Is Urban Heat Island intensity higher during hot spells and heat waves (Dijon, France, 2014–2019)?

The Urban Heat Island (UHI) phenomenon is frequently associated with heat waves, both in scientific literature and in the media. Health problems and other issues often result from the co-occurrence of these two phenomena. Previous studies have not yet provided sufficient evidence to validate potential links between UHIs and heat waves. The MUSTARDijon network was set up in 2014, in Dijon, France. Its spatial density (60 sensors) and temporal depth (6 summers) provide information about several Hot Spells and Heat Waves (HS&HW), as well as the daily intensity of UHIs. For the period from June to August (the warmer months), no statistical relationship could be established between temperature and UHI intensity. The UHI phenomenon is strong when days are sunny and not very windy; these two conditions are frequently met during HS&HW, but not systematically. By contrast, these conditions can be fulfilled even without high temperatures. Strong UHIs can thus develop outside Hot Spell/Heat Wave (HS/HW) events. These HS&HW are not necessarily in phase with UHIs: maximum UHI intensity tends to occur before the onset or during the first few days of HS&HW. The intensity of the UHI decreases during HS&HW because nocturnal urban temperature generally remains stationary, whereas rural temperature tends to increase at night. The hypothesis proposed is that this increase in rural night temperature during HS&HW could be linked to the decrease in Actual Evapotranspiration (ETa) and soil wetness. •MUSTARDijon is a dense new meteorological network in a medium-sized European city.•Six long Hot Spells & Heat Waves (HS&HW) are identified in Dijon from 2014 to 2019.•Observational and reanalysis data are combined to study UHI intensity during HS&HW.•No synchronicity is observed between UHI intensity and temperature during HS&HW.•Low soil moisture and low evapotranspiration favor high rural temperatures at night.