Classifying the seasons is mainly carried out based on the daily movement of the sun. Around the world and especially within the middle latitudes as in Iran, there is not a full coordination between the seasons of a year and planetary seasons. Accordingly, in addition to planetary definitions,numerous definitions have been offered for seasons based on meteorology and climatology criteria. Therefore,season is part of a year which is distinct from other parts, because of a regular frequency of the planetary or climatologic phenomena. In other word, natural season is a time period of calendar year distinguished by homogenous and congruent weather. The natural seasons i.e. spring, summer, fall and winter are calendar time intervals which are naturally determined through different types of the congruent climatic regimes like temperature and wind speed changes. In fact, in such a classifying the seasons are defined based upon climatic elements e.g. temperature and wind representing the climatic facts of that place, with or without connection to the calendar seasons. Based on the above statement, this paper aims to find out a real definition for each season in the west of Iran, in which the seasons will be defined based on the climatic facts of the region, not on the basis of a planetary definition.
The studiedregion consisted of Kermanshah, Kurdistan, Hamedan, Ilam and Lorestan provinces (Fig. 1). The area of the region is about 121000 Km2 and situated between 31o and 51' to 36o and 28' north latitudes, and 45 o and 27' to 50 o and 4' east longitudes. In this research, we have employed the average weekly maps of pressure, relative humidity and wind direction for surface and geopotential height, temperature, relative humidity and wind direction at the level of 500 hPa. The main structure of 7-days average data have been arranged based on the Jalaali calendar as well. For example, periods of 21st -27th March, have been selected as the first week in accordance with 1st-7thFarvardin in Jalali calendar. In this division, the last week for each year has been considered from 12th - 20th March, regardless of the calculation of Leap years, equals with 22nd – 29thEsfand.
Cluster analysis has been used for statistical analyses. For cluster analysis, the mean surface pressure and 500 hpageopotential height maps were employed as the most important climatic variables.Finally, were obtained weeks and climatic season’s duration by computing the prevailing weekly pattern in each of these clusters. At first, the prevailing pattern of air-mass for each season was computed by calculating the air-mass frequency in every climatic season, and presented in terms of percentage. Then, the prevailing air-mass frequency and flow patterns of the surface and 500 hPa maps were determined and one pattern was discussed as a representative for each season.
Results and Discussion
The obtained results of charts show that the climatic 4-seasons control the western parts of Iran. Winter with 18-weeks duration, was recognized as the longest and the main season, while spring with 7-weeks duration identified as the shortest season in the west of Iran. The summer is considered as the second important and long season in the region, as it takes around 16-weeks long, and fall with 11-weeks, is the third season in the view of duration in the study area. The climatic summer lasts from 11-weeks to the end of 26-weeks in the west of Iran, and generally it takes 16-weeks long.In fact,this season starts around 3-weeks earlier than the planetary summer time in the northern hemisphere which is the first of Tir.Furthermore, the end ofsummer usually takes place about 4-days sooner than 31st of Shahrivar, which is the end of planetary summer season. The most percentage of the summer season’s air-masses is dedicated to the continental-tropical air-mass (CT) around 72.7%.
The winter is the most important and the longest climatic season in the region and the continental polar air-mass was identified as the most air-mass of this season. This air-mass represents the arrival of air-masses from higher latitudes towards the west of Iran. In winter,topography and high-pressure centers in the west of Iran intensifythe cooling of air-masses and their impacts. The results show that winter is the longest season in the west of Iran, and their prevailing patterns include CP and CT air-masses respectively. The reason for CP air-mass frequency during the winter season is the Siberia high reinforcement and extension of its troughs towards the west of Iran where the movement is much more during the winter rather than fall. In addition, the reason for CT air-mass frequency during the summer season is the Azores dynamic high-pressure predominant in the middle levels of the atmosphere. The prevailing air-masses of the spring are similar to those of the summers’ and the fall season also looks highly alike to the climatic winter’s air-masses in the region.