مناطق ظل المطر Rain shadow ظواهر تحدث عند اعتراض سلاسل الجبال مجرى الرياح الرطبة، التي تهب على شاطئ البحر. فعندما ترتفع الرياح لتعبر سلسلة الجبال يسقط المطر على السهول الساحلية، ومنحدرات الجبال وتهب الرياح الجافة بعد ذلك إلى داخل اليابسة، مسببة أمطاراً خفيفة خلف سلسلة الجبال. وتُسمى المنطقة التي تقع خلف الجبال منطقة ظل المطر.

أبرز مناطق ظل المطر

تعد المنطقة الشمالية الشرقية، من كوينزلاند بأستراليا، والتي تقع غرب سلسلة الجبال العظمى الفاصلة، من مناطق ظل المطر، حيث تقل الأمطار فيها عن 750ملم في العام. وهناك منطقة أخرى لظل المطر، تقع في ولاية نيوساوث ويلز، لأن الرياح الشرقية الممطرة لا تصل إلى المنحدرات الغربية منها. وفي أسكتلندا تسبب الأراضي المرتفعة، منطقة لظل المطر، تحجز الأمطار عن المناطق الواقعة إلى الشرق منها.

أمثلة مناطق ظل المطر الشهيرة تضم:

آسيا

• قمم جبال القوقاز إلى الغرب، جبال البرز إلى الجنوب والسلاسل متصلة بالهيمالايا إلى الشرق تظِل مطرياً صحاري قرةقم وقزل‌قم شرق بحر قزوين، وكذلك semi-arid Kazakh Steppe.
• Judean Desert والبحر الميت are rain shadowed by the جبال الخليل.
• دشت لوت في إيران هي في الظل المطري لجبال البرز وجبال زاگروس وهي واحدة من أموت بقاع الأرض.
• The Himalaya and connecting ranges also contribute to arid conditions in آسيا الوسطى بما فيهم صحراء گوبي،بمنغوليا وكذلك semi-arid steppes of Mongolia and north-central to north western China.
• The Ordos Desert is rain shadowed by mountain chains including the Kara-naryn-ula, the Sheitenula, and the جبال يين، which link on to the south end of the Great Khingan Mountains.
• الصحراء الهندية الكبرى or Thar desert is bounded and rain shadowed by the Aravalli ranges to the south-east, the Himalaya to the northeast, and the Kirthar and Sulaiman ranges to the west.
• الجانب الشرقي من سلسلة ساهيادري في هضبة الدكن في الهند.
• الجانب الشرقي من الغاط الغربي في جنوب الهند (Nilgris, Anamalai and Agastiya Malai Ranges) semi arid desert.

أمريكا الجنوبية

• صحراء أتاكاما في تشيلي is the driest non-Antarctic desert on Earth because it is blocked from moisture on both sides (by the جبال الأنديز to the east and high pressure over the Pacific at a latitude which keeps moisture from coming in from the west).
• The Argentinian wine region of Mendoza is almost completely dependent on irrigation, using water drawn from the many rivers that drain glacial ice from the Andes. The nearby Chilean wine region of Valle Central on the other hand, is situated on the Chilean side of the Andes and experiences a maritime climate.
• پتاگونيا is rain shadowed from the prevailing westerly winds by the Andes range and is arid (e.g., in Santa Cruz few spots are capable of cultivation, the pastures being poor, water insufficient and salt lagoons fairly numerous).

أمريكا الشمالية

Most rainshadows in the western الولايات المتحدة are due to mountain ranges, notably the Sierra Nevada and Cascades, that intercept rain and snowfall that would otherwise reach a valley in the lee of the mid-latitude prevailing westerlies.

• The deserts of the Basin and Range Province in the الولايات المتحدة and Mexico, which includes the dry areas east of the Cascade Mountains of Oregon and Washington and the Great Basin, which covers almost all of Nevada and parts of Utah are rain shadowed.
• The dry precipitation regime of the Great Plains of Western Canada and the central USA can be attributed in large part to the rain shadow of the Western Cordillera ranges.
• The Mojave, Black Rock, Sonoran, and Chihuahuan deserts all are in regions which are rain shadowed.
• The aptly-named Death Valley in the United States, behind both the Pacific Coast Ranges of كاليفورنيا and the Sierra Nevada range, is one of the driest places on the planet.
• The Colorado Front Range is limited to the rainfall that makes it over the continental divide. While many places west of the divide receive as much as 40 بوصة (1,000 mم) of precipitation throughout the year, some places on the east side, notably إڤرگرين، كولورادوودنڤر، كولورادوonly receive about 17-19 inches. Thus, the القاسم القاري acts as a barrier for the precipitation. This effect only applies to west to east storms. When low pressure areas skirt the جبال روكي to the south, they can give high precipitation to the east side and none to the west side. This happens rarely.
• The Rocky Mountain Front Range in Montana shows areas of limited precipitation as long as the systems passing over the mountain range come from the west. The clouds will dry out considerably by the time they reach the peaks of the mountain range.
• The east slopes of the Coast Ranges in central and southern California also cut off the southern San Joaquin Valley from enough precipitation to ensure desert-like conditions in areas around Bakersfield.
• The areas to the east of the Coast Mountains and the Cascade Range in the province of British Columbia, كندا and the American states of Washington, Oregon and آيداهوare located in a rain shadow.
• The Dungeness Valley around Sequim, Washington lies in the rain shadow of the Olympic Mountains. The area averages 10–15 inches of rain per year, less than half of the amount received in nearby Port Angeles and approximately 10% of that which falls in Forks on the western side of the mountains.
• The effect appears even in the eastern United States. Although much more humid than any obvious deserts or steppes, the Shenandoah Valley mostly in western ڤرجينيا, lying between the Blue Ridge and the Appalachian Mountains, is drier than areas to the east and west because the modest mountains reduce rainfall within the valley. The lesser rainfall than that of surrounding areas makes this long valley paradoxically a richer agricultural area than rainier areas nearby. [1][2]

