Orographic lift 377177 225630455 2008-07-14T17:16:31Z 170.213.132.253 /* Effects of orographic lifting */ [[Image:Wave cloud.jpg|thumb|320 px|This wave cloud pattern formed off the [[Île Amsterdam]] in the far southern [[Indian Ocean]], due to orographic lift of an airmass by the island, producing alternating bands&ndash;([[Standing wave]]s), of condensed and invisible humidity downwind of the island as the moist air moves in vertical waves and the moisture successively condenses and evaporates.]] '''Orographic lift''' occurs when an [[air mass]] is forced from a low [[elevation]] to a higher elevation as it moves over rising terrain. As the air mass gains [[altitude]] it expands and cools [[Adiabatic cooling|adiabatically]]. This cooler air cannot hold the moisture as well as warm air can, which effectively raises the [[relative humidity]] to 100%, creating [[cloud]]s and frequent [[precipitation (meteorology)|precipitation]]. ==Effects of orographic lifting== ===Precipitation=== Precipitation induced by orographic lift occurs in [[Rain shadow#Regions of notable rain shadow|many places throughout the world]]. Examples include: * The Mountains near Baja California North - specifically La Bocana. * The eastern seaboard of Australia, which faces prevailing easterly winds. * The mountains of [[New Zealand]], which faces a prevailing westerly flow off the [[Tasman Sea]]. * The southern [[Andes]], which faces a prevailing westerly flow off the [[Pacific Ocean]]. * The [[Northwestern United States]] and [[Canada]] ([[Oregon]], [[Washington]] and [[British Columbia]]) see prevailing westerly flow off the northern [[Pacific Ocean]]. Places on the sea-facing side of coastal mountains see over 100 inches (over 2.5 m) of [[precipitation (meteorology)|precipitation]] per year. These locales are on the side of the [[mountain]]s which are in the path of [[storm]] systems, and therefore receive the moisture which is effectively squeezed from the clouds. * The [[ski country]] region of [[New York]] and [[Pennsylvania]], particularly with [[lake effect snow]]s. * The [[Ecoregions of Madagascar|Eastern seaboard of]] [[Madagascar]]. ===Rain shadowing=== [[Image:New-Mexico-Lenticular.jpg|thumb|200px|right|A [[lenticular cloud]] in New Mexico.]] [[Image:Orographic lifting of the air - NOAA.jpg|thumb|200px|right|A [[cap cloud]].]] [[Image:Tadrart01.JPG|thumb|200px|right|[[Wave cloud]]s.]] [[Image:Koryaksky Volcano.jpg|thumb|200px|right|Koryaksy volcano, Kamchatka, Russia, showing banner clouds streaming to the right from the peaks.]] <!-- Deleted image removed: [[Image:chinook19.11.05.JPG|thumb|200px|right|Chinook arch in Calgary, Alberta, November 19, 2005]] --> [[Image:Mountains from westlands.jpg|thumb|400px|right|A view of the Front Range of the Rockies capped by a föhn wall.]] :''Main article: [[Rain shadow]]'' The highest precipitation amounts are found slightly upwind from the prevailing winds at the crests of mountain ranges, where they relieve and therefore the upward lifting is greatest. As the air descends the lee side of the mountain, it warms and dries, creating a rain shadow. On the lee side of the mountains, sometimes as little as 15 miles (25 km) away from high precipitation zones, annual [[precipitation (meteorology)|precipitation]] can be as low as 8 inches (200 mm) per year.<ref name="Whiteman">{{cite book|author=Whiteman, C. David|title=Mountain Meteorology: Fundamentals and Applications |publisher=Oxford University Press|year=2000|id=ISBN 0-19-513271-8}}</ref> Areas where this effect is observed include: * The [[Himalayas]] block moisture from the [[Tibetan Plateau]]. * The [[Atacama]] Desert in [[Peru]] and [[Chile]]. * [[Switzerland]]'s [[Rhône River|Rhone valley]]. * Areas east of the [[Cascade range]] in the Pacific Northwest ([[Washington]] and [[Oregon]]). * Areas east of the [[Olympic Mountains]] in Washington state. * Areas east of the [[Sierra Nevada]], (the [[Sierra Nevada]] are the eastern limit of [[California]]'s [[Central Valley (California)|Central Valley]]). **[[Geography of the United States