Onset and End of Natural Seasons in Iran

Onset and End of Natural Seasons in Iran Introduction:  Season is the natural pattern of change in nature, which is related to the movement of the sun, the temperature cycle, the life cycle of the earth (phenology) and human culture. In astronomical and climatic seasons, a year divided into four seasons, spring, and summer, autumn and winter (Alsop, ۲۰۰۵), (Trenberth, ۱۹۸۳). Season is a period of the year with a homogeneous climate (Alsop, ۱۹۸۹), that is difficult to determine exactly when to start and end. The methods of determining of the seasons are: change in the face of the earth (Cayan et al, ۲۰۰۱), (Wang et al., ۲۰۲۱), constant temperature threshold, (Jaagus et al, ۲۰۰۳; Kitowski et al, ۲۰۱۹; Ruosteenoja et al, ۲۰۱۹; Alijani,۱۹۹۸), Air Masses, (Lamb, ۱۹۵۰; Cheng et al, ۱۹۹۷; Pielke et al, ۱۹۸۷; Kalinicky,۱۹۸۷; Alpert et al, ۲۰۰۴). What is a natural constant sign is the key to determining change and starting a new season. Organisms react to the onset and end of natural seasons by changing their behavior. Naturally, plants and animals adjust and adapt their phonological stages to temperature changes and jumps (Sparks et al, ۲۰۰۲), Plants germinate and flower in spring,fruit in summer, reduced activity and leaf in autumn and in winter fall asleep (Menzel et al, ۱۹۹۹ Animals are also adapted to reproduction, nesting and childbirth, And their phonological period is also related to vegetation conditions. In other words, the life stages of living organisms are adapted and dependent on these natural changes (Schwartz et al, ۲۰۰۰). Some organisms also migrate in order to adapt (Smith et al, ۲۰۱۲). The genetic response of organisms to rapid climate change and seasons associated with winter warming across the north, the early onset of spring and a long growing season is a factor in impairing the physiological response (reproduction, dormancy or migration time) of species(Bradshaw et al, ۲۰۰۸). On the other hand, the sensible temperature of organisms is affected by radiation, wind, air temperature and humidity. As appearance temperature is an important heat factor (heat and cold) in nature, to which animals, plants and humans react. Ruosteenoja et al (۲۰۱۹), showed the length and onset seasons of European with thresholds of ۰ and ۱۰ ° C focusing on the scenario of a ۲ ° increase in temperature, an increase in summer length and a decrease in winter compared to pre-industrialization. The length of summer increases by ۱ degree, increases by ۱۰ days, and the length of winters decreases by ۱۰ to ۲۴ days. Kitowski et al, (۲۰۱۹), showed the onset of summer earlier, the shorter autumn, the longer summer and the shorter winter in Poland with zero-, ۵- and ۱۵-degree temperature thresholds. Wang et al, (۲۰۲۱) change the onset time and length of natural and summer seasons from ۷۸ days to ۹۵ days, and spring, autumn and winter, ۱۲۴ to ۱۱۵, ۸۷ to ۸۲, and ۷۶ to ۷۳ days, respectively. Also, summer is halfway through the year and winter is less than two months to ۲۱۰۰ in the middle of the Northern Hemisphere. Dong (۲۰۰۹) showed that in most parts of China since ۱۹۵۰, summers have been longer and winters shorter, with the onset of summer ۵.۸ days earlier and the length of the season ۹ days longer and the winter ۵.۶ days later and the length of the season ۱۱ days. Changes in transition seasons are less. Season start, end and season length changes studied in Oregon and Washington (Alsop, ۱۹۸۹), in the United States (Barry and Perry, ۱۹۷۳), Europe (Jaagus et al, ۲۰۰۳), Estonia (Jaagus et al, ۲۰۰۰), South Korea (Choi et al, ۲۰۰۶), China (Ma et al, ۲۰۲۰), Xinjiang in northwestern China (Jiang et al, ۲۰۱۱; Cheng et al, ۱۹۹۷), Eastern Mediterranean (Alpert et al, ۲۰۰۴), Iran (Alijani ,۱۳۷۷). Therefore, with the increasing trend of temperature in different regions of Iran (Alijani et al, ۲۰۱۲), study of change of the start and end dates of natural seasons in connection with life in nature is necessary (Penuelas et al, ۲۰۰۲). The aim of this study is determine the time of onset, end and length of natural and significant seasons and its difference with astronomical and climatic seasons in Iran with highlands, inland and coastal