The polarity of a lightning flash is defined by the polarity of the charge which is transferred from the cloud to ground during the discharge. There is no difference in the optical appearance of a positive and a negative flash.

During summer only a small fraction of positive flashes is observed. Depending on the thunderstorm the fraction is in the range from 5% to 10%. Contrary to that the fraction of positive flashes can be more than 50 % during winter thunderstorms. This becomes obvious in a plot of the polarity distribution of the years 1992-2001. The percentage of positive flashes during the summer months is about 10 %.

The increase of sensitivity of the Austrian network during the last decade becomes obvious in following figure. The increase of detection efficiency (DE) of the LLS due to the integration of ALDIS in the European LLS EUCLID results in an increasing number of detected cloud to cloud strokes misclassified as positive cloud to ground strokes. Often attempts are made to limit the influence of those misclassified CC strokes by neglecting positive strokes with amplitudes of less than 10 kA. Although this approach seems too simple, because the misclassification of CC strokes most likely occurs over a wider range of amplitudes, we are using the same approach. As a result of the exclusion of small amplitude events the maximum percentage for positive flashes (year 2001) decreases from about 23% to about 13%.