Mountain Waves

Over the plains air moves more or less horizontally. When air blows against a mountain or a hill, it climbs over its slope and descends on the other side. Thereafter, for some distance it does not flow horizontally even when the ground below has once again become plain. It acquires a wavy motion in a deep layer. The wave is known as a mountain wave or lee wave. A pilot flying along these waves will experience alternating
regions of lift and sink. 

Meteorological Conditions
Pronounced mountain waves form under the following meteorological conditions:

Vertical Currents in Mountain Waves
An aircraft flying on the leeward side of a ridge may experience alternate regions of lift and sink. In powerful waves the associated vertical currents may attain a speed of 40 km/h. While in extreme cases the speed may be 100 km/h.
4. The speed of vertical currents in a mountain wave depends on the amplitude, wavelength and the wind speed. Strong up and downdraughts are favoured by:

Visible Effects
When the atmosphere is comparatively dry, the updraughts in a mountain wave may not result in condensation. The waves then show no visible evidence. On the other hand if the air is humid enough, lens shaped clouds (known as lenticular clouds) may form at the crests of the waves. The shape, horizontal extent and thickness of these clouds depend on the variation of the humidity with height. If moist air overlies very dry air, the clouds are thin and sharp and appear only at the crests. If very moist air overlies very dry air, the clouds may be thick and may extend into the troughs also, except the trough immediately next to the ridge. This is known as the Fohn gap and is the best
visible evidence of mountain waves under such conditions.

Turbulence in Mountain Waves
When the amplitude is small and the wavelength large, flying is very smooth in a mountain wave inspite of changes in altitude due to lift and sink. However, when the amplitude is large and the wavelength small, eddies may be created in the crests and troughs. These eddies may give rise to bumpiness in addition to large-scale lift and sink due to the vertical currents of the wave. When there are several ranges, the mountain wave may be so complex that the turbulence due to eddies may be actually more prominent than the wave motion itself and the flow on the leeward side becomes disorganised upto some level. In the Ladakh area such turbulence is common in the post-monsoon months. Clear air turbulence is more frequent over mountainous terrain than over the plains.

Well-developed eddies under the crests of a mountain wave may give rise to rotor clouds. From the air they look like a line of cumulus clouds. Unlike lenticular clouds they may be associated with violent bumpiness. The base of the clouds is near the peak of the ridge, but the tops may extend much higher and merge with the lenticular clouds.

Aviation Hazards of Mountain Waves
Mountain waves are associated with the following hazards to flying:
(a) Changes in Altitude. These may be large and not necessarily accompanied by bumpiness. If the aviator is unwary and does not keep a good watch over the altimeter readings, the aircraft may sink in a trough dangerously close to mountainous terrain.
(b) Vertical Currents. The stronger downdraughts may prevent a powered aircraft from gaining height, especially when flying parallel to mountain ridge. 
(c) Altimeter Errors. It is known that, apart from physical lift and sink of an aircraft due to vertical current, there are rapid variations of pressure in a mountain wave, which give rise to sizable altimeter errors. These are generally such as to indicate high altimeter readings.
Thus an aviator has to keep sufficient ground clearance to compensate for these errors. Thus the errors increase considerably when the wind speeds are high.
(d) Wind Speed Variations. The up and down motion in a mountain wave is accompanied by large changes in horizontal wind speed. In extreme cases this could lead to stalling of conventional low speed aircraft.
(e) Turbulence. Flying conditions may be exceptionally smooth in many mountain waves, but when the amplitude is large and the wavelength small, localised areas of violent turbulence may exist. The zone of the rotor cloud is the worst in this regard. On many
occasions the air may not be sufficiently humid to result in rotor clouds, severe turbulence then occurs without any visual warning and may be encountered suddenly after a spell of smooth flying.
(f) Icing. The chances of icing in mountain wave clouds are greater for two reasons:
(i) The freezing level has a wavy structure and may be much lower in the wave troughs than the undisturbed one.
(ii) The concentration of liquid water is more in mountain clouds than in similar ones over the plains. 

Recommended Flying Techniques

If it is practicable, flight into an area where pronounced mountain waves are forecast should be avoided. Otherwise the following precautions should be observed:
(a) Maintain a close watch on the altimeter. Remember that the altimeter may over-read in a mountain wave.
(b) Approach the mountain range at a 45° angle rather than directly, particularly when flying upwind, so that quick turn can be made away from the ridge if conditions appear dangerous.
(c) In case of sustained loss of altitude when flying parallel to a ridge, rising air will probably be found by changing course so as to fly a few miles towards or away from the high ground up-wind. If the aircraft is very near the lee slope, the down current is obviously caused by air flowing down this slope; then look for rising air further downstream.
(d) Look for rising currents up-wind of a rotor cloud and also of lenticular clouds if they are near flight level.

(e) Avoid flying into rotor clouds and into lenticular clouds which have torn and irregular edges

(f) Avoid flying on instruments.

(g) Avoid flying into a cap cloud which is an orographic cloud near the top of the ridge and projecting towards the windward slope. This cloud is also known as Fohn wall or Banner cloud.