The urban heat island of Basel – seen from different perspectives

AbstractFor decades thermal infrared satellite imagery has been used for climate studies of a variety of geosystems, includingurban areas. Additionally, airborne thermal remotely sensed data can provide high resolution information about urbanland surface temperatures (LST). Numerous studies make use of these data for the investigation of urban-rural LST differences,commonly known as the urban heat island (UHI) phenomenon. Most of these studies try to analyse the urbanheat island by means of the LST distribution. It seems that the UHI is easy to measure, easy to explain, easy to find, andeasy to illustrate. Due to this apparent simplicity some people seem to jump into UHI studies without fully understandingthe nature of the phenomenon as far as time and spatial scales, physical processes and the numerous methodologicalpitfalls inherent to UHI studies are concerned. In this study the use of thermal infrared satellite data with respectto the assessment of the surface UHI is investigated. The need to clearly distinguish between different types of UHI isemphasised by recalling the (surface) temperature and the UHI terminology. The pretended simplicity of UHI effectsis in reality a result of complex interactions between local radiation conditions, earth surface heat budget, the urbanstructure and the boundary layer atmosphere. Different methods may provide completely different results. This paperaims to bring more clearness into the subject by assessing the urban heat island of the city of Basel, Switzerland, by theuse of thermal data provided by satellites (Landsat TM/ETM+), helicopter-borne infrared camera (InfraTec VarioCAM®) and ground-based measurements of air temperature profiles. It is shown that UHIs vary essentially with the choice ofthe respective temperature (LST, air temperature) and height (surface level, street/canopy level, roof level).