Variation in windy days 1966–2013 for October. The horizontal line is the average number of days (5.2) that exceed the selected 60km/h threshold over the 1981–2010 period. The calculation is based on daily 9am msl pressure differences between Auckland and Christchurch, from which the geostrophic (west–east) wind is calculated (note that this will not identify strong southerlies). The AK-CH pressure gradient corresponding to 60km/h is approximately 14 hectopascals.