Effects of urban form and atmospheric stability on local microclimate

Although proper urban planning options could help minimize the effects of UHI, studies to adapt the built environment to climate changes in urban areas are rare, particularly in the context of cool climate cities where urban warming is typically not seen as a current problem. This will change as the background climate continues to warm. While the exploration of the influence of urban form on local microclimate is helpful, it is necessary to untangle this effect from background atmospheric conditions that lead to such effects. The present paper evaluates the effect of urban morphology (as measured by the Sky View Factor – SVF) on local climate according to atmospheric conditions exemplified by atmospheric stability (modified Pasquill-Gifford-Turner [PGT] classification system) in a cold climate city. The aim is to highlight their combined importance and to make preliminary investigations on the local warming effect of urban morphology under specific atmospheric stability classes. Forty-nine locations were selected in the city centre, on the basis of SVF to represent a wide variety of urban forms (narrow streets, neighbourhood green spaces, urban parks, typical street canyons and public squares) and seven of these were assigned as locations for fixed weather stations. Thirty one temperature measurement campaigns were made during spring and summer 2013, using a ‘traverse’ method on a ‘Meteobike’ and on foot. The locations were chosen to represent a variety of urban formation (narrow streets, neighborhood greenspaces, urban parks, uniform and non-uniform street canyons and public squares). The visualization of local temperature variations was accomplished using Arc-Map tool from Arc-GIS package. The present work indicates that the maximum intra-urban temperature differences (i.e. temperature difference between the coolest and the warmest spots in a given urban region) is strongly correlated with atmospheric stability. It appears that atmospheric stability has larger effect on intra-urban temperature variations than urban morphology in a cold climate city. The combined effect of the two provides interesting variations in local temperatures that may have urban planning implications, especially as the background climate continues to warm.