Fohn Wind

A Föhn wind occurs when a deep layer of prevailing wind is forced over a mountain range. As the wind moves upslope, it expands and cools, causing water vapour to precipitate out. This dehydrated air then passes over the crest and begins to move down-slope. As the wind descends to lower levels on the leeward side of the mountains, the air heats as it comes under greater atmospheric pressure creating strong, gusty, warm and dry winds. Föhn winds can raise temperatures as much as 30°C in just a matter of hours. Winds of this type are called "snow-eaters" for their ability to make snow melt rapidly. This ability is based not only on high temperature, but also the low relative humidity of the air mass. Föhn winds are also associated with the rapid spread of wildfires, making some regions which experience these winds particularly fire-prone. The natural occurrence is when a weather system moves across a mountain range.

1. Relatively stable, mild (e.g. 20 °C) but moist air starts at sea-level and has to cross a 10,000ft mountain range situated parallel to it. The air is now forced to ascend and to glide up and over the windward mountain slopes. While initially decompressing and thus cooling at the dry adiabatic lapse rate of 3oC/1000ft (DALR), the air will eventually become saturated and forming visible condensation. Thus clouds will form and rain is setting in. The height where this happens is called condensation-level (at 2000ft AMSL using example).
2. Condensation releases heat and the further lifted air will cool somewhat slower now, at the typical saturated lapse rate of 1.5 °C/1000ft (SALR). This is called the wet or saturated adiabatic lapse rate. Once over the mountain crest, the air will also have reached its lowest temperature - somewhat around 1°C in our example.
3. A so-called lee wave forms downwind of the obstacle. The downward motion of the initial wave forces the now-dry air to plummet from relatively high levels to the foothills, now warming by compression at the dry adiabatic lapse rate. Thus for every 1000ft drop in elevation, temperature raises at the DALR 3 °C again. The wind speed increases and the vigorous Föhn storm can easily reach gale force.
4. Air temperature on the lee-side is now significantly hotter than the wind ward-side of the mountains.

A Föhn wind like situation can last from less than an hour to several days. The high crest of the wave creates a distinctive elongated cloud parallel to the mountains, known as a Föhn wall. Further wave crests more distant to the obstacle form the popular lenticularis clouds.

Mountain Waves

The development of features often associated with a mountain wave system.

Notice the wind flow, with a strong component perpendicular to the primary ridge line. This is a typical condition for mountain wave development, as is a stable atmosphere. Since the U.S. Standard Atmosphere is stable, favourable conditions for wave development usually exist when there is significant cross-barrier flow. If air is being forced over the terrain, it will move downward along the lee slopes and then oscillate in a series of waves as it moves downstream. Sometimes these waves can propagate long distances in "lee wave trains."

Cap clouds indicate likely wave activity downstream. They often appear along mountain ridges as air is forced up the windward side. If the flow is sufficiently humid, the moisture will condense into a cloud bank that follows mountain contours. Quite often, heavy orographic precipitation occurs on the upwind side of the barrier, particularly for barriers located near the sea. As the flow descends in the lee of the mountain ridge, the cloud evaporates. Viewed from downstream, cap clouds frequently appear as a wall of clouds hanging over the ridge top.

It is important to remember that while cap clouds indicate likely wave activity, their absence does not mean that waves are absent. Under drier conditions, waves may be present without cap clouds.

The vertically-propagating wave is often most severe within the first wavelength downwind of the mountain barrier. These waves frequently become more amplified and tilt upwind with height. Tilting, amplified waves can cause aircraft to experience turbulence at very high altitudes. Clear air turbulence often occurs near the tropopause due to vertically-propagating waves. Incredibly, these waves have been documented up to 200,000 feet and higher.

Vertically-propagating waves with sufficient amplitude may break in the troposphere or lower stratosphere. Wave-breaking can result in severe to extreme turbulence within the wave-breaking region and nearby, typically between 20,000 and 40,000 feet. If a vertically-propagating wave doesn't break an aircraft would likely experience considerable wave action and little turbulence.

At times, strong down-slope winds accompany mountain wave systems. Strong down-slope wind cases are usually associated with strong cross-barrier flow, waves breaking aloft, and an inversion near the barrier top. In extreme cases, such as in our Alps scenario, winds can exceed 100 knots. This may be double or triple the wind speed at mountaintop level. These high winds frequently lead to turbulence and wind shear at the surface, causing significant danger to aircraft and damage at the surface. Down-slope windstorms often abruptly end at the "jump region," although more moderate turbulence can exist downstream. The jump region is an extremely turbulent area that can extend up to 10,000 feet.

Rotors are part of a low-level turbulent zone that often forms in association with a mountain wave system. Rotors are also called horizontal roll vortices because they form a complete rotational pattern, with the axis of rotation parallel to the ground. The low-level turbulent zone is another region of potentially significant turbulence. It exists immediately downstream of the jump region and under a wave crest. Rotor axes typically occur at altitude equal to or below mountain-top level and within 20 nautical miles of the ridge line. Smaller-scale rotations embedded within the low-level turbulent zone can cause rolling that exceeds an aircrafts ability to stay level. This occurs most frequently when development of a convective boundary layer aids powerful upward motions in the jump region.

Rotor location can often be identified if sufficient moisture is available to form an associated rotor cloud. Rotor clouds are found near the top of the rotor circulation and under higher lenticular clouds. Immediately above the rotor cloud, smooth, wavy air is likely.

The rotor cloud can look innocuous, but does contain strong turbulence and should be avoided by pilots.

Lee waves whose energy does not propagate vertically because of strong wind shear or low stability above are said to be "trapped." Trapped lee waves are often found downstream of the rotor zone, although a weak rotor may exist under each lee wave. These waves are typically at an altitude within a few thousand feet of the mountain ridge crest and turbulence is generally restricted to altitudes below 25,000 feet. Strong turbulence can develop between the bases of associated lenticular clouds and the ground.

Lenticular clouds form near the crests of mountain waves. As air ascends and cools, moisture condenses, forming the cloud. As that air descends in the lee of the wave crest, the cloud evaporates. Because air flows through the cloud while the cloud itself is relatively stationary, many people refer to these clouds as standing lenticular.