Urban heat island (UHI) is one of the environmental hazards that has emerged due to the urbanization and is often defined by the difference in the air temperature of a station in the city center with that of the station outside the city (Stone 2007), which is called air urban heat island(Arnfield, 2003, Peng et al. 2011).
In practice, the urban and suburban temperatures can be compared by different methods. Some of these methods are: the difference in temperature of one or more pairs of urban and suburban stations (e.g. Nasrallah et al. 1990), difference in city temperature between weekdays and weekend (Figuerola & Mazzeo, 1998), comparing the time series of temperature in one or more urban stations with one or more rural ones (e.g. Magee et al. 1999), time series trend analysis of temperature in an urban weather station (e.g. Tereshchenko & Filonov, 2001), a network of fixed weather stations inside and outside the urban areas (e.g. Morris et al. 2001, Golroudbary et al. 2018), and measurement of temperature cross-section from one city corner to another (Unger et al., 2001). Such heat island identification methods represent the traditional approach to the study of the urban heat island.
In the traditional approach, the definition of urban heat island is based on the comparison between the temperatures of urban and suburban areas (Cai & Du, 2009, Shigeta et al. 2009), but it is not easy to distinguish between the urban and suburban areas (Schwarz et al. 2011). Besides, it is controversial in the traditional approach that the temperature difference between a pair of weather stations one in the city and the other outside the city is the basis for the definition of the urban heat island. This is because the temperature difference between two stations (urban and suburban) depends on their location, distance, and geographical conditions. By choosing pairs of stations different from the urban heat island index in a single city, different values will be obtained. Because the urban land cover is very heterogeneous, one or even several stations cannot be the representative of city temperature. On the other hand, the land cover type outside the city may greatly vary, and depending on the land cover type the station outside the city is located on, different temperatures will be obtained. Hence, not only we cannot achieve a reliable and consistent UHI index for a city, but also the heat island obtained in this way will not be comparable for different cities (Jin, 2012).
There is no consensus among the researchers that the difference in temperature between which urban district can be the basis for the definition of heat island. Some researchers considered the difference between the temperature of the fully developed and the less developed parts of the city, while others considered the difference between the two different built-up areas. Furthermore, in the traditional heat island survey method that is based on the station data, the spatial structure of the heat island cannot be identified.
Due to the city size, scarcity of stations in the city, and impact of surrounding conditions on the station data, the comprehensive information on the urban heat island cannot be achieved only by relying on such method. For this reason, the remote sensing data has been increasingly used for the analysis of heat island (Majkowska et al. 2016).
Using the satellite data, the temporal and spatial structure of the land surface temperature (LST) can be revealed by such a resolution that can well distinguish between the urban and suburban areas (Peng et al. 2011).
Satellite data covers large areas with high spatial resolution, and thus, is a better tool to define the urban heat island intensity (Jin, 2012). The heat island calculated from the LST data derived from remote sensing is called surface urban heat island (Arnfield 2003, Voogt & Oke, 2003). Accordingly, the urban heat island intensity is the difference between the mean spatial temperature of urban and suburban/rural areas (Rizwan et al. 2008). Since the air temperature behavior is different from the LST, the time of measuring the temperature and the approach chosen to calculate the heat island index are both effective in the characteristics of the resulting heat island. In this case, the surface urban heat island reflects the difference in the LST patterns in different urban areas (Schwarz et al. 2011).
The advances made over the past two decades show that the urban heat island is much more diverse than what was previously thought. Depending on where the (air or surface) temperature is measured by what tool (station or satellite sensor), the resulting heat island will show different characteristics (Arnfield 2003). In the clear sky conditions, the higher the surface temperature, the higher the air temperature and the thicker the urban boundary layer. Since the land surface temperature fluctuations in the daytime is higher than the air temperature and there is a lag time between them, the surface heat island is stronger than the air heat island and its fluctuations are greater (Peng et al. 2011, Li et al. 2017). Although the LST measured by satellites is correlated to the air temperature measured at the weather stations, the seasonal and diurnal variations are not necessarily identical, as the LST is linked to the energy exchange processes in the land surface and has little dependence on its upper air column. As a result, the heat islands relying on the land surface temperature are stronger during the night and day, but the heat islands relying on the air temperature are only strong at night (Jin, 2012). This is because the shading of buildings is an important reason for the weak UHI in the daytime (Li et al. 2019). However, the surface urban heat island, like the air urban heat island, relies on the temperature difference between the urban and suburban areas.
In recent years, the modeling has been significantly developed with the techniques such as regression model (e.g. Guo et al. 2015), spatial regression (e.g. Chun et al. 2014), geographically weighted regression (e.g. Su et al. 2012), Gaussian volume model (e.g. Quan et al. 2014), mesoscale model, WRF model (e.g. Chen et al. 2014), and artificial neural networks model (e.g. Bozorgi et al. 2018) for the study of UHI spatial-temporal variations.
Many indices have been proposed for quantifying the urban heat island using the remote sensing data (Schwarz et al. 2011, Li et al. 2018, 2019). Although these indices allow to calculate the heat island intensity in a uniform manner around the world, the credibility of these methods should be established for the cities in the arid regions (Jin, 2012, Li et al. 2018). This is mainly resulted from the fact that in the arid regions, there is a remarkable difference in water budget between the urban areas and the surroundings deserts. Therefore, in order to investigate the heat island in Isfahan, which is located in the arid biome, it is necessary to define a proper index based on the background climate characteristics of the metropolitan area of Isfahan. This is the main purpose of this research.