An investigation has been made into the frequency distribution of errors in equivalent tail-winds on the great-circle air-route Shannon to Gander, as forecast at London Airport. In the course of the analysis the frequency-distribution of ‘actual’ equivalent head-winds as measured from the synoptic upper-air charts was also examined, and compared with seasonal means already published. The monthly extremes are tabulated and quarterly mean and standard errors are given, with frequency curves for the summer and winter half-years.