how high do hills/mountains have to be to create serious mountain waves that should be avoided?

Is there a ball park figure, 1000ft+? or does it just ..."all depend".

This might help.

Wave Theory for Glider Pilots

Wave can be a useful performance enhancement tool as can cloud streets as all soaring pilots know.

Pay attention to any mention of rotor or lee side eddy clouds! To be avoided at all costs!

You can see lenticulars in southern England with the right wind direction (westerly) - certainly weak wave can set up over Oxfordshire as a result of the winds hitting south Wales.

Wave is not exclusive to the immediate vicinity of big mountains.

But wave strength & height is certainly related to wind strength & mountain height - hence Scotland which gets uninterrupted airflow from across the Atlantic + the Cairngorms can equal wave to +35 000 feet in the Aboyne area. But there are other known good wave sites for gliders in Yorkshire (Sutton Bank), the Long Mynd (Shropshire) and Talgarth (South Wales) to name but a few. The general indications for the UK are steady westerly winds. The wave "seasons" for gliding tend to be October - May but it can set up at any time of the year. 500 - 800 feet/minute up or down and very strong turbulence at the bottom in the rotor are all possible.

In extreme conditions the rotor seems to be able to get down to ground level and certainly down wave has been known to cancel out what should have been good ridge lift at the Long Mynd.

But if you know where and when to look for it, it's great! (21 000 feet best so far)

Very true Guppy and thanks to pilots like yourself for spending the time putting your experence into print.

A few years ago I was going to Crete, straight across the Alps from Swizerland, with the Alpine route being direct from Wangen-Lachen LSPV to Corfu LGKR.

I found tons of stuff on "mountain flying" which, to most people, seem to mean flying inside the Alpine canyons, often below cloud, and the emphasis is to avoid the mountain waves and other turbulence, while not getting into a canyon too narrow to turn around in.

However I was going straight over the top of the Alps. As is normal in VFR, a route had to be planned OCAS and in this case it was FL130 because above that was Swiss Class C airspace (and as it turned out the Swiss would not let me into it) so I would be flying only about 1500ft above the mountain peaks. It was obviously important for there to not be much turbulence due to wind.

I found very little material on how high one needs to be to do this. The best I found was that one needs to be 1000ft above the peaks, for every 10kt of wind flowing across them, to be sure any downdraught is no bigger than a few hundred fpm. It has proved to be a good rule.

Ovee Wales etc it works OK because there isn't much low level airspace there so one can fly high.

On a recent flight in Spain I was crossing the Pyrenees, on a track of about 040, towards Biarritz LFBZ, from Granada LEGR. There was about 20kt of wind flowing across the terrain. I could not get above 17500ft no matter what I did, whereas normally the TB20 will go to 20,000 without any great trouble. Quite obviously there was a descending airflow of about -200fpm... not much but bad enough if trying to stay above the cloud and above icing. In this case the terrain was about 8,000ft below!

The above was an IFR flight so not limited by CAS but I would never again fly across mountains VFR, when limited by airspace above, through which a transit can potentially be denied. It's a way to get snookered, and ATC won't give a damn until you declare a mayday.

In a mountain wave, vertical currents can exceed several thousand feet per minute, and can extend twenty or thirty thousand feet above the mountain, as well as several hundred miles down range. Typical magnitudes are less, in the order of 1,000 to 1,500 fpm, but they can be higher.

When approaching a ridgeline close to the ridge altitude in a light airplane, the 10 knot/1000 ft. rule is a good one.

I've spent a great deal of time approaching ridges from about 15 feet above the ridge, in strong winds...knowing where to go in the even that downdrafts and rotors exceed aircraft capabilities is far more important than having sufficient altitude above the ridge. Having that altitude is more about giving you options for turning and exiting, than it is for being in an area where updrafts and downdrafts won't affect you. You'll have to fly far higher than that to escape the effects of a mountain wave.

