# Satellite Remote Sensing of Surface Urban Heat Islands: Progress, Challenges, and Perspectives
## 1 Introduction  
The urban heat island (UHI)--a phenomenon in which temperature tends to be higher in urban zones than surrounding non-urban areas--represents a major anthropogenic alteration to the Earth's environments [1]. The UHI phenomenon has been observed worldwide [2; 3; 4; 5; 6]. It has been widely attributed to the change in the energy exchange between urban land surfaces and the atmosphere, which is primarily caused by increased impervious surface cover replacing evaporative vegetation surfaces, and by anthropogenic heat releases [1; 7]. The UHI has contributed to a suite of environmental changes such as regional climate [8; 9; 10], vegetation growth [11; 12], and water and air quality [13]. These factors, in turn, affect human health and well-being dramatically and may potentially lead to increases in morbidity and mortality [14], energy consumption [15], and even violent incidents [16] in urban areas, which are home to 55% of the world's population today [17]. The UHI effect and the associated consequences are expected to be more severe under a warming climate [10] and a rapidly urbanizing world [18]. This is particularly true in China, India, and Nigeria, which are projected to occupy 35% of the global urban population growth between 2018 and 2050 [17]. UHI, therefore, has gained considerable research interest and has been the subject of active investigation, especially in the last decade. Intensive studies have been conducted to understand UHI, which, in general, can be classified into two broad categories based on the ways and heights that they are formed [1]: \"air\" (or \"atmospheric\") and \"surface\" UHIs. Air UHI refers to UHI effects in the canopy (CLHI) or boundary (BLHI) layer. The CLHI is usually measured by in situ sensors mounted on fixed meteorological stations or traverses of vehicles [19; 20; 21; 22], while more special platforms such as tall towers, radiosondes, and aircraft are needed to measure the BLHI [23]. The development and installation of these measurement devices are usually very time-consuming and expensive, and, thus, are only available in a few large cities worldwide to date [24]. Due to limited monitoring stations, the measured air UHIs usually fail to provide sufficient spatial details for urban land use planning and climate change research [25; 26; 27; 28]. In contrast, the surface UHI (SUHI), which represents the radiative temperature difference between urban and non-urban surfaces, is primarily measured by satellite thermal remote sensing data [29]. It can provide consistent and repeatable observations of the Earth's surface, which offers the ability to study the urban thermal environment at various spatial (from local to global scales) and temporal (diurnal, seasonal, and inter-annual) scales [28; 30]. It can also be used to predict air UHI in a spatially-explicit manner [31; 32] (see Section 5.4). [PERSON] [33] reported in 1972 the first satellite-based observations of the SUHI phenomenon in the Eastern United States. Since then, advancements in remote sensing and spatial science have considerably boosted the number and quality of SUHI studies, which currently form the major body of the UHI literature. A thorough knowledge of past literature on SUHI can not only provide a basis for future research but also is crucial for formulating effective UHI mitigation and adaptation strategies. Previous reviews of SUHI research have concentrated mostly on summarizing the methods, applications, and limitations of thermal remote sensing techniques based on representative literature [28; 29; 34; 35; 36] and/or a more general topic that only partially covered SUHI [7; 8; 24; 37]. For example, [PERSON] et al. [34] reviewed several satellite-based SUHI studies between 1972 and 1993 and emphasized the application of Normalized Difference Vegetation Index (NDVI) data in SUHI research. [PERSON] [8] compared different types of UHI briefly while synthesizing progress in urban climatology during the 1980s and 1990s.