Canopy Urban Heat Island Research Articles

Surface and canopy urban heat island disparities across 2064 urban clusters in China

The vast majority of urban heat island (UHI) studies are now derived from surface temperatures, substituting for the original air temperature-based definition. The disparities in hourly surface-canopy UHI effects (SUHI, CUHI) and the contrasting mechanisms are currently poorly understood. Here, we use high-resolution hourly LST and air temperature data from 2064 urban clusters in China to estimate SUHI and CUHI intensities and their driving mechanisms during the summer and winter of 2022. Across all urban clusters, we find that SUHI is on average ten times higher than the CUHI in the summer (0.97 VS. 0.09 °C) yet nearly triple that in the winter (0.30 VS. 0.11 °C), with SUHI exceeding CUHI in 91.5 % and 65.7 % of urban clusters, respectively. Seasonal and hourly analyses on SUHI/CUHI confirm typically opposite hysteresis variations (magnitude, peak, and timing of occurrence) and more correlated surface-canopy UHIs patterns during the night. We further demonstrate that SUHI magnitude can be largely explained by biophysical factors, urban attributes, and climate contexts, whereas CUHI interferes with additional constraints linked to ground-air energy transfer and advective dissipation. The improvement of urban greenery aids summer cooling efficiently in equatorial and boreal regions, while albedo measures are relevant in mitigating nocturnal warming in arid regions. Our findings support multiple technologies as ideas for urban three-dimensional UHIs (surface, canopy and boundary) and energy mechanisms, and the urgent need for ambitious urban heat mitigation strategies to minimize future climate change impacts.

Study on the Impact of Urban Morphologies on Urban Canopy Heat Islands Based on Relocated Meteorological Stations

This study addresses a crucial gap in understanding the impact of urban morphologies on the canopy urban heat islands (CUHI) effect. The selection of reference stations lacks a unified standard, and their surface air temperature (SAT) sequences are also inevitably influenced by urbanization. However, synchronous observational data from relocated meteorological stations could provide high-quality sample data for studying CUHI. Utilizing remote sensing techniques, the findings of this paper revealed that the observation environment of stations after relocation exhibited remarkable representativeness, with their observation sequences accurately reflecting the local climatic background. The differences in synchronized observation sequences could characterize the CUHI intensity (CUHII). Among the various factors, land use parameters and landscape parameters played particularly significant roles. Furthermore, the fitting performance of the random forest (RF) model for both training and testing data was significantly superior to that of the linear model and support vector regression (SVR) model. Additionally, the influence of local circulation on CUHI could not be overlooked. The mechanisms by which urban morphologies affect CUHII under different circulation backgrounds deserve further investigation.

Meta-pragmatic investigation of passive strategies from ‘UHI– climatology’ nexus perspective with digital twin as assessment mechanism

A global phenomenon identified 200 years ago as Urban Heat Island (UHI) effect gained popularity as the sheer contributor to the precipitous temperature gradient between rural and urban interface, instigating excess heat gain and associated ill effects on the urban dwellers. UHI is a function of many interrelated geographical, ecological, and economic parameters that require differential treatment in determining the antecedent impacts. This transdisciplinary review assessed the passive strategies (vegetation, cool roofs, cool pavements, and green roofs) from 83 studies that employed a numerical simulation approach to combat UHI. On average, vegetation and cool/green roofs can reduce ambient temperature by 3–5 ​°C, while cool pavements help to reduce surface temperature by 5 ​°C. All passive strategies also reveal it can reduce buildings' energy demand by 4–10%. However, the current methodological framework for evaluating UHI is quite fragmented, using multiple software and estimates only Surface Urban Heat Island (SUHI), ignoring Canopy Urban Heat Island (CUHI), Boundary Urban Heat Island (BUHI), and the nexus of ‘UHI-Climatology,’ which is linked to regional and global climate change, failing to model UHI and its complex connection to climate change accurately. The review found that the efficacy of passive strategies is a function of factors ranging from location, cloud cover, and soil type to simulation accuracy; hence, while these passive strategies alleviate outdoor temperature in one place, they can cause counterproductive impacts in another region. Therefore, as a postlude, the paper explores an alternative methodological framework for evaluating the nexus of UHI-Climatology using digital twin technology, thus espousing better mitigation strategies.

