Since 2006, half of the world's population lives in cities. In the age of climate change, designing for quality environmental living conditions and sustainability is a topical concern.  However, on the one hand, designers and city planners operate with their three dimensional city morphological data such as building shapes and volumes, forms and their spacings, and functional attributes and definition signatures. On the other hand, urban climatologists operate with their numbers and equations, quantities and signals, and normals and anomalies. Traditionally the two camps do not meet. It is a challenge to develop design tools that they can work together. Map based information system based on computational geographic information system (GIS) that is properly structured and represented offers a common language, so to speak, for the two professional groups to work together. Urban climatic map is a spatial and graphical tool with information embedded in defined layers that are collated so that planners and urban climatologists can dialogue over design issues. With various planning and meteorological data coded in defined grid resolutions onto the GIS map system, data can be synergized and collated for various understandings. This papers explains the formulation of Hong Kong's GIS based Urban Climatic Map as an example of how the map works in practice. Using the map, zonal and district based planning decisions can be made by planners and urban climatologists that lead to new designs and policy changes.