Urban heat islands

By their very existence, towns and cities are subject to the urban heat island phenomenon. This is commonly defined as a temperature difference between an urban environment and its non-urbanized periphery.

Following the numerous heatwaves experienced by metropolitan France over the past decade, this phenomenon is beginning to be identified by decision-makers, elected representatives and engineering consultants as part of climate change adaptation policies. However, there is a need to improve general understanding of urban heat islands, so that they can be taken into account with greater relevance and effectiveness in urban projects and developments.

The aim of this article is to introduce a number of essential elements relating to urban heat islands, in particular the notion of urban microclimate, and the horizontal and vertical spatial scales used in this context. It defines this climatic phenomenon more precisely, distinguishing the four different types of urban heat island identified in the scientific literature. A theoretical framework is presented, namely the energy balance method for rural and urban surfaces. The final section focuses on the urban heat island of the urban canopy boundary layer. Fixed and mobile observation methods used to characterize this type of heat island are presented. The urban canopy boundary layer heat island is analyzed in terms of its causes, controlling factors, daily dynamics and genesis.