# Satellite Remote Sensing of Surface Urban Heat Islands: Progress, Challenges, and Perspectives
## 6 Research Challenges
### Differences between Satellite-Derived LST and Air Temperatures  
Remotely-sensed LST is completely different from air temperature in terms of observation principles and altitudes, which makes SUHI not directly comparable to air UHI [29]. Thermal sensorsmeasure surface temperature indirectly by detecting upward long-wave radiation from the Earth surface at a narrow solid angle of view (Figure 6). Thermodynamic and radiative properties, including surface energy input from the atmosphere and sun, surface moisture, and thermal admittance, surface emissivity, near-surface atmospheric conditions, and surface turbulent transfer can seriously affect the upwelling surface radiation [197]. The radiances received by remote sensors are further influenced by the sensor-viewing angle [29]. A large fraction of the urban surface may not be viewed by sensors because of the three-dimensional nature of the urban structure obscuring some of the surface from view, and the atmosphere may absorb a significant portion of the radiation [24]. At the same time, some atmospheric radiation may be received by sensors directly without interacting with the Earth surface or the target objects (Figure 6). The accuracy of LST estimates, therefore, depends strongly on corrections for atmospheric effects and an accurate estimate of surface emissivity [28; 35; 54; 198]. These two, however, are often difficult to assess given large areas covered by remotely sensed imagery and the lack of detailed atmospheric parameters at the time of satellite overpass [38]. For example, surface emissivity was usually assumed to be the same across a heterogeneous urban area [35]. Furthermore, the interpretation of satellite-derived LST data is hampered because urban pixels are generally not homogeneous and isothermal. They usually include several active surfaces that have different thermodynamic and radiative properties and, thus, different temperatures. In addition to the internal errors of sensors and cosmic background noise, the pixel-based LST estimate is an ambiguous quantity found by inverting [PERSON]'s law.  
Therefore, a considerable gap exists between the retrieved LST and air temperature, on which turbulence and wind have a significant effect [24]. Given the closer relationship of air temperature with the urban environment and human health than LST [31; 32], various sensor-view models have been proposed to predict air temperature from LST data [199]. However, model performance needs to be examined, because of the highly complex interrelationships between surface and air temperatures.  
Figure 6: Illustration of the infrared radiance received by a satellite sensor. Path 1: radiance emitted directly by the target surface. Path 2: upward atmospheric thermal radiance. Path 3: upward solar diffusion radiance scattered by the atmosphere. Paths 4: downward atmospheric thermal radiance. Path 5: downward solar diffusion radiance. Path 6: direct solar radiance reflected by the surface. The radiance from paths 1, 3, 5, and 6 will be attenuated by the atmosphere before reaching the sensor. Among these channels, only the radiance from path 1 is directly related to land surface temperature and the emissivity of interest. The solar-related items (paths 3, 5, and 6) are usually ignored in LST retrieval due to a negligible solar radiation infrared thermal window of 8 to 14 μm and 3 to 5 μm. Further details can be found in a review article by [PERSON] et al. [38].  
It remains a grand challenge to apply the satellite-derived LSTs directly in air UHI detection, attribution, and modeling [28].