lowlands in the north and south with a new approach based on biological physiology. Material and methods: To determine the onset and end of natural seasons, daily data of relative humidity, water vapor pressure, and wind speed and air temperature over a ۶۰-year period for ۳۲ synoptic stations in Iran from ۱۹۵۹ to ۲۰۱۸ were used. Selected stations cover all areas of Iran (coastal, low and highlands). In the first step, the apparent daily temperature of each station was calculated (Formula ۱). In the second stage, with the knowledge of the direct effect of atmospheric circulation factors in the occurrence of natural phenomena (Alijani, ۲۰۱۱) And rapid changes in temperature (season), the ۴-day moving averages of apparent temperature (average life of cyclone and anticyclone) at each station were calculated and was the basis of study. The onset and end of the season are with a natural and biological approach related to the stages of bio phenology and the natural part's reaction to temperature changes. Therefore, the apparent temperature of zero and below zero with the reduction or cessation of biological activity in nature, is the onset of winter. On the other hand, the time required by nature to adapt to new temperature conditions, is at least ۱۰ days (Joy, ۲۰۱۷). Therefore, the temperature of zero degrees and non-return to zero Up to at least the next ۱۰ days, is the basis for the onset of winter. In fact, with the continuation of sub-zero temperatures for ۱۰ days, the living part of nature receives the signal of change. If after that, for a period of less than ۱۰ days, the temperature goes above zero, the situation will not return to the previous state (nature did not react and adaptation occurred). On the other hand, the best temperature for the growth period is from at least zero degrees to a maximum of ۳۰ degrees in nature (Abrami, ۱۹۷۲). The second key indicator is the temperature of the onset of summer and the warm period. For the onset of the summer season, the temperature of ۲۰ degrees was base with the previous conditions. Because at this temperature, the reproductive period in plants and animals has started, most animals and plants have children and humans also feel warm. As plants begin to fill grain at this temperature, including wheat (Jenner, ۱۹۹۱ and Dupont et al, ۲۰۰۳) as the world's oldest grain. Here, the same condiction as before, don’t return to ۲۰ degrees for at least the next ۱۰ days was the basis. So at the onset of both seasons, if the temperature returns to zero and ۲۰ in the ۱۰-day period, the season has not begun, and in that year the station does not have winter and summer, respectively. Then, the temperature of ۱۹ degrees and less with the above conditions, the onset of autumn and the temperature of ۱ degree and more with the above conditions, are the basis for the onset of spring. Formula ۱: Calculate the apparent temperature                      AT = T + ۰.۳۳ PV - ۰.۷ WS – ۴ T = air temperature in Celsius, PV = water vapor pressure in hPa, WS = wind speed in meters per second, AT = apparent temperature in Celsius Results and discussion:  The onset and end of natural seasons are different in the geographical and topographical location of Iran. Southern regions and the northern coasts are two seasons with a warm summer season and a transitional season (cool). Other parts of Iran, like the temperate regions of the globe, have four seasons, but the start, end and length varies. The longest winter in the northwest and the western heights and the length of winter to the east and south is short and vice versa, the longest summer in the south and center of Iran. Spring season in below ۲۹ degrees orbit, Khuzestan and the shores of the Caspian Sea is not a separate season, but with the absence of winter, it merges with autumn. In other regions, spring begins in the south and northwest, respectively, from ۳۱ January to ۸ March. In most parts of Iran, the onset of spring coincides with the traditional date of Nowruz, after small chelleh of winter. This month coincides with the rise in temperature and the revival of nature and the introduction of the New Year. The end of spring in the central regions, ۱۰ May and in