In response to astir 8's post,

Westerlies still produce excellent wave in Scotland, however, some of the most extreme wave we have experienced in the gliding world here in recent years is actually produced in southerlies. To the north of the Cairngorms and towards the Inverness area we have managed fantastic wave climbs. I have enjoyed several 20k plus climbs and excellent cross countries. A number of diamond heights have now been claimed in this area, with wave systems seen working even higher(possibly 40k plus) towards the west coast. On most occasions, a 2000feet aerotow is more than sufficient to contact the wave.

I think Nibbler was saying that is was taught in ground school, but that was his first practical experience of a mountain wave. And if you're training in flatland, what do you do? Take the student on a 400 mile navex to some hills in the hope of catching a good example of a mountain wave?

How are mountain waves "taught throughout the flight training" in East Anglia?

Other than what I learnt from the oxford CD rom package I don't recall mountain wave coming up at all in my course. I suppose the instructors are going to focus on what they see as relevant. On the other hand learning in essex, CAS was a very topical discussion.

Do we have mountain in southern England :}

Last mountain waves I came across were in Southern California when the Santa Ana's were blowing.....40-60 kts kts across the 11000 peaks. Everyone [as in every commercial airliner and everyone airborne] was reporting severe turbulence.

I learned to fly at Oxford, mountain flying was not a real consideration and waves were only briefly mentioned in Met.

When I came to Canada and started flying again I was shocked to find I was below the tops of the mountains, very strange sensation for us flatlanders. Then people started mentioning mountain flying courses etc. Several accidents around here in the last seven or eight years attributed vaguely (not usually much evidence after the event) to wave action.

Getting transition training in my amphibian from a local sightseeing pilot we were flying thirty or forty feet above hill top lakes at pretty well the aircraft`s ceiling and encountered turbulence as we flew back over the valleys (Slow down, slow down!) I fly nice days, low winds only, with a relatively low climb cabability I have already seen 400ft per min updraft on final and do not want to find its evil twin.

There are several people doing mountain flying instruction in BC. I am planning on getting some before I get more adventurous.

In August I was flying westwards approaching the Rockies just north of Salt lake City when I hit mounatain wave. I was 2,000ft above the highest peak and approaching at the recommended 45 degree angle, my VSI maxed out downwards and I turned tail and fled to my alternate airport on the east side of the range. It was a clear day and there were no clouds to provide warning, my alternate airport was reporting calm winds.

My theory may be a little bit rusty, but wave generally requires:

A stable layer low down (i.e. a temperature inversion)
Barrier (mountain range etc.) which protrudes above the inversion.
Ideally a steadily increasing windspeed with altitude above the layer (laminar flow)
Wind direction within 45deg either side of perpendicular to the barrier

The waves set up parallel to the barrier (not perpendicular to the wind) - the inversion allows the wave to 'bounce' back up on the downwind side of the barrier. You'll find wave is extremely smooth. (again, the rotor underneath the wave is not..) They are also geographically static - they do not blow downwind. If there are a series of barriers, like another mountain range 1 wavelength downwind, even relatively small barriers can re-inforce to produce huge wave (NZ being a good example)

So to answer liam548, it all depends - You can however predict from the forecast info. You want to look at a skew-T chart, aka a temperature sounding, and the terrain profile around your area, though they're a bit of a specialist subject.

It would seem to make sense that the fastest way out of sink is to turn downwind, perpendicular to the barrier - go upwind and you have a lower groundspeed, and escape it into the rotor. If you parallel it - good luck!

Of course to have massive sink and turbulence downwind of a ridge does not require wave...

Wave orientation is roughly perpendicular to the prevailing wind, but modified by the obstacle causing the wave (and it need not be a mountain). A solitary mountain will have a crescent shaped wave pattern roughly akin to the mountain shape layed down facing downwind...it will curve and be lower in magnitude and closer to the mountain at the lower ends of the mountain, and be farthest with the greatest magnitidue, downwind at a point corresponding roughly with the highest point in the mountain.

