Design guidelines for natural ventilation (NV) in buildings focus on the potential hourly air change (ACH) rates based on the building space parameters. Critically, external airflow data is often assumed on the basis of a single mean wind speed and an associated prevailing wind direction. This can result in significant variation in ventilation rates and comfort conditions when non-design external wind conditions prevail. This paper describes a CFD study aimed at examining the influence of variations in external wind speed and direction on the air change rate for the atrium space of a two-storey naturally ventilated building. The building atrium is ventilated by a series of entry vents on one wall of the building in conjunction with roof vents. External wind speeds from 25 to 250% of the mean site wind speed (5.7 m/s) were examined and found to result in an almost linear increase in the ACH rate. For a single wind speed, the relationship between wind direction and the ACH rate was also found to be approximately linear for wind directions between 0 degrees and 90 degrees (orthogonal and parallel) to the wall vent openings, but non-linear for other wind directions (90-135 degrees). More generally, the significant variation in the atrium ACH rate with changes in external wind conditions, evident in this particular building model, illustrates the importance of considering non-design wind conditions when designing NV buildings. (C) 2008 Elsevier B.V. All rights reserved.