The sky view factor (SVF) is a commonly used indicator in research fields of urban microclimate such as the urban heat island (UHI). Analytical methods have been widely adapted in SVF estimation with the development of accurate three dimensional (3D) data, in which, however, the sky radiance is usually assumed to be isotropic. In this study, we discuss the sky temperature distribution and its influence on the SVF calculation based on its definition and differential geometry, and then according to the nocturnal net outgoing radiation ratio of cloudy skies to that of clear skies, the effective SVF (ESVF) is presented and defined to explain UHI intensity variance. The results of numerical computation combined with calculations from true 3D building data for Adelaide, Australia, demonstrate that the sky temperature variance has little effect on SVF estimation. While for totally overcast or cloudy sky conditions, Ts is less different from the sky temperature than for clear skies, the proposed ESVF is sensitive to the sky temperature distribution and cloud cover fraction, providing the potential ability to analyse UHI intensity for various cloudy conditions. Copyright © 2013 Royal Meteorological Society
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