Because a mountain isn't a continuous, even obstacle with even neight and gradient, neither are it's effects downwind. Additionally, most of the time multiple mountains exist, and the wave form isn't solitary or a singular repeating wave, but a series of mixed waves more akin the the swells and wave pattens one might see in the surf. Amid these waves can be turbulent rotors which may or may not coincide with the crests and/or troughs of each wave.

Quite correct.

In some cases this is true, in some cases it's not. A wave can form a distinct shear which can produce severe clear air turbulence.

If wind magnitude remains constant, this is true, unless atmospheric properties change (airmass change, frontal passage, etc); the waves do move then. If the magnitude of the wind changes, so do the positions of the waves.

It's common to have calm winds near the surface and strong winds aloft, particularly near or above the peaks, in the Rockies. Also strong winds in and around passes or gaps between mountains or saddles...these act as venturis where you'll find a pressure drop and a wind velocity increase.

Very often the best place to be is near the peak; get on the upwind side and enjoy the lift it provides.

I did a lot of fire flying around SLC, much of it in single engine air tankers. We were often maxed out in load, meaning minimal performance...airplanes capability of doing only one or two hundred feet per minute climb. Less than the Skyhawk. What I did to get to altitude, on my way to a fire, was hug the hill, usually with about half a wingspan's distance from the surface or less, and let the rising air carry me to altitude. Approaches to drops are planned the same way; knowing where the rising air is and where the descending air will be found is a key to coming out the other side of the drop alive.

Same thing in the Grand Canyon. When flying from some of the canyon airstrips in heavy loads, often the airplane didn't have the capability to do better than maintain altitude, initially. Takeoffs would be made which lead to flight over a drop-off of one to three thousand feet, and the rising air at the drop off was then used as the lift necessary to climb out of the canyon. Many of those flights were made in 172's, often from dirt strips in high density altitude situations; successfully operating there was all about finding and using the available orographic lift.

Sometimes if you find yourself approaching the lee or downwind side of a ridge and sinking, you can tuck in closer to the ridge to avoid the sink. This places you into rotors, and you have to know where you're going to go and always have a generous escape path...this is something to try after you've got some training and experience in the mountains...but a wave with descending air is a sure sign that somewhere not too far away is rising air of the same value or better...if you can find it you can really boost your performance, reduce your fuel burn, and even enjoy a quieter ride.

Again, waves or even rotors don't trap you. There's no cage there. If you don't like the performance where you are, fly to a different location and see what you get. Find the lift...it's all about looking for lift.

http://%3Ca%20href=%22http://s389.ph...%22%3E%3C/a%3EA lenticular cloud over southern California

http://i389.photobucket.com/albums/o...g?t=1227593438

Another a couple days earlier!http://i389.photobucket.com/albums/o...g?t=1227594609

Thank you everyone for your advice, links and experiences all of which will add to my (slowly growing) knowledge.

For clarity my FTO and instructors did in fact cover everything (like using the mixture etc) however as others mentioned, some subjects are more relevant to the airfield you are learning at - mountain wave hardly ever affected the airfield and it was never experienced in flight so it was never discussed in any detail. Having said that I now wonder if the topic should be covered better.

SNS3Guppy - couple of great posts on the subject so thank you for your time and effort explaining and in particular the 'what to do if...'!

Getting out of trouble
 

Many years ago I did a flight in the southern french Alps, with an instructor, landing at a few airfields like Barcelonette.

You know how pragmatic the french can be..... He tried to climb close to a ridge but there was a downdraft. He simply said: If we have a downdraft here, there will be an updraft at the other side of the valley, and there was!

It is something that always stuck in my mind regarding valley flying as a potential escape: If there 's a downdraft one side, there must be an updraft on the other side!

How do you find the other side (with the updraught) if the terrain is totally covered in cloud?

The strange thing about the lenticular type clouds is that one often sees them where there is absolutely zero turbulence or wind....