climat indicate that for the present climate: (i) observed nighttime UHI is enhanced under HW by 1.5°C (0.6°C ERA-Interim), (ii) due to a dry summer bias in ARPEGE-Climat, the control simulation was not able to reproduce the UHI intensification during HW, and (iii) in the ERA-Interim runs, HW affects urban and rural surface energy balance differently. Hereby, the HW definition and heat stress index are based upon the warnings issued by the Public Health of Belgium. For the 2050s, climate change increases, statistically significant, the intensity of HW for urban (rural) areas during the night (day). Under future HW, the biggest and statistically significant increase of the ratio of number of hours to number of days while the Humidex is above dangerous (>40) thresholds occurs over urban areas.

Daytime and nighttime urban heat islands statistical models for Atlanta

In this study, we empirically model the interactions between 2D and 3D geospatial information and both daytime and nighttime urban heat islands, and estimate the relative importance of various urban heat islands drivers. While previous studies have explored the relationship between the urban heat islands and 2D urban features, the interactions with 3D urban features and neighboring surface characteristics have not been adequately explored. This paper specifies the impacts of these urban features on the urban heat islands intensity during daytime and nighttime, which tend to be quite different. The empirical evidence from this study suggests that while vegetation is the dominant factor for urban heat islands intensity during daytime, the urban canyon has stronger impacts on the urban heat islands than vegetation at night. In addition, adjacent surfaces are more likely to influence nighttime surface temperatures. These results could be used to develop urban design solutions for mitigating the urban heat islands.

Influences of land cover types, meteorological conditions, anthropogenic heat and urban area on surface urban heat island in the Yangtze River Delta Urban Agglomeration

Urban heat islands (UHIs) reflect the localized impact of human activities on thermal fields. In this study, we assessed the surface UHI and its relationship with types of land, meteorological conditions, anthropogenic heat sources and urban areas in the Yangtze River Delta Urban Agglomeration (YRDUA) with the aid of remote sensing data, statistical data and meteorological data. The results showed that the UHI intensity in YRDUA was the strongest (0.84°C) in summer, followed by 0.81°C in autumn, 0.78°C in spring and 0.53°C in winter. The daytime UHI intensity is 0.98°C, which is higher than the nighttime UHI intensity of 0.50°C. Then, the relationship between the UHI intensity and several factors such as meteorological conditions, anthropogenic heat sources and the urban area were analysed. The results indicated that there was an insignificant correlation between population density and the UHI intensity. Energy consumption, average temperature and urban area had a significant positive correlation with UHI intensity. However, the average wind speed and average precipitation were significantly negatively correlated with UHI intensity. This study provides insight into the regional climate characteristics and a scientific basis for city layout.