اوروبا

• The Pennines of Northern England, the Welsh Mountains, and the Highlands of Scotland create a large rain shadow that covers almost the entirety of the Eastern United Kingdom, with گلاسگوand Manchester for example receiving around double the rainfall of إدنبرة and York respectively. The contrast is even stronger further north, where Aberdeen gets around a third the rainfall of Fort William or Skye. The Fens of East Anglia receive similar rainfall amounts to Seville.
• The Cantabrian Mountains make a sharp divide between "Green Spain" to the north and the dry central plateau. The northern-facing slopes receive heavy rainfall from the Bay of Biscay, but the southern slopes are in rain shadow. The most evident effect on the Iberian Peninsula occurs in the Almería, Murcia and Alicante areas, each with an average rainfall of 300 mm and the driest spot in Europe (see Cabo de Gata) mostly a result of the mountainous range running through their western side, which blocks the westerlies.
• Some valleys in the inner Alps are also strongly rainshadowed by the high surrounding mountains.
• The Plains of Limagne and Forez in the northern Massif Central, France, are also relatively rainshadowed (mostly the plain of Limagne, shadowed by the Chaîne des Puys (up to 2000mm of rain a year on the summits and below 600mm on Clermont-Ferrand, which is one of the driest places in the country).
• The Piedmont wine region of northern Italy is rainshadowed by the mountains that surround it on nearly every side; Asti receives only 527 mm of precipitation each year, making it one of the driest places in mainland Italy.
• Athens is shielded strongly by mountains from the strong moisture-bearing winds of the Adriatic Sea and receives only a quarter the rainfall of most of Albania.
• The Scandinavian Mountains create a rain shadow for lowland areas east of the mountain chain and prevents the Oceanic climate from penetrating further east; thus Bergen west of the mountains receives 2,250 mm precipitation annually while اوسلوreceives only 760 mm, and Skjåk, a municipality situated in a deep valley, receives only 280 mm.

أفريقيا

• The windward side of the island of مدغشقر, which sees easterly on-shore winds, is wet tropical, while the western and southern sides of the island lie in the rain shadow of the central highlands and are home to thorn forests and deserts. The same is true for the island of ريونيون.

• تشكيل جبال أطلس has been deemed at least partially responsible for the climatic change which eventually created the Sahara. There is a strong rain shadow effect to the south side of the mountains.

اوقيانوسية

• كاليدونيا الجديدة تقع على مدار الجدي، بين خطي عرض 19° و23° جنوب. The climate of the islands is tropical, and rainfall is brought by trade winds from the east. The western side of the Grande Terre lies in the rain shadow of the central mountains, and rainfall averages are significantly lower.
• هاواي also has rain areas of the islands being desert, much to the surprise of many tourists.^[] Orographic lifting produces the world's second-highest annual precipitation record, 12.7 meters (500 inches), on the island of Kauai; the leeward side is understandably rain-shadowed. The entire island of Kahoolawe lies in the rain shadow of Maui's East Maui Volcano.
• In New Zealand is to be found one of the most remarkableنطقب:Says who rain shadows anywhere on Earth. On the South Island, the Southern Alps intercept moisture coming off the Tasman Sea. The mountain range is home to significant glaciers and about 6,300 mm (250 in) to 8,900 mm (350 in) liquid water equivalent per year. To the east and down slope of the Southern Alps, scarcely 50 km (30 mi) from the snowy peaks, yearly rainfall drops to less than 760 mm (30 in) and some areas less than 380 mm (15 in).
• In Tasmania, one of the states of Australia, the central Midlands region is in a strong rain shadow and receives only about a fifth as much rainfall as the highlands to the west.
• في نيوساوث ويلز وڤيكتوريا (both states of أستراليا), the Monaro is shielded by both the Snowy Mountains to the northwest and coastal ranges to the southeast. Consequently, parts of it are as dry as the wheat-growing lands of those states.
• Also in Victoria, the area around Port Phillip Bay is in the rain shadow of the Otway Ranges. The area between Geelong and Werribee is the driest part of southern Victoria: whereas the crest of Otway Ranges receives 2,000 milliمترs (79 بوصة) of rain per year, the area around Little River receives as little as 420 milliمترs (17 بوصة) annually, which is as little as Nhill or Longreach.

انظر أيضاً

• Orographic precipitation
• Foehn wind

الهامش

وصلات خارجية

• USA Today on rain shadows
• Weather pages on rain shadows