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  • Mountain waves can cause turbulence at high levels in the atmosphere, typically around common cruising altitudes for large aircraft. This turbulence can be severe and even extreme and is a significant threat to aviation as it can cause abrupt changes in altitude and speed of aircraft.
  • Mountain waves cause very strong winds, closer to the mountain ridges and downstream of the mountains. These strong winds can cause speed wind shear (a sudden change in wind speed) downwind of the mountain barrier, which can be hazardous to aircraft.

About

Definition

Mountain waves are a type of mountain induced turbulence that occurs at or near the tropopause. They are caused by strong winds that encounter a barrier such as a mountain range, which forces the air upwards. As the air rises, it experiences compression and expansion, resulting in the formation of alternating regions of upward and downward motion. These upward and downward motions move vertically and horizontally, creating a series of waves in the atmosphere.

Mountain waves are more correctly called Vertically Propagating Mountain Waves, but are abbreviated to Mountain Wave or MTWV in aviation. They occur at similar altitudes to clear air turbulence however they are directly related to a mountain barrier so are stationary with respect to that barrier. They can occur with downslope windstorms, like chinooks, and may happen directly above a region of lower-level mechanical or lee wave turbulence.

Associated terms coming soon:

An inversion is a term also associated with mountain waves that will be coming soon to the Aviation Meteorology Reference.

Associated Terms

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Mountain Wave

Visualization

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Dissipation

Mountain waves dissipate when either the stability profile changes, or the winds weaken or change direction. Mountain waves can accompany a passing low-pressure system, with conditions that change as the system advances and winds shift.

Duration

Mountain waves can last for several hours and sometimes longer depending on several factors such as atmospheric conditions (wind speed, wind direction and stability of the atmosphere), topography (size and direction of the mountain range), and the strength of the mountain waves themselves.

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The main challenge with forecasting mountain waves is evaluating if the wind speed, direction, and stability will be the right combination to produce mountain waves that are able to travel vertically. If so, whether conditions will be enough for severe turbulence that would warrant a SIGMET is another challenge.

Mountain waves are not indicated on the GFA. They can occur with lee waves and downslope windstorms, so the presence of lee wave on the GFA is an indication that mountain waves could occur. Because they can accompany lee waves, a severe mountain wave SIGMET might be stacked on one for lee waves. An example of a mountain wave SIGMET is provided under the SIGMET tab.

MAIN CONCERNS

Turbulence is the main hazard associated with mountain wave activity. Loss/gain of speed and/or altitude, difficulty in maintaining aircraft stability, discomfort among passengers/crew, and potential damage to the aircraft in cases of severe or extreme turbulence. 

Service Providers

Operations Duty Managers

  • Near the Canadian Rockies, mountain wave turbulence can have a significant impact at almost all levels. This includes increased workload for ATC due to issuing turbulence reports and finding alternate altitudes.
  • Edmonton Operations Duty Managers will utilize the Canadian turbulence forecast, SIGMETs and/or PIREPs to understand where the problem areas are.
  • Mountain wave activity is far less frequent in eastern regions than western regions and thus is less of a focus point for other Operations Duty Managers.
  • Montreal Operations Duty Managers may be on the lookout for MWA impacting ATC controlling far northern Quebec airspace. No impact on CYUL traffic, but aircraft can see delays if changes in altitude or lateral deviations are required to avoid severe conditions.

Mountain waves are a serious threat to aviation safety, and FSS keep a sharp out for forecast and observed conditions that might support the development of MWA. 

MWA commonly generate both updrafts and downdrafts that exceed the design limitations of light and medium aircraft, leading to unintended flight into terrain or even structural failure of the aircraft itself. For IFR aircraft in Class A airspace over the Rocky Mountains, MWA commonly generate turbulence that impacts crew fatigue levels by demanding manual flight (versus Autopilot) and can also seriously affect cabin crew operational safety and passenger comfort. 

FIC

Other than telling a pilot that mountain waves (and all associated hazardous phenomena) are occurring, a briefer might suggest alternate routing through the mountainous area, if the pilot is willing to entertain suggestions. Of course, the extent of strong flow aloft must be considered prior to offering this service, as a detour around a very large airmass may be impractical for the pilot of a light aircraft.

AAS

Advisory specialists at sites in the vicinity and downstream of mountainous terrain will be on the lookout for lenticular and roll clouds that are a sure sign of an existing mountain wave. This information will always be disseminated in METAR/SPECIs.

NTMU places less importance on MWA unless we are advised of specific weather conditions at major airports that would affect capacity.

