The impact of inclement weather to urban traffic has been widely observed and studied for many years, with focus primarily on individual road segments by analyzing data from roadside deployed monitors. However, two fundamental questions are still open: (i) how to identify regional weather-traffic sensitivity index throughout a city, that indicates the degree to which the region traffic in a city is impacted by weather changes, (ii) among complex regional features, such as road structure and population density, how to dissect the most influential regional features that drive the urban region traffic to be more vulnerable to weather changes. Answering these questions is unprecedentedly important for urban planners to understand the functional characteristics of various urban regions throughout a city, and to improve traffic prediction and learn the key factors in urban planning. However, these two questions are nontrivial to answer, because urban traffic changes dynamically over time and is essentially affected by many other factors, which may dominate the overall impact. In this work, we make the first study on these questions, by developing a weather-traffic index (WTI) system. The system includes two main components: WTI establishment and key factor analysis. Using the proposed system, we conducted comprehensive empirical study in Shanghai, and the WTI extracted have been validated to be surprisingly consistent with real world observations. Further regional key factor analysis yields interesting results. For example, house age has significant impact on WTI, which sheds light on future urban planning and reconstruction. © 2015 IEEE.