the northwest, ۱۸ June, and its length varies from ۱۰۳ to ۹۶ days in the northwest and northeast, respectively. In the temperate regions of Iran, it is about three months with a ۱۰-day spatial fluctuation (Table ۱). The onset of summer is with a new stage of phenology in nature. The onset of summer is from ۱۵ April on the southern coasts with high tropical arrival and the latest onset of summer in the northwestern part is ۱۹ June (Table ۱). In the south of the orbit of ۲۹ degrees and the region of Khuzestan, until ۸ May, in the central and northeastern regions of Iran from ۲۲ May to ۲۹ May and the west and northwest region, from mid-June to the end of June. The end of summer, as opposed to the onset, is the earliest time of ۱۷ September in the northwest, and in the southern regions of Iran, the end of ۸ October is in the ۲۹ degree orbit. The southern regions of Iran, the longest summer that shows the role of latitude and slower exit of the tropical system (Alijani, ۱۳۹۰). The length of the summer season in temperate regions varies from ۹۰ to ۱۳۹ days, approximately three to five months, respectively in the northwest and the ۲۹-degree geographical orbit, respectively. Therefore, the spatial trend of summer length from east and south of Iran to north and northwest is decreasing and there are the shortest summers in northwest of Iran. Naturally, this spatial trend is related to the high-altitude inbound and outbound routes of the subcontinent and the western systems from the south and northwest, respectively. The month of October and November is the onset of autumn in Iran, in the northwest and northeast, with the arrival of cold atmospheric circulation from above, the angle of radiation and altitude, is ۱۸ September. The latest start of autumn in Hormozgan is ۱۲ November (Table ۱). The end of autumn is the first of April to the first of June in the south and north coasts, respectively. In the northeast of Iran, ۲۴ to ۲۸ December, and in the central regions, ۲۸ to ۳۱ December, is the end of the autumn season. The earliest end in the northwestern regions of Iran at the end of December is ۱۰-۱۷ December. The length of the autumn season in temperate regions is ۸۳ to ۹۷ days, respectively, in the northwest and northeast, that’s an average of nearly three months. With the onset of winter, decreases in temperature (frost) and winter during the year below the ۲۹ degree orbit are rare, but on the northern coast, with the influence of atmospheric systems, it is a coincidence. In other regions of Iran, northwest, west and east of Zagros and south of Alborz, above ۲۹ degree orbit, from ۱۱ December to ۱ January, is the time of winter. Respectively, the earliest onset of winter is in the northwest, and the latest onset in the central regions (Table ۱). As the westerly winds of the extraterrestrial latitudes with cyclones and anticyclones dominate the Iranian atmosphere, also, the angle of radiation and the amount of radiation received at the earth's surface at this time, reaches a minimum during the year. The end of winter in temperate regions is from ۳۰ January in the ۲۹ degree orbit to ۷ March in the northwestern regions. Winter length reaches ۸۶ days in northwestern Iran, ۲۹ days in central regions (above ۲۹ degree orbit) and ۵۸ days in northeastern Iran, Therefore, there are only three winter months in northwestern Iran and in other parts of Iran, it is the shortest season during the year. Spatial trend of winter length from northwest of Iran to east and south is decreasing. Figure۱: Date of onset, end and duration of natural seasons in different regions of Iran Fall Summer Spring Winter Season Length End onset Length End onset Length End onset Length End onset ۸۳ ۱۰ Dec ۱۸ Sep ۲۲۷ ۱۷ Sep ۱۵ Apr ۱۰۰ ۱۰ may ۳۱ Jan ۸۶ ۳۰ Jan ۱۱ Dec Earlier ۱۶۰ ۲۱ Apr ۱۳ Nov ۹۰ ۱۲ Nov ۱۹ Jun ۱۰۳ ۱۸ Jun ۸ Mar ۲۹ ۷ Mar ۱ Jan Later ۷۷ ۱۳۳ ۵۶ ۱۳۷ ۵۵ ۶۵ ۳ ۳۹ ۳۶ ۵۷ ۳۶ ۲۱ Fluctuation Conclusion: The time of the onset, end and length of natural seasons in Iran are different from astronomical and calendar seasons. The slow decreasing and increasing trend of temperature at the onset and end of the seasons is initially a function of the angle of radiation and the length of day