Urban heat islands (UHIs) pose a significant challenge in cities worldwide, exacerbating energy use, air pollution, and health risks. This paper reviews the role of water bodies in mitigating UHI effects, which is vital for informed urban planning and climate adaptation. We analyze how water features, particularly when combined with green spaces and strategic urban design, can significantly cool urban environments. The effectiveness of water bodies in reducing temperatures is influenced by their size, shape, surrounding land use, climatic conditions, and vegetation. Empirical research and case studies indicate that larger and well-shaped water bodies, due to their extensive surface area and continuous evaporation, are more effective. Furthermore, the integration of water bodies with green spaces enhances cooling through increased evapotranspiration and shading. This review highlights the strategic placement and design of water bodies within urban landscapes as crucial for maximizing their cooling benefits. By integrating water features with other urban cooling strategies, such as tree planting and expanded greenery, cities can effectively counter UHI effects, leading to more sustainable and resilient urban environments.

Canada is warming at double the rate of the global average caused in part to a fast-growing population and large land transformations, where urban surfaces contribute significantly to the urban heat island (UHI) phenomenon. The federal government released the strengthened climate plan in 2020, which emphasizes using nature-based solutions (NBSs) to combat the effects of UHI phenomenon. Here, the effects of two NBSs techniques are reviewed and analysed: increasing surface greenery/vegetation (ISG) and increasing surface reflectivity (ISR). Policymakers have the challenge of selecting appropriate NBSs to meet a wide range of objectives within the urban environment and Canadian-specific knowledge of how NBSs can perform at various scales is lacking. As such, this state-of-the-art review intends to provide a snapshot of the current understanding of the benefits and risks associated with the implantation of NBSs in urban spaces as well as a review of the current techniques used to model, and evaluate the potential effectiveness of UHI under evolving climate conditions. Thus, if NBSs are to be adopted to mitigate UHI effects and extreme summertime temperatures in Canadian municipalities, an integrated, comprehensive analysis of their contributions is needed. As such, developing methods to quantify and evaluate NBSs’ performance and tools for the effective implementation of NBSs are required.

The Urban Heat Island (UHI), a consequence of urban development, leads to elevated temperatures within cities compared to their rural counterparts. This phenomenon results from factors such as urban designs, anthropogenic heat emissions, and materials that absorb and retain solar radiation in the built environment. Materials commonly used in cities, like concrete, asphalt, and stone, capture solar energy and subsequently emit it as heat into the surroundings. Consequently, this phenomenon amplifies summertime cooling energy demands in buildings. To mitigate the UHI impacts, various mitigation strategies have emerged that include but are not limited to using higher solar reflectivity materials, known as “cool materials”, and increasing vegetation and greenery in urban areas. Cool materials have high reflectivity and emissivity, effectively reflecting solar radiation while emitting absorbed heat through radiative cooling. Increasing the solar reflectivity of building envelope materials is a promising sustainable solution to lessen the UHI effects. This state-of-the-art review summarizes the UHI causes and effects, states the mitigation strategies, describes the cool building envelope materials, explains the solar reflectivity index measurements, indicates the building and micro-climate simulations, highlights the performance evaluation of using cool building envelope materials, points out the research gaps, and proposes future research opportunities.

Increasing surface reflectivity in urban areas can decrease ambient temperature, resulting in reducing photochemical reaction rates, reducing cooling energy demands and thus improving air quality and human health. The weather research and forecasting model with chemistry (WRF-Chem) is coupled with the multi-layer of the urban canopy model (ML-UCM) to investigate the effects of surface modification on urban climate in a two-way nested approach over North America focusing on Sacramento, Houston, and Chicago during the 2011 heat wave period. This approach decreases the uncertainties associated with scale separation and grid resolution and equip us with an integrated simulation setup to capture the full impacts of meteorological and photochemical reactions. WRF-ChemV3.6.1 simulated the diurnal variation of air temperature reasonably well, overpredicted wind speed and dew point temperature, underpredicted relative humidity, overpredicted ozone and nitrogen dioxide concentrations, and underpredicted fine particular matters (PM_2.5). The performance of PM_2.5 is a combination of overprediction of particulate sulfate and underprediction of particulate nitrate and organic carbon. Increasing the surface albedo of roofs, walls, and pavements from 0.2 to 0.65, 0.60, and 0.45, respectively, resulted in a decrease in air temperature by 2.3 °C in urban areas and 0.7 °C in suburban areas; a slight increase in wind speed; an increase in relative humidity (3%) and dew point temperature (0.3 °C); a decrease of PM_2.5 and O₃ concentrations by 2.7 µg/m³ and 6.3 ppb in urban areas and 1.4 µg/m³ and 2.5 ppb in suburban areas, respectively; minimal changes in PM_2.5 subspecies; and a decrease of nitrogen dioxide (1 ppb) in urban areas.

The Solar Reflectance Index (SRI) is a standardized metric used to assess the reflective properties of materials in relation to solar radiation and their capacity to emit absorbed heat, particularly within the infrared spectrum. Materials with high SRI values, often referred to as “cool materials”, contribute to ambient temperature regulation, Urban Heat Island (UHI) mitigation, and cooling energy demand reduction. The effectiveness of SRI depends on factors such as solar incidence angles, intrinsic material properties, and varying environmental conditions. Accurate assessments require the implementation of standardized testing and rating methodologies. This paper reviews and compares North American (ASTM E1980, ASTM C1549, ASTM C1371) and European (EN 15976) standards to determine SRI, focusing on the impacts of weathering and climatic factors on material aging. The study highlights the inadequacy of current practices, which typically measure SRI after only three years of exposure, and advocates for long-term performance monitoring across diverse climates. Key findings reveal that high-SRI materials can reduce surface temperatures by up to 20 °C, significantly lowering cooling energy demands. The study recommends the development of comparable standards to measure solar reflectivity on vertical surfaces, emphasizing the importance of assessing long-term performance across various climatic conditions. Findings underscore the importance of advanced modeling, innovative materials development, and effective maintenance strategies to extend the durability and efficacy of cool materials. The novelty of this work lies in its comprehensive framework for SRI assessment, integrating advanced modeling, innovative materials development, and real-world performance monitoring. This study provides actionable insights for policymakers, urban planners, and architects to enhance building energy efficiency and urban resilience.