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In order to determine the natural seasons in Iran, mean daily temperatures and relative humidity often stations during. 1961 - 90 period were analysed via pentad means. Application of Harmonic analysis to the pentad means showed a complete harmonic during year. At first, pentads were grouped by SPSS Clustering program. The clustering resulted in climatic classes in each station. Thermal characteristics of the resulted classes did not coincide with people’s common impression of the seasons, especially in winter. Thus, pentads grouped according to two winter threshold (10C) and summer threshold (22C).
The results showed only long summer and short spring in the Southern coastal areas. At the rest of the country, spring began generally in March and lasted about two months. Summer began as early as May 1st in South and as late as June 21st in the North and continued generally up to end of September all over the country. Fall season is short over the country and lasts about two months.
Winter starts about late October in the North and about early December in the South. The longest season is winter in the North (lasting about 5 months) and Summer in the South (lasting about 10 monthes).