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A gravity wave cloud pattern--analogous to a ship wake--in the downwind zone behind the Île Amsterdam, in 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.
Table Mountain, Cape Town, South Africa. The cold Atlantic air mass flows up over the north western face to 3,500 feet (1,100 m) above sea level and is met by the warm Indian Ocean air mass from the south eastern back side of the mountain forming the famous "Table Cloth".
The Front Range Foothills of Northern Colorado - west of Boulder to Golden as storms pass by. Winter storms can produce 5-6 feet (1.5-1.8 m) of snow.
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.
North East England is in the rain shadow of the Pennines, this combined with Britain's prevailing wind from the South West. This explains the significant differences between the rainfall in the North West and North East.
The Monaro and South Coast regions in South Eastern Australia; as snow-bearing westerlies are forced upwind the inland slopes of the Snowy Mountains, the coastal plain remains dry, and is significantly warmer than on the inland slopes at equivalent altitudes. This is evident when comparing Batlow on the windward slopes to Cooma on the leeward coastal plain, both around 800 metres (2,600 ft).
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.
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. 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.
The leeward edge of an extensive mass of orographic clouds may be quite distinct. On the leeward side of the mountain, the air flowing downward is known as a foehn wind. Because some of the moisture that has condensed on the top of the mountain has precipitated, the foehn (or föhn) is drier, and the lower moisture content causes the descending air mass to warm up more than it had cooled down during ascent. 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.:676–677 A foehn wall is a common feature along the Front Range of the ColoradoRockies.
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, i.e. 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 12,000 metres (39,370 ft).
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 it 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.