Rapid urbanization and new global construction estimated to be 250x NYC by 2050 is increasing traffic congestion, pollution, and related health threats. Thus, it is imperative that we develop new modeling capabilities that allow urban designers to quantify the performance of mobility solutions, sustainability, public health impacts, pedestrian thermal comfort and pollution exposure during the earliest stages of a design process. Embedded in a generative, performance-driven design process, such a tool can significantly facilitate the design of healthy and sustainable urban habitats that promote active mobility. Outdoor thermal comfort simulation simulations rely on the mean radiant temperature (MRT) seen by pedestrians as an important input that remains difficult to compute. Especially for large urban models, computing relevant surface temperatures and radiation fluxes that make up the MRT is a daunting task in terms of simulation setup and the computational overhead. We propose a new algorithm to estimate exterior surface temperatures of building facades, roofs, and ground surfaces in an arbitrary urban 3D model.