{{short description|Strong summer wind in the east and southeast of the Iranian Plateau}} The '''120-day wind''' or '''wind of 120 days''' ({{langx|fa|باد صد و بیست روزه|translit=bād sad ve bist ruzeh|lit=one hundred and twenty days wind}}) is a strong summer wind occurring from late May to late September in the east and southeast of the [[Iranian Plateau]],<ref name="dePlanhol">{{Cite encyclopedia |last=de Planhol |first=X. |date=19 August 2011 <!--|orig-date=15 December 1988 -->|title=bād (1) |encyclopedia=[[Encyclopaedia Iranica]] |url=http://www.iranicaonline.org/articles/bad-wind-phenomenon-at-ground-level-resulting-from-modifications-of-general-airflows-by-local-topographic-factors |access-date=22 October 2019 |volume=3 |pages=349–350 |issn=2330-4804}}</ref> particularly the [[Sistan Basin]]. It is so called because it lasts for four months.<ref>{{Cite book |last1=Spilsbury |first1=Louise |last2=Spilsbury |first2=Richard |year=2011 |series=Countries Around the World |title=Iran |url=https://books.google.com/books?id=AGulkkNHopcC&pg=PA13 |publisher=[[Raintree Publishers]] |isbn= 978-1406233490 |language=en}}</ref> The typical wind speed is {{convert|30–40|km/h|mph}} or less, but it can occasionally exceed {{convert|100–110|km/h|mph|-1}}.<ref name="dePlanhol"/> Strong speeds are caused by the [[topography]] surrounding the region.<ref name="atmos"/> The wind moves fairly consistently south-to-southeastward;<ref name="Ganji M"/><ref name="atmos"/> along with the ''[[shamal (wind)|shamal]]'', it is one of two well-known winds in Iran.<ref name="Ganji M">{{Cite book |last=Ganji |first=M. H. |editor-last=Fisher |editor-first=William Payne |year=1968 |chapter=Climate |series=[[The Cambridge History of Iran]] |volume=1 |title=The Land of Iran |chapter-url=https://books.google.com/books?id=I2pO-mttL50C&pg=PA219 |location=Cambridge |publisher=[[Cambridge University Press]] |isbn=978-0521069359 |pages=219 |language=en}}</ref>
During the "depression of Sistan", the four months when the wind is strongest, winds from northern Afghanistan and from the deserts of eastern Iran and western Afghanistan combine, resulting in accelerated high-pressure winds blowing from the central Iranian deserts toward [[Sistan and Baluchestan Province]].<ref name="Ganji M"/> The 120-day wind affects all of the Helmand Basin, but [[Sistan]] receives stronger winds as they intensify between the mountains of Iran and Afghanistan.<ref name="Whitney">{{Cite book |last=Whitney |first=John W. |year=2006 |series=Scientific investigations report |title=Geology, water, and wind in the Lower Helmand Basin, Southern Afghanistan |location=Reston, VA |publisher=[[Department of the Interior]], [[U.S. Geological Survey]] |oclc=1049741054}}</ref> The wind is relatively hot and carries abrasive sand particles.<ref>{{Cite book |last=Kheirabadi |first=Masoud |date=2009 |series=Modern World Nations |title=Iran |url=https://books.google.com/books?id=C-PU-0jBmdIC&pg=PA22 |location=Philadelphia, PA |publisher=[[Chelsea House Publishers]] |page=22 |isbn=978-1438105123 |language=en}}</ref> It causes evaporation in the Sistan Basin, contributing to drought in the region.<ref>{{Cite book |last=Mojtahed-Zadeh |first=Pirouz |author-link=Pirouz Mojtahedzadeh |editor-last=Mojtahed-Zadeh |editor-first=Pirouz |date=2006 |chapter=Hydropolitics of Hirmand and Hamun |title=Boundary Politics and International Boundaries of Iran |chapter-url=https://books.google.com/books?id=9o0IRPpvDjkC&pg=PA246 |location=Boca Raton, FL |publisher=[[Universal Publishers (United States)|Universal Publishers]] |page=246 |isbn=978-1581129335|language=en}}</ref>
== Environmental impact ==
=== Wind intensity === [[Flora]] in the Sistan Basin is negatively affected by the intensity of the 120-day wind and by blown sand associated with it. The wind can reach speeds of {{convert|10|m/s|km/h mph}} at {{convert|10|m|ft}} above ground level and up to {{convert|20|m/s|km/h mph}} {{convert|300–500|m|ft|-2}} above ground level.<ref name="atmos">{{cite journal |last1=Alizadeh-Choobari |first1=Omid |last2=Zawar-Reza |first2=Peyman |last3=Sturman |first3=Andrew Philip |date=15 June 2014 |title=The "wind of 120 days" and dust storm activity over the Sistan Basin |url=https://www.researchgate.net/publication/261099898 |journal=[[Atmospheric Research]] |volume=143 |pages=328–341 |publisher=[[Elsevier]] |issn=0169-8095 |doi=10.1016/j.atmosres.2014.02.001 |bibcode=2014AtmRe.143..328A |via=[[ResearchGate]]}}</ref> This can have a significant effect on crop [[transpiration]].