Shadows, the common phenomena in most outdoor scenes, bring many problems in image processing and computer vision. In this paper, we present a novel method focusing on extracting shadows from a single outdoor image. The proposed tricolor attenuation model (TAM) that describe the attenuation relationship between shadow and its nonshadow background is derived based on image formation theory. The parameters of the TAM are fixed by using the spectral power distribution (SPD) of daylight and skylight, which are estimated according to Planck's blackbody irradiance law. Based on the TAM, a multistep shadow detection algorithm is proposed to extract shadows. Compared with previous methods, the algorithm can be applied to process single images gotten in real complex scenes without prior knowledge. The experimental results validate the performance of the model.