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A statistical analysis of the changes of the average wind direction after one or two hours, using data from Schiphol airport weather station, was made in order to be able to predict the effect of suddenly liberated noxious gases. Values of wind direction differences after one or two hours (delta(1) resp. delta(2)) were determined. The frequency distribution of delta and the normal distribution of x = sin delta were established. The variances of x mean, of the standard deviation of x and of q, i.e. the number of wind direction changes greater than 90 deg, were calculated. Finally, the possibility of delta absolute being greater than or equal to the angle with which 80 % (resp. 90 %) certainty the wind direction will vary was determined.