## 1 Introduction  
Urban heating is usually quantified by means of the urban heat island (UHI), a well-known anthropogenic thermal modification whose effects have implications for human comfort and health, ecosystem function, local weather, air pollution, urban planning, and energy management [1; 2; 3; 4]. The UHI phenomenon is linked to urban land transformations, especially when vegetated and rural zones are substituted by impervious surfaces, causing an increase in sensible heat flux and a decrease in latent heat in urban areas [5]. Urban heating is mainly ascribed to the trapping and absorption of solar irradiation in built-up areas connected to the thermal capacity of construction materials and the urban canyon phenomenon [6]. As a result, a difference in air and surface temperature between urban area and rural surroundings arises [7], quantified as UHI intensity or magnitude. Many studies have estimated the UHI intensity by using ground-based weather stations observing air temperature in urban and rural locations [8], however, the sparsely distributed stations do not allow a global view over the urban area. Air temperature has a diurnal cycle with a nighttime UHIformation, while surface temperatures typically exhibit a surface urban heat island (SUHI) throughout the diurnal cycle, with a maximum in the daytime [9; 10; 11]. Advances in space-based remote sensing and satellite data availability have contributed to the growing number of studies on SUHI in several cities around the world, exploiting the global spatial coverage and the revisit time of satellite remote sensing missions [12]. SUHI map images were extensively retrieved by using Landsat data, providing spatiotemporal analysis and allowing the modeling of the urban thermal patterns with the land cover classes [13; 14; 15]. The MODerate Resolution Imaging Spectroradiometer (MODIS), aboard Terra and Aqua satellites, allows monitoring SUHI daily variations but with a lower spatial resolution (1 km) [16; 17; 18]. SUHI mapping can also satisfy the growing demand for integrating urban planning and landscape ecosystems, supplying scientific support for urban development policies [19]. In this context, remotely sensed imagery is often used to map impervious surfaces together with SUHI, the former widely recognized as a significant urbanization indicator affecting the thermal pattern. Instead, it is well known that the increase of vegetation and green spaces in the urban texture is one of the main strategies to mitigate SUHI intensity [20; 21]. In this work, we provide a SUHI insight on different Thailand urban areas. Even though a great number of papers have explored the SUHI maps retrieved by remote sensing data in several cities worldwide, a complete study in Thailand is missing. For instance, different works were carried out with ground-based weather stations measuring air temperature, including: investigation of the UHI intensity in three big cities in Thailand (Bangkok, Chiang Mai and Songkhla) by using meteorological weather stations for the period 2004-2008 [22]; analysis of the air temperature data from four weather stations for a UHI insight in Bangkok area [23; 24]. Spaceborne imagery in Thailand was used with limited spatial and temporal coverage: study of the land surface temperature (LST) pattern in Chiang Mai city (northern Thailand) with only two Landsat Enhanced Thematic Mapper Plus (ETM+) images (one in 2000 and one in 2006) [25]; Kantarawichai district analysis in the northeastern Thailand with only two Landsat images (one in 2006 and one in 2009) [26]; SUHI monitoring on Bangkok with few MODIS images in the period 2001-2003 [27]. Recently, a SUHI insight over the land use zoning plan of Bangkok using seven Landsat 8 scenes was proposed [28]. To bridge this gap, this work provides, for the first time, an extensive temporal and spatial investigation in Thailand by examining the SUHI of six major cities located in different geographic regions during a long-time interval (2003-2016) using 728 MODIS images, allowing daytime and nighttime monitoring.