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It is well known that air pollution is one of the major factors affecting human health and the activities in a town. Among the various air pollutants an interesting one, especially for sunny climates, is the reactive O3. O3 varies significantly through the year; it is therefore very important to know the onset and the duration of the warm period of the year since at that time O3 concentrations increase significantly. In this work, an attempt was made to determine the onset of warm season of the year as well as its duration in various sites located in Northeastern Mediterranean. For this purpose, a fuzzy clustering method is used. The method was applied on the isobaric thickness between 1000 and 500 hPa for the period from 1974 to 1997. The results were compared to conventional methods based on synoptic classification and were found to be in good agreement.