A comparison of daytime and night-time thermal satellite images of Hong Kong for urban climate studies Janet Nichol, Law Kin Hang,Au Yeung Wai-Shun Department of Land Surveying and GeoInformatics The Hong Kong Polytechnic University Hunghom, Kowloon Hong Kong email:lsjanet@polyu.edu.hk tel: 852 2766 5952 fax: 852 2330 2994 Introduction The urban heat island refers to the elevation of urban temperatures over those in surrounding rural areas, the difference generally being greater at night than during the day For people living and working in cities, both day- and night-time air temperatures are important, particularly in tropical and sub-tropical cities where the climate may often exceed the threshold of human comfort for large parts of the year. Heat island studies are additionally significant due to the perceived relationship between air pollution and heat islands. A number of satellite-based studies using thermal infra-red imagery have been carried out since satellite data provide a dense grid of almost instantaneous temperature measurements over a city and can permit visualisation of spatial relationships between temperature patterns and urban land use and infrastructural features. Satellite-based studies can therefore provide recommendations for building design and landscaping of urban developments, to minimise heat accumulation and retention by urban surfaces, and thus the accumulation of warm, polluted and stale air in the city. This study is a multi-spectral and multi-sensor evaluation of thermal infra-red imagery for microclimate monitoring in two densely-built high rise towns in the New Territories of Hong Kong. Objectives and study area The main objectives of the research are to supplement the existing body of research on satellite-derived urban heat islands by examining the enhanced spatial and spectral resolution of LANDSAT 7's Enhanced Thematic Mapper+ (ETM+) sensor as well as the multispectral thermal wavebands from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). The fact that the ASTER image studied is a night-time image can also provide a meaningful comparison of satellite-derived surface temperatures of urban surfaces between day and night. The towns, Tuen Mun and Yuen Long are located in the western New Territories, Hong Kong and were selected because the night-time ASTER image is cloud-free only over this part of Hong Kong. Tuen Mun, to be discussed in this paper is coastal, located in a narrow, steep-sided valley with grass and tree-covered slopes (Figure 1). Remnants of the original village settlements are located between the modern, high-rise town in the valley, and the mountain slopes. Methods The ASTER and LANDSAT images were obtained in the relatively warm part of the year (Table 1) September and October, 2001. Mean daily air temperatures on 17th September and 6th October were similar (29° and 27° respectively) suggesting similar climatic conditions on the two image dates thus permitting the day (9.15am) and night (9.40pm) images to represent a continuum of relative air and surface temperatures. 
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