Pacific Mountain System]]. **[[Pacific Cordillera]]. * [[California]]'s [[Central Valley (California)|Central Valley]]. * The [[Canadian Prairies]]. * The [[Hawaii|Hawaiian]] [[island]] of [[Kauai]]. * The [[Great Basin]]. ===Leeward winds=== Downslope winds occur on the leeward side of mountain barriers when a stable air mass is carried over the mountain by strong winds that increase in strength with height. Moisture is removed and latent heat released as the air mass is orographically lifted. As the air mass descends, it is compression heated. The warm [[foehn wind]], locally known as the [[Chinook wind]], [[Bergwind]] or [[Diablo wind]] or "Nor-Wester" depending on the region, provide examples of this type of wind, and are driven in part by latent heat released by orographic-lifting-induced precipitation. A similar class of winds, the [[Sirocco]], the [[Bora]] and [[Santa Ana wind]]s, are examples where orographic lifting has limited effect since there is limited moisture to remove in the [[Sahara]]n or other air masses; the Sirocco, Bora and Santa Ana are driven primarily by ([[adiabatic]]) compression heating. ===Associated clouds=== As air flows over mountain barriers, orographic lift can create a variety of cloud effects. * ''Orographic fog'' is formed as the air rises up the slope and will often envelope the summit. When the air is humid, some of the moisture will fall on the windward slope and on the summit of the mountain. * When there is a high wind, a ''banner cloud'' is formed downwind of the upper slopes of isolated, steep-sided mountains. This cloud is similar to the condensation observed off the tips of high-performance aircraft wings when they operate in humid conditions; it is created by the [[vortex|vortices]] and local uplifting in the air caused by the orographic lifting as the wind passes the mountain. The most famous such cloud forms routinely in the lee of the [[Matterhorn]].<ref name="Whiteman" /><ref>[http://www.atmos.washington.edu/gcg/Atlas/phot_oro03.html Example of a banner cloud forming in the lee of the Matterhorn.]</ref>. * The leeward edge of an extensive mass of orographic clouds may be quite distinct. On the leeward side of the mountain, the air flows downwards. Because some of the moisture has condensed on the top of the mountain, the leeward air is drier. The remaining moisture quickly evaporates as the air descends. The distinct cut-off line which forms along and parallel to the ridge line is sometimes known as a ''foehn wall''-(or föhn wall). This is because the edge appears stationary and it often appears to have an abrupt wall-like edge. A foehn wall is a common feature along the [[Front Range]] of the [[Colorado]] [[Rocky Mountains|Rockies]].<ref name="Whiteman" /> * A ''Rotor cloud'' is sometimes formed downwind and below the level of the ridge. It has the appearance of the ragged [[cumulus]] cloud type but it is caused by a turbulent horizontal vortex, ie the air is very rough. * ''[[Lenticular cloud]]s'' are stationary lens-shaped clouds that are formed downwind of mountains by [[lee waves]] if the air mass is close to the dew point<ref name="Whiteman" />. They are normally aligned at right-angles to the wind direction and are formed at altitudes up to {{convert|12000|m|ft|0}}. * A ''cap cloud'' is a special form of the lenticular cloud with a base low enough that it forms around and covers the peak, capping it.<ref name="Whiteman" /> * A ''[[Chinook wind#Chinook arch|chinook arch cloud]]'' is an extensive wave cloud. It has this special name in North America where is is associated with the [[Chinook wind]]. 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.<ref name="Whiteman" /> ==References== <!--See http://en.wikipedia.org/wiki/Wikipedia:Footnotes for an explanation of how to generate footnotes using the <ref(erences/)> tags--> <references/> [[Category:Climate forcing agents]] [[es:Nube orográfica]] [[fr:Onde orographique]] [[it:Sollevamento orografico]] [[no:Orografisk heving]] [[nn:Orografisk heving]] [[fi:Orografinen pilvi]] [[sv:Orografisk nederbörd]]