Investigations of the nocturnal wind climate in urban parks during clear and calm weather have been carried out during a period of two years. The main purpose with the study was to seek field evidence for the hypothesis that the temperature difference between large parks and their surrounding built-up areas induce a thermal wind system, the park breeze. The study was carried out in two large parks, Slottsskogen in Göteborg (57° 42´N, 11°58´E), Sweden and Fælledparken in København (55° 38´N, 12°40´E), Denmark. Both parks have a diversified vegetation with both trees and bushes of different densities but are different according to the topography. Altogether 21 case studies of the nocturnal wind pattern were carried out in the two parks during 1996 and 1997. The field surveys were made through hand-carried instruments in and around the parks. The measurement points, arranged in profiles, were chosen with respect to the topography and vegetation and the measurements were carried out in intervals during the night. The wind speed was measured with anemometers and the wind direction with a wool-thread and a compass, both at a height of 2 metres. In addition to these measurements, data from permanent meteorological stations have been used in the analysis.
The results strongly indicate that a park breeze develops in both parks during certain conditions. The windpattern, interpreted as park breeze, was characterized by calm in the middle of the park and a distinct and steady outflow of air at the park borders. The park breeze system started 1 to 2 hours after sunset and was best developed during the first part of the night. The results also indicate that the system continues during a period of at least 3-5 hours but additional field data are needed before a more exact determination of the start and break down of the system could be made. The general pattern of the park breeze does not differ between the summer and winter examples. Neither was the general wind pattern different in the flat Fælledparken compared to the topographical Slottsskogen.
The prerequisite for park breeze, the possible influence of topography and other thermal windsystems (urban heat island circulation and land breeze) as well as the frequency and importance of park breeze system will be further discussed at the conference.
The study was financially supported by the Swedish Environmental Protection Agency, the Swedish Council for Building Research and the City of Göteborg.