Research progress on the synergies between heat waves and canopy urban heat island and their driving factors

Under the background of global warming and accelerating urbanization, the interaction between heat waves (HWs) and canopy urban heat island (CUHI) has become one of the focuses in the field of global climate change research. This paper comprehensively reviewed and summarized the research process on the synergies of HWs and CUHI and their influencing mechanism. The coupling effect between HWs and CUHI remains debated, which may be related to the use of different standards to define heat wave events. The spatiotemporal differences in the synergies between HWs and CUHI was also influenced by climate background and local circulation. For instance, scholars have reached different conclusions regarding the stronger synergistic effect between daytime and nighttime in cities with different climate backgrounds. In addition, the modulation of urban morphological structure to the synergies between HWs and CUHI cannot be ignored. In the future, it is necessary to adopt different definitions of HWs to systematically study the formation mechanism of urban excess warming from different climatic backgrounds, local circulation conditions, and urban morphologies.

Remote Sensing and Field Measurements for the Analysis of the Thermal Environment in the “Bosco Verticale” Area in Milan City

The trend of urbanization nowadays has caused serious issues related to climate. One of the most important ones is that of the ‘Urban Heat Island (UHI)’ and it occurs in major cities throughout the world. The most important categories, and therefore the most studied ones, are the canopy urban heat island (CUHI) and surface heat island (SUHI). The aim and the novelty of the current study was to assess different remote sensing approaches to detect the thermal environment of an open area inside a large city. The study was undertaken in an urban area with green spaces, in the Bosco Verticale area in the city of Milan, during the spring and summer period of 2021. The area is characterized by different types of cover materials, which were investigated in terms of surface temperature under shaded and non-shaded conditions. Both field measurements and remote sensing techniques were applied. Remote sensing techniques included downscaling techniques and the usage of different split-window algorithms applied on the Landsat8 satellite sensor data. The land surface temperature (LST) extracted from remote sensing methods was compared with the surface temperature derived from in situ measurements. For the needs of the study, both in situ measurements and the collection of meteorological data from different fixed meteorological stations throughout the city of Milan were carried out. The results revealed the significance of greenery presence inside the urban environment, as a comparison of the meteorological data across the urban area of Milan showed that the areas with a low presence of greenery were found to be warmer than those with a higher presence of green elements. Concerning the field measurements in the study area, the results showed a significant reduction in both surface and air temperature in shaded places. On the other hand, the presence of conventional artificial materials in sunny areas led to relatively high values of both surface and air temperature. The downscaling method showed satisfying results in terms of average LST values; however, some discrepancies appeared in terms of the RMSE index. The application of split-window algorithms has shown that some forms of the ‘Generalized split-window algorithm’ and some forms of the ‘Jimenez-Munoz algorithm’ presented better performance among the studied algorithms. Comparing the LST values derived from the most representative algorithm, the ‘Du, Wan algorithm’, with those derived from downscaling methods, it was found to be quite close. However, under shaded conditions, the results derived from the ‘Split-window algorithm’ were found to be more precise. The application of remote sensing techniques in microscale in urban regions should be further studied in future, as they could be an essential tool for observing microclimatic conditions in urban areas and on building scale.

Different explanations for surface and canopy urban heat island effects in relation to background climate

The background climatic conditions and urban morphology greatly influence urban heat island effects (UHIs), but one-size-fits-all solutions are frequently employed to mitigate UHIs. Here, attribution models for surface UHIs (SUHIs) and canopy UHIs (CUHIs) were developed to describe UHI formation. The contribution of factors to SUHIs and CUHIs shows similar dependencies on background climate and urban morphology. Furthermore, the factors that mainly contributed to CUHIs were more complex, and anthropogenic heat was the more critical factor. Influence from urban morphology also highlights that there is no one-size-fit-all solution for heat mitigation at the neighborhood. In particular, maintaining a low building density should be prioritized, especially mitigating CUHIs. Moreover, it is more effective to prioritize urban irrigation maintenance over increasing green cover in arid regions but the opposite in humid regions. The work can provide scientific evidence to support developing general and regional guidelines for urban heat mitigation.

