Abstract
This paper attempts to illustrate the complexity of thermal infrared (TIR) data analysis for urban heat island studies. While a certain shift regarding the use of correct scientific nomenclature (using the term “surface urban heat island”) could be observed, the literature is full of incorrect conclusions and results using erroneous terminology. This seems to be the result of the ease of such literature implicitly suggesting that “warm surfaces” result in “high air temperatures”, ultimately drawing conclusions for urban planning authorities. It seems that the UHI is easy to measure, easy to explain, easy to find, and easy to illustrate—simply take a TIR-image. Due to this apparent simplicity, many authors seem to jump into UHI studies without fully understanding the nature of the phenomenon as far as time and spatial scales, physical processes, and the numerous methodological pitfalls inherent to UHI studies are concerned. This paper attempts to point out some of the many pitfalls in UHI studies, beginning with a proper correction of longwave emission data, the consideration of the source area of a thermal signal in an urban system—which is predominantly at the roof level—demonstrating the physics and interactions of radiation and heat fluxes, especially in relation to the importance of urban storage heat flux, and ending with an examination of examples from the Basel study area in Switzerland. Attention is then turned to the analysis of spatially distributed net radiation in the day- and at nighttime as a minimum requirement for urban heat island studies. The integration of nocturnal TIR images is notably recommended, as satellite data and the UHI-phenomenon cover the same time period.
Highlights
Accepted: 3 September 2021One major interest in urban climatology is the study of the so-called “urban heat island”effect (UHI)
Scientific contributions regarding this effect, which is typical for urban climates in most cities and urban agglomerations worldwide, have developed and, over the last two centuries, new and innovative methods for the detection and spatio-temporal distribution of the urban heat island”effect (UHI) effect have been integrated into the field
A clear distinction has been made between the following: the urban heat island effect (UHI), which is related to air temperature differences between urban and rural sites and even sub-dividing between a boundary layers urban heat island (UHIUBL ) and a canopy layer urban heat island (UHIUCL ); the surface urban heat island effect (SUHI or UHISurf ), which considers the land surface temperature distribution (LST) between urban and rural areas, and is mostly derived from remote sensing data; and the subsurface urban heat island effect (SSUHI or UHISub ), which is based on soil temperature measurements under urban versus rural surfaces [3]
Summary
One major interest in urban climatology is the study of the so-called “urban heat island”. A clear distinction has been made between the following: the urban heat island effect (UHI), which is related to air temperature differences between urban and rural sites and even sub-dividing between a boundary layers urban heat island (UHIUBL ) and a canopy layer urban heat island (UHIUCL ); the surface urban heat island effect (SUHI or UHISurf ), which considers the land surface temperature distribution (LST) between urban and rural areas, and is mostly derived from remote sensing data; and the subsurface urban heat island effect (SSUHI or UHISub ), which is based on soil temperature measurements under urban versus rural surfaces [3] These differences are important to clarify before establishing the scientific target of this investigation. In recent years, a trend towards correct terminology has been observed in the international literature
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