## 2 Study Area  
Six major cities in Thailand were selected for this study (Figure 1), including: (1) Bangkok (BG), the capital of Thailand, located in the central region; (2) Chiang Mai (CM), the largest city in the northern region, and four large cities located in northeastern region: (3) Nakhon Ratchasima (NR); (4) Ubon Ratchathani (UR); (5) Khon Kaen (KK); and (6) Udon Thani (UT).  
Bangkok has a population of about 6.3 million within municipality and the largest urban extension in Thailand. It is located in the Chao Phraya River delta in Thailand central plains. The area is flat and low-laying, with a mean elevation of 1.5 m above sea level. It has a tropical wet and dry climate with three seasons--namely, summer, rainy season, and winter. The air temperature ranges from an average value of 22.0 \({}^{\circ}\)C in December to 35.4 \({}^{\circ}\)C in April. The rainy season begins around mid-May until October, with the dry and cool northeast monsoon occurring until February, when the summer season starts until mid-May.  
Chiang Mai (136,700 inhabitants within municipality) is the capital of Chiang Mai Province, situated at about 300 m above sea level, 696 km north of Bangkok. About 70% of the Chiang Mai province consists of mountains covered with forests. The coolest months are December and January. The temperature throughout the year varies between 14 and 30 \({}^{\circ}\)C, with an average value of 26 \({}^{\circ}\)C.  
Finally, four of the largest cities located in the northeastern Thailand were selected: Nakhon Ratchasima (135,300 inhabitants within municipality), Ubon Ratchathani (82,800), Khon Kaen (110,000), and Udon Thani (135,500). They are located mostly in flat plains between latitudes 14\({}^{\circ}\)0\({}^{\prime}\)E and 18\({}^{\circ}\)30\({}^{\prime}\)N and longitudes 101\({}^{\circ}\)0\({}^{\prime}\)E and 105\({}^{\circ}\)0\({}^{\prime}\)E. Average temperatures range from 19.6 \({}^{\circ}\)C to 30.2 \({}^{\circ}\)C. Northeastern Thailand is the focal point of the Greater Mekong Subregion, which operates as an economic center with Laos, China (Yunnan), and Vietnam. Development projects at large scales have sprung up, including construction of highways to provide access to the northeastern Thailand area for trade increasing. All of these aspects have expanded the northeastern economy and fostered rapid growth of the urban territories.  
Figure 2 shows the urban surface evolution for the six cities in the years 2004, 2008, 2012, and 2016. These maps have been obtained from Landsat images as described in Section 3.1. The overview of Figure 2 shows the urban expansion across the years, which makes the study of the surface thermal impact interesting in terms of spatial and temporal evolution. Concerning the land use of urban areas for the six cities, the city core always includes high- and medium-density residential areas,  
Figure 1: Location of the analyzed six cities on a Thailand administrative boundary map.  
commercial zones, and government institutes, while outskirts are mainly made up of medium- and low-density residential areas and industrial zones.  
Figure 2: Maps of urban surface (red) evolution for the six cities in the years 2004, 2008, 2012, and 2016 obtained from Landsat data (see Section 3.1). From top: Bangkok (BG), Chiang Mai (CM), Nakhon Ratchasima (NR), Ubon Ratchathani (UR), Khon Kaen (KK), and Udon Thani (UT). All panels represent a 30 \(\times\) 30 km area except for Bangkok, 90 \(\times\) 90 km.