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It forms above the mountain range, usually at the beginning of a chinook wind as a result of orographic lifting over the range. It appears when seen from downwind to form an arch over the mountain range. A layer of clear air separates it from the mountain. [3] A view of the Front Range of the Rockies capped by a föhn wall.
Earth systems across mountain belts include the asthenosphere (ductile region of the upper mantle), lithosphere (crust and uppermost upper mantle), surface, atmosphere, hydrosphere, cryosphere, and biosphere. Across mountain belts these Earth systems each have their own processes which interact within the system they belong.
Orographic or relief rainfall is caused when masses of air are forced up the side of elevated land formations, such as large mountains or plateaus (often referred to as an upslope effect). The lift of the air up the side of the mountain results in adiabatic cooling with altitude, and ultimately condensation and precipitation.
Effect of a rain shadow The Tibetan Plateau (center), perhaps the best example of a rain shadow. Rainfalls from the southern South Asian monsoon do not make it far past the Himalayas (seen by the snow line at the bottom), leading to an arid climate on the leeward (north) side of the mountain range and the desertification of the Tarim Basin (top).
The orographic effect causes rainfall when moist air originating in the Gulf of Mexico and western Atlantic Ocean is forced upwards by the mountains. [29] [30] Winter and spring months see a gradient precipitation pattern, with higher rainfall concentrated in the south. [29]
When the moist air meets mountains, it rises and upon subsequent cooling, its moisture condenses and is released as rain, typically heavy rain. [48] The wet summer monsoon winds cause precipitation in India and all along the layered southern slopes of the Himalayas. This forced lifting of air is called the orographic effect. [48]
Altadena averages 21.09 inches (535.7 mm) of rain a year, over 6 inches (150 mm) more than nearby Los Angeles due to the orographic effect created by the San Gabriel Mountains. Because of the slope on which the city is built, sewer lines in the city's northern section have been known to overflow significantly.
The causes of the Foehn effect in the lee of mountains (adapted from: [1]) Dissolving Föhn clouds over Cumbre Nueva, La Palma, at an elevation of 1,400 m (4,600 ft) A Foehn, or Föhn (UK: / f ɜː n /, US: / f eɪ n / fayn, [2] [3] US also / f ʌ n, f ɜːr n / fu(r)n [4] [5]), is a type of dry, relatively warm downslope wind in the lee of a mountain range.