thumb|320 px|A [[gravity wave cloud pattern—analogous to a ship wake—in the downwind zone behind the Île Amsterdam, seen from above over the far southern Indian Ocean. The island generates wave motion in the wind passing over it, creating regularly spaced orographic clouds. The wave crests raise and cool the air to form clouds, while the troughs remain too low for cloud formation. Note that while the wave motion is generated by orographic lift, it is not required. In other words, one cloud often forms at the peak. See wave cloud.]]

Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. Orography is the study of the topographic relief of mountains. As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and create clouds and, under the right conditions, precipitation.

Rain shadowing

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 can be as low as 8 inches (200 mm) per year. This sharp decrease in moisture is caused by adiabatic warming, which inhibits condensation and thus enhances the rain shadow effect.

Areas where this effect is observed include:

  • The Himalayas block moisture from the Tibetan Plateau
  • The Atacama Desert in Chile
  • The Argentine side of Patagonia, or southern Argentina
  • 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, (i.e. Sequim, Washington) Conversely, if the polar front or rain event arrives from the south-east (the Tasman Sea), then the coastal plain will be on the windward side and the inland slopes are on the leeward side.
  • The Judean Desert in the Land of Israel and the Dead Sea.
  • The Southern Alps of New Zealand

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 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 (adiabatic) compression heating.

Associated clouds

As air flows over mountain barriers, orographic lift can create a variety of cloud effects.

  • thumb|[[Lenticular cloud over Arenal Volcano]]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 wind is strong, a banner cloud is formed downwind of the upper slopes of isolated, steep-sided mountains. It is created by the low pressure areas in the downwind vortices drawing in relatively humid air from the lower slopes of the mountain. This reduction in pressure compared to the surrounding air increases condensation, in the same manner as an aircraft's wingtip vortices. The most famous such cloud forms routinely in the lee of the Matterhorn.

thumb|center|800px|A view of the [[Front Range of the Rockies capped by a föhn wall.]]

See also

  • Atmospheric convection
  • Weather front
  • Lake-effect rain

References

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