: Frequency and intensity of urban heat islands in Rennes: assessment of 16 years of monitoring (2004-2019).

International audienceThis article focuses on the observation of the urban heat island (UHI) in Rennes from 2004 to 2019. The existence of a relatively old and dense urban observation network makes it possible to quantify the frequency and intensity of the UHI according to the hour of the day and the season: the mean UHI thus reaches 1.3°C but it approaches 3°C on average at night in late summer and can sometimes exceed 7°C. Strong UHI situations, above 4°C, represent more than 17% of observations on average but up to almost 30% in September. In 2019 as in 2011, a record number of high intensity UHIs were noticed, essentially resulting from a high frequency of radiative situations (strong sunshine, weak wind). The spatialization of the UHI shows that the classic gradient between downtown and the periurban area is not symmetrical between the south and the north of the agglomeration: indeed, the presence of parks and valleys to the north generates islands of freshness whose intensity varies according to their size