and night, but the real onset of a season with temperature jumps associated with the migratory atmospheric system (cyclone and anticyclone), Siberian hypertension, It is from the north and high in the subtropics from the south. Areas below ۲۹ degree orbit in the south of Iran and Khuzestan and the northern coasts, have only two seasons of autumn (cool) and summer (warm) and the temperature decreases to zero and less (occurrence of winter), in the southern regions, rare and on the northern coasts is accidental and short. The apparent temperature in these areas has been decreasing since late summer and in the middle of the cold period, it is decreasing to the maximum (lowest temperature during the year) and increasing again until the onset of summer. Therefore, the above areas are two periods, with a cool season and a hot and hot season. The southern coasts of Iran and Khuzestan have short cooling seasons and long hot and hot summers, and the northern coasts, on the contrary, have shorter summers and longer and cooler autumns, that The influence of water temperature, latitude, topography and atmospheric systems are effective in these differences. In other regions of Iran, except the mentioned regions, four natural seasons occur (spring, summer, autumn and winter). In connection with the role of latitude, altitude, the arrival of migratory and high pressure Siberian atmospheric systems, the time of onset, end and length of the season has a change of location. As the length of summer is more in the southern, eastern and central regions of Iran and decreases in the northwest and west of Iran, and the length of winter is the opposite. The length of the transitional seasons (autumn and spring) in the temperate regions of Iran is not different and the three months in the season are similar to the astronomical and calendar seasons. The most important spatial difference is during winter and summer. Winter decreases from three months in the northwest of Iran to the south and east of Iran and reaches a month in the ۲۹ degree orbit. On the other hand, the length of summer, on the contrary, varies from five and three months from east and south of Iran to northwest of Iran. Therefore, in temperate regions of Iran, the length of natural seasons from the south and east of Iran to the west and northwest of Iran is more regular and approaches to three months in each season. This spatial trend indicates the climatic similarity of western and northwestern Iran with temperate regions of the globe in higher latitudes and but to the center, south and east of Iran, this similarity decreases and to hot and cold dry desert climate in the Middle East and central Asia region is similar, respectively. This indicates regularity and order in nature, which is related to the geographical principle of Tobler’s law, the spatial correlation of climates and the onset, end and length of their seasons. Therefore, if we consider three months in a season as a natural feature of the temperate regions of the earth and two seasons (climatic period) as a feature of the subtropical regions, Iran is in the transition zone of these two climates. As from three months, the length of each season in the northwest to less than a month in the range of orbit ۲۹ degrees, and then the subtropical conditions with two seasons (warm and cool) appear. Therefore, from northwest to east and south of Iran, the climatic moderation decreases and its tropical sub-characteristic (longer summer and shorter winter) heat and dryness to heat and humidity in southern Iran is added. Naturally, in this spatial process, primarily large-scale atmospheric rotations and secondly, geographical phenomena (their shape and position) play a pivotal role. The Caspian Sea coast is an exception to this rule due to its higher latitude and complexity of geographical phenomena and the role of water, because the climate systems related to the Caspian climate are different from other regions of Iran. Key words: Natural Seasons, Apparent Temperature, Plant and Animal Phenology, Iran.

کلیدواژه ها:

Natural Seasons ، Apparent Temperature ، Plant and Animal Phenology ، Iran ، فصول طبیعی ، دمای ظاهری ، فنولوژی گیاهی و جانوری ، ایران