The current climate changes imply the rise of the mean temperature of the cities during hot periods. To help the public policy to be more efficient about Urban Heat Island (UHI) reduction we aim to define a UHI risk indicator. To be useful at a local scale, the resolution of this index must be at the scale of the buildings – i.e. about 5 m.The UHI risk indicator can be mapped with the use of standard methods like direct or indirect measurements, meso or micro scale simulations, or via climate zones classifications. All these methods are not applicable over a whole territory or cannot produce map with a fine resolution as expected.We choose here to use an intermediate approach by developing a physically based simplified micro scale model to produce a UHI index (ranging between 0 and 1). This model is based on data of the territory (vector data and imagery analysis) and physical laws governing heat exchanges between sun, city and atmosphere; in ideal climatic conditions (anticyclonic, without winds and clouds).The objective is to develop an algorithm integrated into a Geographic Information System (GIS) software like QGis with a reasonable computational time to be executed on a personal computer.