Remote sensing of urban heat islands by day and night

A night-time thermal image from the ASTER satellite sensor, of the western New territories of Hong Kong is compared with a daytime Landsat Enhanced Thematic Mapper Plus (ETM+) thermal image obtained nineteen days earlier. Densely built high rise areas which appear cool on daytime images are conversely, relatively warm on nighttime images, though the temperature differences are not well developed at night. Lower temperature gradients between different land cover types observed on the night time image result in meso-scale, rather than micro-scale climatic patterns being dominant, suggestive of processes operating in the Urban Boundary Layer (UBL), as opposed to the Urban Canopy Layer (UCL) which is dominant in the daytime. Thus, at night, proximity to extensive cool surfaces such as forested mountain slopes appears to be influential in maintaining cooler building temperatures. The relevance of satellite-derived surface temperatures for studies of urban microclimate is supported by field data of surface and air temperatures collected in the study area. Comparison of the ASTER Kinetic Temperature standard product with a thermal image processed using locally derived emissivity and atmospheric data indicated higher accuracy for the latter.