A comparison of daytime and night-time thermal satellite images of Hong Kong for urban climate studies Results Both air and sea surface temperatures were 2°C cooler on 6th October 2001 (ASTER image) than on 17th September (ETM+ image) (Table 1). However, the ASTER night-time Ts values were found to be much (approximately 12°C) cooler than for daytime ETM+ (Figures 3 and 4). The range of Ts values were also smaller: 6° for ASTER, as opposed to 11°C for ETM+. Most spatial variations in Ts on both images can be explained by differences in urban morphology, and are illustrated in sections 5.1 and 5.2 and Figs 3-9. 
LANDSAT daytime image (Figure 3) * High rise areas are relatively cool ("37°C) if buildings are closely spaced (Figure 5) * Densely-built low rise areas are relatively warm ("39-40°) (Figure 6) * Car parks and open spaces are the hottest areas (Tuen Mun bus station forecourt "45°C) * Aspect of buildings in relation to sun elevation and azimuth has significant influence on Ts (Figure 7) 
Vegetated mountain slopes appear to have a cooling effect on urban temperatures but only in urban areas immediately adjacent to them. A corridor of cool surfaces extending northwards from the coast along the river is interrupted by a shopping complex built over the river, thus blocking any potential funnelling effect of winds and sea breezes during the daytime, for dispersal of warm, stale and polluted air. The UHI effect is readily visible on images of both day and night, but the satellite-derived UHI based on Ts, is more pronounced during the day, as observed here. Densely-built high rise areas having a high building mass are relatively cool on daytime images due to the lag in heating of building materials, but the reverse is true at night. The daytime UHI is most intense in open, non-built areas, as well as in low rise developments which heat rapidly due to a larger proportion of their surface (roof) exposed to direct solar radiation. The daytime ETM+ image, after processing, is shown to be of adequate spatial resolution to show the effects of aspect and shadow due to building spacing and orientation, and both ASTER and ETM+ indicate the meso-scale climatic effects of vegetated open spaces and waterbodies in ameliorating heating especially during the daytime. The ASTER image additionally suggests that proximity to large waterbodies such as the sea may actually increase the temperatures of urban areas at night, even in open areas such as car parks where night-time heat loss would be expected to be rapid. References
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