Elucidating the Multi‐Timescale Variability of a Canopy Urban Heat Island by Using the Short‐Time Fourier Transform

AbstractTaking the megacity of Beijing as an example, a short‐time Fourier transform (STFT) method was employed to extract the multi‐timescale evolution pattern of the canopy urban heat island intensity (CUHII) during 2000–2020. The STFT of CUHII showed a close relationship between the evolution of the CUHII in Beijing and the background meteorological forcing at intra‐annual, weather and intra‐daily scales. The intra‐annual‐scale spectrum of CUHII exhibited an increasing trend with obvious seasonal variation of the canopy urban heat island (CUHI). The intra‐daily‐scale spectrum of CUHII showed an increasing trend with the nighttime CUHI developing faster. Increasing Western Pacific Subtropical High intensity can enhance the seasonal and diurnal fluctuations of CUHII. The weather‐scale spectrum of CUHII is controlled by weather system evolution, showing that the frequency of cold/heat waves (CWs/HWs) in Beijing was significantly negatively correlated with the weather‐scale spectral intensity of the CUHII. CWs and HWs can increase the CUHII for a long duration.

Diurnal Variation in Urban Heat Island Intensity in Birmingham: The Relationship between Nocturnal Surface and Canopy Heat Islands

Urban heat islands have garnered significant attention due to their potential impact on human life. Previous studies on urban heat islands have focused on characterizing temporal and spatial variations over longer periods of time. In this study, we investigated the urban heat island (UHI) in Birmingham from September 2013 to August 2014 using higher temporal resolution SEVIRI satellite surface temperature data along with data from the Birmingham Urban Climate Laboratory (BUCL) meteorological station and the UK Meteorological Office meteorological station. Our aim was to characterize the diurnal variations in the surface urban heat island intensity (SUHII) and canopy urban heat island intensity (CUHII) and to explore their relationship under the influence of three factors (day/nighttime, season, and wind speed) using regression analysis. Our findings reveal that SUHII and CUHII exhibit relatively stable patterns at night but vary significantly during the day with opposite diurnal trends. In addition, SUHII and CUHII were more variable in spring and summer but less variable in winter. During the nighttime, SUHII represents CUHII with high confidence, especially during spring and summer, but less so during the cold season. In addition, SUHII represents CUHII with greater confidence under low-wind conditions. This study deepens our understanding of the diurnal dynamics of urban heat islands and the influence of atmospheric conditions on the relationship between surface and canopy heat islands in urban areas. The results of this study can be used for heat island studies in cities that lack high-precision observation networks and to guide sustainable urban development.

To aspirate or not to aspirate – Impact of active versus passive ventilation on urban heat (island) indicators

The present paper reports on the impact of ventilation mode – active versus passive – of instrument screens for air temperature measurements on canopy urban heat (island) indicators. This is done using air temperature data gathered in the Antwerp (Belgium) area in July 2013, in an experimental set-up composed of an urban and a nearby rural climate station, each equipped with both actively and passively ventilated temperature sensors. The resulting data shows an air temperature bias between the passively and actively ventilated measurements of up to approximately 2 °C, the highest values occurring under conditions of a high solar radiation load and low wind speed. Yet, the canopy urban heat island (UHI) increment, i.e., the urban-rural air temperature difference, is hardly affected by the ventilation method, which is ascribed to the fact that high UHI increments occur mainly at night when, in absence of solar radiation, active ventilation has a lesser influence on measured temperature. Conversely, urban heat indicators involving daytime air temperature, such as heatwave degree days (HWDD) or Steadman's apparent temperature, tend to produce overly high values when use is made of passively ventilated temperature measurements.

Investigation on Air Ventilation within Idealised Urban Wind Corridors and the Influence of Structural Factors with Numerical Simulations

Wind corridors are expected to be effective in alleviating the canopy urban heat island effect and air pollution. However, investigations on airflow characteristics within wind corridors, especially the influences of structural factors, are still limited. This current work performed numerical simulations on a group of idealised wind corridor models with different aspect ratios (ARs) and varying heights and/or widths along the corridors. Simulations revealed that the AR value had a vital influence on the wind speed, and an AR value of 0.1 facilitated the best ventilation conditions within the wind corridor. Structural variations along the corridor have a critical influence on ventilation, where the width contraction (contraction structure) and high-rise buildings (protrusion structure) would considerably weaken the wind speed within the corridors. The results suggested that wider and step-up structural design along the corridor should be encouraged in urban wind corridor planning, which would be helpful in promoting ventilation efficiency; but contraction structures should be prevented for primary wind corridor design.

