Directional persistence of low wind speed observations

Low wind speed data is presented for stable, high-pressure sunny weather for an urban environment in the UK. In these climatologies, ventilation of buildings and dispersal of pollutants is often considered inadequate. Persistence, the length of time in which the wind blows in a specific direction and for a given angular range, is analysed. The observational data shows that in these low wind speed conditions, the wind blows in a consistent direction, for significant periods of time, and that truly calm conditions are very infrequent. The analysis of data for hot, dry, sunny and high-pressure conditions at one UK urban site, indicates that there are larger than expected air movements in these light wind scenarios. Values for mean duration of persistent wind episodes commonly exceed 30s, with similar values for the standard deviations. The Weibull statistical structure equally applies to these low wind speed observations. In this urban environment, for 45% of the time, the persistent wind events last longer than one standard deviation above the mean. At wind speeds lower than 2 m s(-1), the duration of persistent wind episodes remains constant and nor does it decrease with wind speed. This may be due to larger scale meteorological influences, perhaps convective activity and eddies in the boundary layer, which create air motions in calm daytime conditions in the UK. (C) 2004 Elsevier Ltd. All rights reserved.