States that zero and low wind speed occurrences are often overestimated in standard meteorological data for use by HVAC engineers because of the use of the robust, rotating cup anemometers. Therefore the data would be expected to underestimate wind-driven natural ventilation as well as pollution dispersal. A comparison of rotating cup and ultrasonic anemometers carried out for this study indicated that the former can indicate zero wind speed over many hours in the day when speeds up to 1.5 m/s can be present. As wind speed drops below 3 m/s the anemometer was shown to be increasingly inaccurate. The authors show how by using a Weibull nonlinear least squares fit to higher speed data, more accurate low wind data can be obtained from the rotating cup anemometer.