Modeling Urban Heat Islands and Thermal Comfort During a Heat Wave Event in East China With CLM5 Incorporating Local Climate Zones

AbstractThe urban expansion‐induced heat can exacerbate heat stress for urban dwellers, especially during heat waves. With a focus on the intra‐urban variability of urban heat islands (UHIs) and thermal comfort, the urban parameterization within the Community Land Model version 5 (CLM5) was modified incorporating the local climate zones (LCZs) framework, named CLM5‐LCZs, to simulate the urban climate during a heat wave (HW) event in the summer of 2013. The evaluation of model performance demonstrated that it did a reasonable job of simulating surface energy balance and thermal regimes in cities against observational fluxes from a flux tower measurement site and temperatures from automatic meteorological stations in Nanjing, China. Then we investigated the characteristics and causes of UHIs associated with local background climate, intra‐urban inhomogeneity and HW intensity in East China. The results exhibited that daytime and nighttime canopy urban heat island intensity (CUHII) were highest in the Compact Low Rise (LCZ3) and the Compact High Rise (LCZ1) areas, respectively, while surface urban heat island intensity (SUHII) peaked in the Large Low Rise (LCZ8) and the Compact High Rise (LCZ1) areas during daytime and nighttime, respectively. Urban dwellers were easier exposed to serious heat environment in LCZ3 and LCZ1 areas over the north subtropical climate zone. Contrasts of CUHII and SUHII among different urban classes could exceed 1.7°C and 5.4°C. The intra‐urban heterogeneity may alter the dominant factors controlling SUHII, which were also modulated by local climate and HW intensity. Unlike other controlling factors, the impact of local climate on the contribution from the urban‐rural contrast of convection efficiency was larger than urban features. Overall, CLM5‐LCZs displayed potential of implementing detailed simulations for inter‐ and intra‐city UHIs at a larger scale, and enhancing the capabilities in modeling urban climate and exploring the causes and controls of UHIs.

Contrasting Trends and Drivers of Global Surface and Canopy Urban Heat Islands

AbstractA comprehensive comparison of the trends and drivers of global surface and canopy urban heat islands (termed Is and Ic trends, respectively) is critical for better designing urban heat mitigation strategies. However, such a global comparison remains largely absent. Using spatially continuous land surface temperatures and surface air temperatures (2003–2020), here we find that the magnitude of the global mean Is trend (0.19 ± 0.006°C/decade, mean ± SE) for 5,643 cities worldwide is nearly six‐times the corresponding Ic trend (0.03 ± 0.002°C/decade) during the day, while the former (0.06 ± 0.004°C/decade) is double the latter (0.03 ± 0.002°C/decade) at night. Variable importance scores indicate that global daytime Is trend is slightly more controlled by surface property, while background climate plays a more dominant role in regulating global daytime Ic trend. At night, both global Is and Ic trends are mainly controlled by background climate.

Simulation of canopy urban heat island at a block scale based on local climate zones and urban weather generator: a case study of Beijing

ABSTRACT The current research on urban heat island (UHI) effect mostly focuses on the analysis of land use type changes and the surface UHI intensity. Few studies on the urban canopy heat island effect at a block scale of local climate zone (LCZ), although the canopy heat island effect is a key factor affecting human thermal comfort. Therefore, this study will combine the LCZ classification system and the urban weather generator (UWG) model to simulate and quantitatively analyse the urban canopy heat island effect in Beijing at the block scale. First, based on Sentinel-2 Multispectral remote sensing images, the residual neural network (ResNet) method was used to obtain the LCZ of Beijing, and the results of LCZs was validated based on the google earth engine (GEE). Then, according to the classification results of local climate zones, the input parameters of the UWG and their corresponding value ranges are calculated. Finally, the UWG model is used to simulate the canopy temperature in different local climate zones, and the urban canopy temperature is validated using the meteorological station dataset. We quantitatively analyse the temperature differences between different types of LCZs. The results shows that the canopy heat island effect in Beijing gradually weakened outward from the city centre. This is mainly due to the relatively dense distribution of compact local climate zones in the centre of Beijing, while the surrounding areas of Beijing have lower building density and better natural coverage. The canopy urban heat island intensity of built-up LCZs is significantly stronger than that of natural-covered LCZs. The heat island intensity of the compact LCZ is higher than that of the open LCZ with the same building height. However, for LCZs with comparable compactness, the heat island intensity of high-level LCZs is higher than that of low-level LCZs.

