The use of weather types in the definition of seasons: the case of southern Balkans

The use of weather types for the definition of seasons is examined. Specifically, an objective definition of seasons is attempted for the southern Balkans based on the intra-annual variations of the frequencies of the objectively defined weather types. The methodology includes a combined application of Principal Component Analysis and k-means Cluster Analysis on daily grid point meteorological data obtained from the ERA5 database, for the 40-year period 1979-2018. The methodology is used at first for the classification of weather types and then for the definition of seasons based on the intra-annual variations of the weather types' frequency. The application of the methodology results in nine weather types and four seasons. The characteristics of the defined weather types are connected to the prevalence of specific atmospheric circulation patterns, the windward or leeward character of the various sub-regions, and the thermal characteristics of the earth's surface. The four seasons generally correspond to the conventional ones, but they present differences regarding their onset and cessation dates and their duration.