Orographic lift
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This wave cloud pattern formed off of the Île Amsterdam in the far southern Indian Ocean, due to orographic lift of an airmass by the island, producing alternating bands 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
adiabatically. This cooler air cannot hold the moisture as well as warm air and this effectively raises the
relative humidity to 100%, creating
clouds and frequently
precipitation.
Precipitation
Precipitation induced by orographic lift occurs in
many places throughout the world. Examples include:
* The eastern seaboard of Australia, which faces prevailing easterly winds,
* The mountains of
New Zealand, which faces a prevailing westerly flow, off the
Pacific Ocean.
* 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 per year. These locales are on the side of the
mountains which are in the path of
storm systems, and therefore receive the moisture which is effectively squeezed from the clouds.
Rain shadowing
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A lenticular cloud in New Mexico. |
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Wave clouds forming over Mount Duval. |
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Koryaksy volcano, Kamchatka, Russia, showing banner clouds streaming to the right from the peaks. |
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Chinook arch in Calgary, Alberta, November 19, 2005 |
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A view of the Front Range of the Rockies capped by a foehn wall. |
Main article: Rain shadow
The highest precipitation amounts are found slightly upwind from the prevailing winds at the crests of mountain ranges, where the relief and therefore the upward lifting is greatest. As the air descends the lee side of the mountain, it warms and dires, 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 can be as low as 8 inches (200 mm) per year.
Areas where this effect is observed include:
* The
Atacama Desert in
Peru and
Chile.
*
Switzerland's
Rhone valley.
* Areas east of the
Cascade range in the Pacific Northwest (
Washington and
Oregon).
* Areas east of the
Olympic Mountains in Washington state.
* The
Hawaiian island of
Kauai.
*
California's
Central Valley.
* The
Great Basin.
Atmospheric waves
As air flows over mountain barriers, orographic lift can create a variety of wave effects, which produce vertical air motion. If the air mass is close to the dew point, the waves may show as a variety of leeward clouds
:
*
Lenticular clouds are stationary lens-shaped clouds that form at high altitudes, normally aligned at right-angles to the wind direction. Orographic lifting creates a wave which creates the condition for cloud formation.
* 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.
*
Wave clouds are lenticular clouds, created when an
air mass passes over a geographic feature and a standing wave forms downwind.
* A
banner cloud is a cloud that forms downstream from the upper lee 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 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.
[Example of a banner cloud forming in the lee of the Matterhorn.].
* A
foehn wall is an extensive cloud formed along and parallel to the ridge line. The wall appears stationary, while the wind flows through; moisture condesenses on the upslope and evaporating when it descends the lee slope. When viewed as one faces it, it often appears to have an abrupt wall like edge. The foehn wall is a common feature along the
Front Range of the
Colorado Rockies.
* A
chinook arch cloud forms above a mountain range, usually at the beginning of a chinook wind as a resulting 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.
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
Föhn wind, locally known as the
Chinook wind,
Bergwind or
Diablo wind 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 winds, are examples where orographic lifting has limited effect since there is limited moisture to remove in the
Saharan or other air masses; the Sirocco, Bora and Santa Ana are driven primarily by compression heating.
*
Orography*
Rain shadow are a result of moisture removal by orographic lifiting.
