Definitions

rotor-cloud

Orographic lift

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 can, which effectively raises the relative humidity to 100%, creating clouds and frequent precipitation.

Effects of orographic lifting

Precipitation

Precipitation induced by orographic lift occurs in many places throughout the world. Examples include:

Rain shadowing

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 can be as low as 8 inches (200 mm) per year.

Areas where this effect is observed include:

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.

  • 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 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..
  • 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 Rockies.
  • 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 clouds are stationary lens-shaped clouds that are formed downwind of mountains by lee waves if the air mass is close to the dew point. They are normally aligned at right-angles to the wind direction and are formed at altitudes up to .
  • 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.
  • A 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.

References

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