Radio frequency identification (RFID) technology is now widely used for tracking a great variety of objects in spatio-temporal dimensions. In the field of urban management and analysis, personal RFID is a powerful tool for tracing both individual pedestrians and crowd dynamics. In this research, we have focused our attention on how public services can use RFID technology, especially from the viewpoint of attractive and real-time visualization. In a trial demonstration, we installed nine RFID-reader gates in an urban district and provided hundreds of passive RFID-tags to pedestrians. Using this basic RFID-infrastructure, we developed a public display service which visualized and reported urban activities in the style of a real-time weather map. A large display shows the density of people at each location by using geometric contour lines and virtual clouds. Accumulated data is automatically updated and the 'weather' map quickly changes as time passes. The weather-like visualization helps the user understand the dynamics, flows, and trends of crowds, and promotes intuitive forecasting of urban activity.