Intra-urban differences in canopy layer air temperature at a mid-latitude city

Detailed meteorological measurements were carried out in two urban street canyons in central Adelaide and at two reference sites in a suburban location and at an exposed site near the middle of the city for an extended period of nearly a year. The meteorological records revealed substantial differences between air temperature in the urban street canyons and both reference sites. The nocturnal urban heat island observed in most cities was noted in Adelaide street canyons, too. However, the frequent occurrence of a daytime cool island during summer, albeit much weaker than the night-time phenomenon, is less expected. Both phenomena are attributed at least in part to the increase in surface area participating in energy exchanges with the atmosphere in an urban street canyon compared to a typical rural site, and hence in an increase in the effective thermal mass. The presence of additional thermal mass is manifested not only in the dampening of the diurnal temperature range in the city, but also in a noticeable time lag in the maximum intensity of the daytime cool island. The measurements also demonstrate an observable difference in the daily progression of air temperature between a north-south street canyon and an adjacent east-west oriented street.