Inter-seasonal characterization and correlation of Surface Urban Heat Island (SUHI) and Canopy Urban Heat Island (CUHI) in the urbanized environment of Delhi

The Intergovernmental Panel on Climate Change's (IPCC) 6th assessment report warns India about excessive heat. As India's population, urbanization, built-up regions, and concrete expand, vulnerable people may face higher heat risks. Delhi, India's capital and second-largest city, is noted for its excessive heat and air pollution. Heat in Delhi is spatially distributed and varies in magnitude due to built-up area density, the absence of a natural landscape, and other geographical aspects. To pinpoint the hotspot region, the study attempts to characterize Surface Urban Heat Islands (SUHIs) and Canopy Urban Heat Islands (CUHIs). Temporal and seasonal assessment of LST (Land Surface Temperature) using MODIS (Moderate Resolution Imaging Spectroradiometers) data and Ambient Temperature (AT) data from 39 Central Pollution Control Board (CPCB) monitoring stations with a representative radius of 1–5 km used to explore the inter-seasonal variation of SUHI and CUHI over the city in terms of magnitude and extent for 2019, 2020, and 2021. To describe SUHI and CUHI, a two-dimensional Gaussian fitting-based image analysis method is developed. The results show that the region's greatest SUHI magnitude is 2.36 °C in the monsoon season and 2.09 °C in the spring season. In comparison, the maximum CUHI magnitude is 4.74 °C and 3.15 °C in the Autumn and Winter seasons, respectively. The impact of SUHI can be experienced up to 82 km2 from the critical location in Winter and CUHI has 67.83 km2 in Summer. Inter seasonal comparison matrix between SUHI and CUHI demonstrates a positive but modest association due to considerable seasonal parameter fluctuation. UHI hotspot zone is at the centre of the urban with little seasonal fluctuation. South-West (SW) is a hotspot for SUHI and CUHI during the monsoon and spring seasons due to its dense residential and industrial areas. During winter, SUHI indicates SW as the critical zone and CUHI depicts NW. The finding shows that Delhi has a longer duration of hot weather. This study could help to understand UHI in Delhi at the local scale. The study shows UHI's seasonality and potential drivers. Critical hotspots and seasonal shifts should be used in urban development to improve Delhi's thermal environment.

Unevenly spatiotemporal distribution of urban excess warming in coastal Shanghai megacity, China: Roles of geophysical environment, ventilation and sea breezes

The complexity and uncertainty of urban excess warming are modulated by the morphology of urban areas, canopy urban heat island (CUHI)–heat wave interactions, the geographical–climatological background and the local circulation. Focusing on the coastal Shanghai megacity, we analyzed the synergistic effects of local climate zones (LCZs), urban ventilation, and sea breezes on the urban excess warming related to the CUHI and heat waves in summer from 2013 to 2018. Over the whole urban areas, the average CUHI intensity (CUHII) increased by 128.91% during heat waves periods relative to non-heat wave periods. Moreover, the spatial-temporal distributions of both CUHII and heat waves exhibited heterogeneously. In dense urban areas (e.g., LCZ 1 and 2), the blocking effect of dense high-rise buildings, and poor ventilation with low wind speeds would exacerbate urban excess warming, which exhibited relatively higher occurrence of heat waves and an increase in the CUHII. By contrast, over open areas of the urban periphery (eg., LCZ5), more heat wave events and stronger CUHII were mainly affected by the horizontal advective transport of urban heat at medium wind speeds. Particularly, the sea breezes could weaken the CUHII and decrease the occurrence frequency of heat wave events during daytime over coastal areas. In addition, the shading effects of high-rise buildings and the evaporative cooling effects of water and vegetation would also mitigate urban warming at the local scale, which would reduce the CUHII as well as the frequency of heat waves.

Role of local climate zones and urban ventilation in canopy urban heat island–heatwave interaction in Nanjing megacity, China

Different local climate zones (LCZs) and wind speeds (WSs) increase the uncertainties related to canopy urban heat island (CUHI), heat waves (HWs), and their interactions. Taking the megacity of Nanjing in China as an example, this paper quantifies and elucidates the spatiotemporal variations of the CUHI intensity (CUHII) and its association with HWs, as well as their potential drivers. Results show a significant diurnal variation in CUHII, showing a valley at daytime and a peak at nighttime. The CUHII is significantly stronger in the HW periods than in the non-heat-wave (NHWs) periods, especially at daytime, which is a positive feedback to HW intensity. The interannual variability of CUHII was mainly dominated by anthropogenic heat emissions and air pollutions. While the spatial patterns of mean CUHII and HWs were shaped by LCZs and urban ventilation. Over LCZs with high densities of buildings in the city center, the CUHII is higher and HW events are more frequent under medium-low WS than high WS. In contrast, over LCZs with an open distribution of buildings on the urban outskirts, under high WS condition, CUHII becomes higher and HW events become more frequent relative to low WS, due to the high wind-driven heat advection.

