Georgia Tech has initiated a research program into the detection of covert personnel in traditionally difficult environments (e.g., urban, caves). This program focuses on a detailed phenomenological analysis of human physiology and signatures with the subsequent identification and characterization of potential observables--particularly in the context of urban environments. For this current effort, several electro-optical sensing modalities have been evaluated for use as a component in an unattended sensor suite designed to detect personnel. These modalities include active sensors (e.g., vibrometry) and passive sensors (e.g., multi-spectral, thermal). Within the urban environment, illumination conditions can vary widely and change dynamically during the course of a day. This paper will discuss those issues and present a computational approach to computing the radiative exchange environment and the corresponding thermal signatures of personnel in these environments. Consideration will also be given to the impact of these variations on thermal signatures of clutter objects.