Dear Colleagues,

One of the classic topics of thermal infrared remote sensing for urban climate studies is the analysis of the urban heat island (UHI) effect, known since the publication of Luke Howard’s “The Climate of London” published in 1820. He already discovered the UHI effect by comparing air temperature measurements between the center of London and the rural environment. He came to the result that the city is warmer during nighttime and cooler during daytime. This publication has been referenced often but only very few scientists had read the original publication prior to its re-publication in 2006, through the International Association of Urban Climate (IAUC). The problem with numerous UHI publications in recent years is that mostly thermal infrared satellite data from daytime orbits are used. But high surface or brightness temperature during daytime is, on the one hand, typical for impervious surfaces that are typical for urban and industrial areas, but, on the other hand, it contradicts Luke Howard’s research result that UHI is a nighttime feature of the urban climate. Simply expressed, most satellite data is taken at the wrong time of day.

Therefore, it is necessary to go deeper into the radiation and heat flux physics to come to consistent conclusions. To overcome the problems with technical terms and to make clear what is meant, some additional technical terms have been developed, like UHI_surf, UHI_UBL, UHI_UCL, and UHI_sub indicating the surface UHI, the urban boundary layer UHI, the canopy layer UHI, and the subsurface UHI (see Oke et al. 2017: Urban Climates, Cambridge University Press, p. 198). These make it clear whether air or surface temperatures are considered. During recent years one could see a significant move towards the right terminology in many existing publications.

Seen from a radiation and heat flux point-of-view the thermal infrared signal of satellite data indicates the strength of the longwave radiative energy loss of a surface. It can easily be expressed by the complete radiation and heat flux equation:

K↓ - K↑ + L↓ - ε σ To⁴ + (1- ε) L↓ - Q_H – Q_E – Q_G + Q_F = 0 [Wm^-2}

where K↓: solar irradiance; K↑: reflection; L↓: atmospheric counter radiation; ε: emissivity; σ: Stefan-Boltzman-constant; To: surface temperature; Q_H: sensible heat flux (responsible for air temperature); Q_E: latent heat flux; Q_G: storage heat flux; Q_F: anthropogenic heat flux

It can easily be seen that the physics behind this are quite complex and ruled by too many variables to simply say: “High surface temperature To = high air temperature”.

High surface temperatures reduce net radiation if not compensated by mostly increased solar irradiance like it is the case in mountain terrain with sun-exposed slopes and those that face away from the sun. Emissivity ε and the anthropogenic heat flux Q_F and also the wind situation during satellite overpass are important players in the urban system.

The special issue of Remote Sensing is committed to the present state of research using satellite data for the analysis of the various UHI studies. There is a particular focus on night IR data from ASTER, LANDSAT, or MODIS.

I want to invite researchers to contribute to this Special Issue of Remote Sensing.

Prof. Dr. Eberhard Parlow
Guest Editor

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• Urban heat Island - local, regional, global
• Surface Urban Heat Island – local, regional, global
• Urban Energy and heat flux studies
• Statistical analysis of UHIsurf
• Physics of UHI/ UHIsurf
• Trigger of UHI by anthropogenic heat flux
• Storage heat flux analysis
• Nighttime IR data analysis and UHI, UHIsurf
• Green roofs and implications for UHI
• Urban parks and forests to reduce UHIsurf
• Urban cooling island effects
• Thermal tomography of UHIsurf
• Time series of UHIsurf using data from MODIS or geosynchronous meteorological satellites
• Local Climate Zones (LCZ) and UHI/ UHIsurf
• Satellite data as predictors to UHI/UHIsurf