The dependence of gust factor probabilities on convective activity: Analysis conducted for Eurotunnel

The ratio of the maximum gust in an hour to the mean wind for the how is the ‘gust factor’. Extreme value analysis of gust factors was conducted as part of the design studies for Eurotunnel's Wind Alert System. This analysis indicated that the population of gust factors was mixed, suggesting that two different types of mechanism cause hourly gust maxima. Various investigations confirmed this indication and identified the two mechanisms. During the majority of hours the gust factor is a measure of the maximum gust due to the turbulent fluctuations of the atmospheric boundary layer. However, during a substantial minority of hours it is a measure of a gust caused by a ‘convective’ meteorological event such as a thunderstorm, squall line or heavy shower. The latter mechanism causes occasional exceptional gust factors, which can be of dominating importance for a transport system which must react when there is a risk of severe gusts.