<ref name="geotech">{{cite conference |last=Ganji |first=Homayoon |display-authors=etal |date=19–21 November 2014 |title=Effect of the "120-Day Wind" on Agricultural and Environmental Conditions in Herat, Afghanistan |url=https://www.academia.edu/9477613 |conference=Fourth International Conference on Geotechnique, Construction Materials and Environment |location=Brisbane, Australia |isbn=978-4990595838 |via=[[Academia.edu]]}}</ref> Natural formations, such as [[gorge]]s, act as shelters or blockades to protect flora. A gorge on the northwest border of the region provides protection for palm groves used by people in the area.<ref name="Huntington">{{Cite journal |last=Huntington |first=Ellsworth |authorlink=Ellsworth Huntington |date=1905 |title=The Depression of Sistan in Eastern Persia |journal=[[Bulletin of the American Geographical Society]] |volume=37 |issue=5 |pages=271–281 |doi=10.2307/198468 |issn=0190-5929 |jstor=198468 |url=https://zenodo.org/record/1624143}}</ref>
The wind has the potential to cause breaches in dams on the [[Helmand River]], leading to flooding. If river levels are too low, this can also contribute to drought.<ref name="Mellville">{{Cite journal |last=Melville |first=Charles|date=1984 |title=Meteorological Hazards and Disasters in Iran: A Preliminary Survey to 1950 |journal=[[Iran: Journal of the British Institute of Persian Studies|Iran]] |publisher=[[Taylor & Francis]] |volume=22 |pages=113–150|doi=10.2307/4299740|issn=0578-6967|jstor=4299740}}</ref> Sistan and Baluchestan Province reaps some benefits from the wind, including the elimination of stagnant water.<ref name="Huntington"/>
=== Dust storms and drought === The region experiences continuous [[dust storm]] activity due to loose soil around wetlands and lakes being blown by the 120-day wind.<ref name="atmos"/> A severe drought from 1999 to 2001 transformed [[Hamun Lake|Lake Hāmūn]] into a desert; it remains one of the major sources of dust in the region. Droughts and their duration directly affect the regularity and intensity of dust storms.
Loose sands displaced by wind create [[yardang]]s and [[dune]] fields in the lower Helmand Basin.<ref name="Ganji M"/> The moving sands intrude on villages in the province.<ref name="Mellville"/> Wind-blown sand threatens to invade fields and orchards, which jeopardizes agriculture and affects irrigation patterns.<ref name="Mellville"/><ref name="Ganji M"/> [[Windbreak]]s have been constructed to keep the sand from being removed from lake beds during droughts.<ref name="unep">{{Cite report |last1=Vekerdy |first1=Zoltán |last2=Dost |first2=Remco |date=May 2006 |title=History of Environmental Change in the Sistan Basin |url=https://postconflict.unep.ch/publications/sistan.pdf |publisher=[[United Nations Environment Programme]], Post-Conflict Branch |location=Geneva, Switzerland |access-date=22 October 2019 |archive-date=11 October 2017 |archive-url=https://web.archive.org/web/20171011103059/http://postconflict.unep.ch/publications/sistan.pdf |url-status=dead }}</ref>
=== Helmand Valley === The Helmand River is the primary water source in the Sistan Basin. Without it, life in the area would be almost impossible. In the Helmand delta, flooding is common and can be a greater hazard than dust storms.<ref name="Whitney"/>
Sistan is generally avoided during summer; there are villages in the Helmand River delta, but most inhabitants are [[nomad]]ic. [[Fauna]] is also scarce, though migratory birds do pass through.<ref name="Huntington"/>
== Architectural impact == In historic times, Sistan inhabitants prepared for possible disasters caused by the 120-day wind—dust, droughts, dry weather—by constructing buildings with extended walls and by placing buildings parallel to the wind.<ref name="Whitney"/> Inscriptions and architectural details became important with the development of [[baked brick]] buildings.<ref name="Whitney"/> <!-- not sure what to make of this sentence: The Wind of 120 Days protects the close details, as opposed to the parts of the buildings exposed to the sun. --> Upper parts of buildings and [[minaret]]s were not affected by wind-blown sands, but the lower portions were vulnerable: the wind is more intense near ground level, causing more structural erosion. Surfaces of buildings up to {{convert|10–15|feet|m}} above ground level were smoothed by these conditions.<ref name="Tate">{{Cite book |last=Tate |first=George Passman |authorlink=George Passman Tate |year=1979 |orig-year=1910 |title=Seistan: A Memoir on the History, Topography, Ruins, and People of the Country |publisher=[[Nisa Traders]] |oclc=254915988 |isbn=978-1535810845}}</ref>
During these four months, the winds also bring higher temperatures. Small intakes on top of homes allow residents to use the 120-day wind as air conditioning.<ref name="Whitney"/>
== References == {{reflist|40em}}
[[Category:Environment of Iran]] [[Category:Geography of Iran]] [[Category:Physical geography]] [[Category:Winds]]