Diurnal and interannual variations of canopy urban heat island (CUHI) effects over a mountain–valley city with a semi-arid climate

Taking Lanzhou (a typical mountain–valley city in Northwest China) as an example, the diurnal and interannual variation characteristics of the canopy urban heat island (CUHI) and its relationship with heat waves (HWs), local climate zones (LCZs), mountain–valley wind circulation and aerosol pollution were explored. Results showed that the peak CUHI intensity (CUHII) in Lanzhou occurred at nighttime, while the minimum appeared in the daytime. The interannual variation of the CUHI fluctuated greatly, showing an upward trend. The average CUHII in HW periods was 103.6% higher than that in non-heat wave (NHW) periods. Longer-lasting HWs amplified the CUHII, in turn stronger CUHII led to more frequent HWs. The spatial variation of CUHIIs mainly varied with LCZ types. In addition, relative to NHW periods, the mountain–valley wind circulation was strengthened during HW periods, favoring an increase in CUHII. On the one hand, the increased urban wind speed contributed to the enhancement of vertical turbulent heat transfer during HW periods. On the other hand, the ventilation conditions were improved, resulting in the reduced aerosol concentration, the urban canopy received more shortwave radiation, and the heat storage increased during HW periods. Both of these situations were conducive to enhanced CUHII.

Modeling urban canopy air temperature at city-block scale based on urban 3D morphology parameters– A study in Tianjin, North China

Urban 3D morphology significantly influences the outdoor thermal environment. Understanding the influence of urban expansion in both horizontal and vertical landscapes helps the canopy urban heat island (CUHI) effect mitigation. However, the microscale numerical CUHI models are difficult to be applied for large-area CUHI effect studies. On the other hand, the mesoscale CUHI models make a wider study area workable but lost some of the model accuracies. To perform a large-area study on the CUHI effect with relatively light computing costs and fine accuracy, this paper builds a canopy air temperature predicting model at city-block scale with urban 3D morphology parameters including building coverage ratio (BCR), grass coverage ratio (GCR) and the mean value of building height (BH) to obtain the citywide block-mean 2-m temperature (T2M). The model accuracy was validated through RMSEs and comparison with the mereological station data. The proposed model shows an RMSE of 0.286 straight °C and an R-square of 0.83. Using the validated model, Tianjin with an area of 647 km 2 was performed to investigate the effects of vertical landscape on the canopy air temperature under different scenarios between 2010 and 2016, including the changes in landcover and building heights. It finds that a 40% increase in BCR may lead to the highest canopy air temperature, and the increase of BH may lead to an increase in the canopy air temperature in low-rise and high-rise building areas, but there is an opposite trend in multi-story and mid-rise building areas.

Impacts of the Urban Spatial Landscape in Beijing on Surface and Canopy Urban Heat Islands

Impacts of the Urban Spatial Landscape in Beijing on Surface and Canopy Urban Heat Islands

Heat Waves Amplify the Urban Canopy Heat Island in Brno, Czechia

This study used homogenised mean, maximum, and minimum daily temperatures from 12 stations located in Brno, Czechia, during the 2011–2020 period to analyse heat waves (HW) and their impact on the canopy urban heat island (UHI). HWs were recognized as at least three consecutive days with Tx ≥ 30 °C and urban–rural and intra-urban differences in their measures were analysed. To express the HWs contribution to UHI, we calculated the UHI intensities (UHII) separately during and outside of HWs to determine the heat magnitude (HM). Our results show that all HW measures are significantly higher in urban areas. UHII is mostly positive, on average 0.65 °C; however, day-time UHII is clearly greater (1.93 °C). Furthermore, day-time UHII is amplified during HWs, since HM is on average almost 0.5 °C and in LCZ 2 it is even 0.9 °C. Land use parameters correlate well with UHII and HM at night, but not during the day, indicating that other factors can affect the air temperature extremity. Considering a long-term context, the air temperature extremity has been significantly increasing recently in the region, together with a higher frequency of circulation types that favour the occurrence of HWs, and the last decade mainly contributed to this increase.