تحليل ويژگي هاي روزهاي شرجي در ايران

Analysis of the Characteristics of Sultry Days in Iran

پديده شرجي به­ واسطۀ اثر تركيبي دما و رطوبت بالا رخ مي­ دهد و همواره باعث بروز مشكلاتي براي فعاليت­هاي انساني مي­ شود. در پژوهش ­هاي قبلي، اين پديده در مناطق محدودي از كشور و با تعداد شاخص­هاي كمتري مطالعه شده است. اين پژوهش با هدف بررسي فراواني، تداوم و شدت روزهاي شرجي در سرتاسر ايران با سه شاخص شدت شرجي، فشار بخار آب جزئي و گرمايش به انجام رسيده است. به اين منظور داده‌هاي روزانه دما و رطوبت‌ نسبي 101 ايستگاه سينوپتيك كشور در يك دوره زماني 28ساله (2014-1987) جمع ­آوري و با سه شاخص يادشده مطالعه شد. نتايج تحقيق نشان داد دو شاخص شدت شرجي و فشار بخار آب جزئي براي تبيين شرايط شرجي در ايران مناسبند و برونداد آنها تفاوت محسوسي با يكديگر ندارد. اما شاخص گرمايش به نتايج مطلوب منتهي نشد. بر اساس شاخص منتخب، يعني شاخص شدت شرجي، سواحل جنوب و سپس سواحل شمال كشور، بيشترين فراواني، تداوم و شدت شرجي را دارند. اما در ساير نقاط كشور شرايط شرجي حاكم نيست و به طور ميانگين حتي يك روز با شرايط شرجي مشاهده نمي­ شود. در سواحل جنوب، به دليل حاكميت پرفشار آزور، وضعيت شرجي زودتر از ساير نقاط كشور، در روز 14 فروردين آغاز مي‌شود و با تداوم 7 ماه و 11 روز، در 25 آبان خاتمه مي‌يابد. اما در سواحل شمالي كشور، با يك تأخير 48روزه، از 1 خرداد آغاز و با تداوم تقريبي 4 ماه و 19 روز، در 20 مهر خاتمه مي‌يابد. بندر چابهار در ساحل درياي عمان با 291 روز شرجي كه 160 روز آن وضعيت شرجي بسيار شديد را داراست، بالاترين و بندر آستارا در سواحل خزر با 126 روز پايين‌ترين تعداد روزهاي شرجي را دارا هستند. در سواحل شمالي، وضعيت شرجي بسيار شديد ديده نمي­ شود. روند سالانۀ تعداد روزهاي شرجي در هيچ ايستگاهي معني­ دار نيست.

Extended Abstract Sultry phenomenon occurs due to the combined effect of high temperature and humidity. Sultry intensity increases with increasing relative humidity and decreases with decreasing temperature. This phenomenon has a tremendous impact on comfort and other human activities. Various indices have been used to study this phenomenon in Iran and in the world. According to previous studies, and as far as information is concerned, there has not been a comprehensive study across Iran on the characteristics of sultry days based on degree of severity. Therefore, the purpose of this study is to investigate the frequency, duration and severity of sultry days and its temporal and spatial analysis throughout Iran. To do this research, daily temperature, relative humidity and partial water vapor pressure of 101 synoptic stations were used for a 28-year period (1987-2014). In choosing the indices of sultriness, the goal was to select indices that show the sultry state on a daily scale. For this purpose, in the first stage, 16 empirical sultry or sultry-related indices were used, all of which used climatic parameters such as temperature, relative humidity, water vapor pressure and cloudiness to calculate the sultry state or comfort. Among them, 13 indices were eliminated because they surveyed the phenomenon on a monthly or annual basis or were not consistent with the objectives of this study. Finally, according to the objectives of the study, three indices were chosen: 1- Sultry Intensity Index (Lancaster-Carstone empirical equation), 2- Partial Water Vapor Pressure Index (partial water vapor pressure equal to or greater than 18.8 hPa), and 3- Heat Index (HI). The results of this study showed that two indices of Sultry Intensity and Partial Water Vapor Pressure are suitable for explaining the conditions in Iran and their outputs are not significantly different. But Heat Index did not lead to desirable results. According to the results of the Sultry Intensity Index, the sultry phenomenon is not comprehensive in the country and is limited to 21 stations adjacent to the Caspian Sea coasts in the north (besides Parsabad Moghan Station) and the Persian Gulf coasts (besides Ahwaz station) and the Oman Sea coasts in the south. In other parts of the country, due to their internal and leeward position, being away from moisture sources, poverty or lack of vegetation and insufficient penetration of wet and rainy systems, there is no sultry condition and, on average, even one day is not seen with sultry circumstances. On the southern coasts, on average, sultry conditions begin on April 3 and end on November 16. Therefore, in this area, 7 months and 11 days of the year have sultry conditions. This is natural due to the lower latitude and the Azores high pressure sovereignty in the south. But on the northern coasts, the sultry period is shorter and with a 48-day delay compared with the southern coasts, the average sultry day begins on May 22 and ends on October 12. Therefore, the duration of the sultry period is on average 4 months and 19 days. In terms of the number of sultry days, the most frequencies belong to the southern coasts stations. The largest number of sultry days related to the Chabahar port on the coasts of the Oman Sea with 291 days, followed by Jask port with 264 days. The lowest number of sultry days is also from Ahwaz station with 1 day and then Mahshahr port with 42 days. Among the stations on the southern coasts, the Oman Sea stations compared with the Persian Gulf stations have more sultry days due to lower latitudes, Azores high pressure sovereignty and Southeast Asian monsoon moisture influence. In contrast, the number of sultry days on the northern coasts is much lower and averages 140 to 150 days a year. Sultry severity is also less, so that there are no extreme severe sultry days in any of the stations on the northern coasts. But the number of extreme sultry days is remarkable on the coasts of the South, to 160 days in the port of Chabahar and 111 days in the port of Jask. At Parsabad Moghan in the north and port of Mahshahr in the south, due to distance from the coast and lack of sufficient moisture, the duration and severity of sultry is much lower and there are basically no days of severe and extreme sultry states. The annual trend of the number of sultry days at any station is not significant.