  • This may spark some questions to the meteorologist about possible turbulence created in sectors that hold for major airports.

Enroute, MWA does not automatically lead to action for the NTMU until another agency makes the request for reroutes and/or activity is directly impacting an aircraft’s ability to fly through affected areas.

High level FL290 and +: Orographic turbulence is by far the one that increases the workload the most.

  • ATC are briefed on the potential for MWA weather briefings. However, MWA can be unpredictable on the ATC side and at times we learn of their existence only with the first reports of the day.
  • This turbulence increases workload significantly. It requires more coordination with the neighboring sectors and above all, a lot more transmissions over frequencies. Transmissions are often associated with variations in speed, or even altitude.
  • Depending on its severity, on rare occasions, ATC may be forced to increase vertical separation between aircraft and/or close part of the airspace to avoid poor flying conditions for users.

Users

Turbulence in the lower altitudes can be of highest concern as aircraft are near the ground in a low energy state. Generally, MWA is determined from using GFAs however there are some weather savvy individuals who might identify a risk of it by looking at FDs or looking for signs on satellite pictures. When MWA is sufficient enough to trigger a SIGMET, graphical products (like HubWX) are used to identify where and to what extent the hazard exists.

MWA tends to impact operations in Central and Southern Alberta. Calgary tends to be most affected in the winter months.

  • Dispatchers will pass along potential or forecast MWA to crews for situational awareness and resultant tactical adjustments in flight.

There are some cases where turbulence may extend far enough to the east as to cause difficulties for arrivals and departures out of YYC which can be problematic:

  • ATC has a number of preferred routes in and out of major airports and deviations from this require increased coordination and workload for DXs, pilots and ATC.
  • Oftentimes, track mileage will increase as a result, increasing fuel burn and cost.

MWA activity can generate higher level turbulence where GFAs are referenced, the Canadian turbulence forecast as well as: high-level significant weather charts and current SIGMETsOpen a new window. 

Significant role in pilot flight planning, in collaboration with flight dispatchers.

  • Mostly trying to avoid forecast mountain wave activity, either by reroutes or varying flight levels.
  • PIREPs, SIGMETs, and ATC reports are relied upon heavily and a big help to understanding where the worst conditions are occurring.

It will be assessed first by our dispatcher, supplemented with internal PIREPs, similar products but from private vendors (when available), and more specialized products to confirm the extent and the severity. The crew will then make their own assessment based on the information available to them. Filing a route which could be 100nm off course to avoid the area doesn’t increase the total flight time by much for a medium to long haul flight and will only result in a minor increase of the flight time. Most often, the problem is mitigated at the flight planning stage.

Mountain wave activity (MWA) can affect flights at altitude in the lee of the Rocky mountains. In mountain wave activity aircraft can become unstable, making it hard to maintain assigned altitudes, seeing loss/gain of 500ft at times. These conditions require a climb or descent to more stable air or a reroute out of the area,

Mountain wave activity is common descending into CYYC from the west when the upper winds are strong. During strong upper wind conditions, it is preferable to delay the descent as long as possible, ensure the cabin is prepared early and that all passengers and crew members are in their seats with seat belts fastened at the top of descent.

ATC will usually pass along turbulence reports from prior flights. For mountain wave activity in the lower levels of the atmosphere, ATC will work with us to keep us out of the worst air as long as possible before starting a descent into an airport such as CYYC.

  • Strong upper winds and pilot and ATC reports are indicators of mountain wave activity.
  • In the rare cases where an area of MWA cannot be avoided, some of the more specialized products will be used. One such product is the Turbulence Auto-PIREP System (TAPS). Aircrafts equipped will send live ride conditions automatically. It will then be available for use by our dispatchers and also us, through our weather briefing app. As with turbulence, we will use multiple sources. The stronger the correlation between all of those sources, the more likely we will tend to mitigate the problem by avoiding the area, at the planning stage.
  • If we witness lenticular clouds or rotor clouds, it will be our strongest indication to actively avoid the area. Our second most reliable source of information will be PIREPs. Not all PIREPs are being transmitted to FICs, although if there are reports given verbally to the air traffic controller, those reports will be passed on by the controller to the FIC. If our dispatcher deems them relevant to our route and altitude, those PIREPs will be relied upon and passed to us.
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  • As a rule of thumb, general aviation pilots are educated and aware to be extremely cautious of MWA due to the possibility of severe turbulence. Aside from that, and the awareness of lenticular and rotor clouds, they don't have much knowledge of detailed conditions.
  • It is commonly recommended that pilots flying in or near mountainous regions should undergo a mountain flying course to cover this and other related topics.