Introduction During the last century, huge movements of people have been observed between rural and urban habitats. Currently, about 50% of the world population lives in urban areas . With the increase of people in cities, the so-called urban heat island (UHI) effect has become a bigger and bigger problem for health and comfort. Indeed, urban materials like asphalt and concrete trap heat fluxes inside the urban centres.  This effect increase locally the temperature, which may lead to several hazards, particularly during summer . A huge amount of studies has tried to describe the role of green roofs as a solution against the heat effect (e.g. Takebayashi & Moriyama, 2007, Kleererkorper et al, 2012, Zinzi & Agnoli, 2012, Susca et al, 2011). Among others, Takebayashi & Moriyama proved by measuring experimental data that the heat flux on green (vegetalized) surfaces would be significantly smaller than on a grey one . Vegetation may then have a reducing effect on heat increase. Susca et al (2011) studied the effect of vegetation and green roofs against UHI in the city of New-York, USA, and used some monitoring stations spread in the city to obtain measures during a complete year. They demonstrated the advantages of having green roofs against UHI, but also for biodiversity and air quality improvement . In this paper, we will focus on the city of Geneva, Switzerland. We will try to measure a reduction of heat measurement in the locations where a green surface is present, and confirm the propositions made previously.