Nocturnal airflow from urban parks-implications for city ventilation

The spatial and temporal pattern of nocturnal airflow in and around two urban parks in Scandinavia were analysed. The results, based on 724 field measurements during 21 case studies, showed that both parks generated a local airflow during clear and calm weather conditions. The spatial pattern was characterised by calm in the middle of the park and a steady airflow towards the surrounding built-up areas at the park borders. The airflow from the park started one to two hours after sunset and continued during a period of four to eight hours. The wind speed was low (< 0.5 ms(-1)) and the local ail flow leached a short distance from the park border. In the flat park in Kobenhavn. Denmark, the air flow from the park was attributed solely to the development of a thermally induced park breeze. The park breeze development was also predominant in the park in Goteborg Sweden, but the influence of topography could not be totally excluded. The origin of the airflow from the Dark and its importance for urban air quality were discussed.