Last month I did a short but indepth advanced thermaling podcast with Judith Mole.
It doesn’t explain how to find thermals but more how to use them efficiently when you found them.
Once these techniques are honed and mastered you’ll nearly always climb out if it’s possible to do so.
Give it a listen at http://www.judithmole.net/blog/?page_id=123
Dates are firming up now for the Bassano trips so check out the dates page on the main website.
KF
The next few weeks will see a new article for the coming season, how to simply thermal better, the cornerstone of cross-country flying. With out going round in circles efficiently you can almost guarantee a short, frustrating flight. Invest the time and effort in being able to launch exactly when you want and nailing that first important thermal.
Over the years I’ve seen many low airtime pilots and had many on XC courses in the past. However although keen many pilots want to skip the learning to thermal part and crack on with trying to fly long XC’s. Mainly with disappointing results due to being unable to work the second or even first tricky thermal. Whether that be trying to get through an inversion to just simply not turning tight enough close to terrain.
The following tips are designed to help pilots get more feel for the wing and the air.
Take a wrap when thermaling
Taking a wrap in the beginning of your flying career can feel very strange. Some pilots believe it to be dangerous as you are in effect shortening the brake lines (by about 10-15cms) and thus bringing the glider closer to stall. This is true if pilots still fly with their hands down by their hips when thermaling.The reason for the wrap is easy to explain.
When thermaling without ‘wrapping’ pilots can find their hands very low and if you think about what muscles you are using to ‘push’ the brakes down the following explain is simple. When ‘pushing’ the brakes you are using your triceps and lats(side-back muscle), this are very powerful muscles and the more powerful the muscle the less sensitive it is to small changes in pressure. Which is what we are looking for when thermaling, esp. in light thermals.
When the brakes are ‘wrapped’ you’ll instantly notice that your hands are higher than before for the same brake pressure. When the brake is applied now you’ll be using the wrist and bicep muscles, much weaker and great for detecting small/large changes in wing pressure. I also balance the line on my index finger for even more feedback. This is the simple physics of taking a wrap, hands are higher and feedback increases. I know of no local pilots here in Zillertal that don’t use this method of glider control. Even on high aspect gliders with short brake travel. Try it on a calm day and see if it helps, as it may not be for everyone.
Stay loose in the Harness
A big help when learning to thermal better is to stay very relaxed in the harness. I sometimes notice that when the thermals are strong and punchy I may stiffen up. Then I’ll find that the air feels nastier than before. Try to relax and roll with the turbulences. It’ll make the glider & the air feel less aggressive.
So if the conditions are strong and you find yourself sitting bolt upright in your harness. Just sit back and control you breathing. Don’t let the air buffer you from side to side, still maintain some balance and the gliders roll authority. Just make sure that you are still leaning on the side you want to and enjoy the ride. The looser the pilot the less gnarly the air will feel and also as you’re relaxed you will feel the surges and pull of the thermal and climb better.
Keep glider in it’s happy place
It sounds obvious but the only thing holding your wing open and flying above your head is the weight of the pilot underneath it. If the pilot allows the wing to oscillate wildly above there is a greater chance for something nasty to happen as there will be less wing loading on the aerofoil and this will require less turbulence to close the wing.
When entering a thermal there is nothing the pilot can do to stop the surge backwards but a lot can be done to stop the subsequent dive as the glider surges forward to regain it’s airspeed. This is a critical moment for the glider, too far forward and a front stall is on the cards. If the glider is halted on it’s dive forward and kept in it’s happy place (with highest wing-loading) there is much less chance anything will happen. Practicing with your wing in calm air by dropping it back and letting it surge forward you’ll understand how much brake is needed to keep the glider open in the strong spring thermals.
Float the Outside Brake
Floating the outside brake is the technique of not allowing the glider to dive too much when coring or searching for lift. It can be a great help in both light and strong conditions, although when it’s light it really comes into it’s own.
Image encountering a climb whilst gliding along a ridge. If the pilot feels that it’s on the right side and quickly applies right hand brake the glider will turn sharply and the swing through if no outside brake is used, this often results in the pilot ending up lower than he started, esp when light.
A second pilot in the same place pulls the same amount of right brake and as the glider starts to turn applies the outside brake, the turn is not so sharp obviously but more obvious is that the wing does not grab and dive. Once the pilot hits the core he can release and apply as needed to turn sharper or arrest the dive.
Once a pilot gets this wired you’ll notice that even weak thermals become usable. This is what normally separates a good and bad thermal-climber. If the average climb is more than your sink rate and radius of the thermal similar to your turning circle there is no reason that you won’t climb out, with patience and feeling.
Coming next, 2-D maping of the climb you’re in…
Recently was discussing some interesting techniques with Adrian Thomas(long time British Team member), mostly about when does it become necessary to push on to the next thermal if you want to fly fast. It opened my eyes to what other pilots are doing when racing.
In the past I´ve always tried to work out an average for the time of day and if there is a good visible clue to lift closeby when the thermal averager dropped below this I´d move on. I often say to myself “ok, if this drops to below 2m/s average I´ll crack on”, but this has nothing to do with what the climb has been and only if I´m in a good position for finding lift on the next glide. A little too conservative really if you want to crack on and push average speeds.
This works but can be a little too theoretical due to conditions and terrain ahead. If there are inversions and a long valley crossing where arriving high is crucial it´s important to milk the climb as much as possible, as is the case if it´s completely blue out front. The following theory relies on the fact that you know where the next climb is likely to be, either there is a huge cloud forming, birds circling or other glides climbing.
Adrian´s theory also relies on the assumption that if you enter a thermal low down it could be weak, then it normally gets stronger with altitude and then once it loses it´s strength will start to drop off again. So the idea would be to once you are established in a thermal low down you should check you average climb(set it to 30seconds), maybe 1.5m/s. As the climbs strengthens remember this figure as if the climb starts to drop and you have altitude enough to push on that would be the most theoretically efficient time to go, assuming you see your next climb. Then repeat this process along the route.
I´ve been working with my own way of pushing on in weaker lift but it´s different to Adrian´s way of thinking. I´m going to experiement the next chances I get and will report on the results whilst flying with my friends whilst flying around the Zillertal tour or up to the Ahorn Spitz.
One other thing to note also is that if the climb doesn´t drop off from entering to base then leaving whenever you have enough to cross next obstacle should be the way to go. This is more like alpine flying with strong climbs from source to base. Perhaps this is more for flatland flying or especially places like Piedrahita, where I had a terrible time. Mainly because I was pushing on too hard, failing to top up in extremely weak climbs and not mincing around a base. Some of the tasks were flown at extremely slow speeds, one task was 18kmh for the winner around 107km, slooooooow. Austrian Open task 2 97km, average speed 44kmh for winner. Very different types of flying and I gotta say my slow mincing style needs some work!!
When I have some results I´ll post them on this site.
Thanks for reading and hope that you get to experiment with this too with your mates.
KF
Thunderstorms.The other way thunderstorms can be formed is from a heat source, if the pressure, lapse rate and humidity are all favorable. Firstly the sun will warm the earth and produce thermals early on in the unstable air mass. Normally on this sort of day, good cumuli forming early on in the day at around 9am is not a great sign that the day is long lasting.
Inversions in the high level can halt the progress of Cbs but once the thermals become strong enough to break through you can expect a rapid change for the worse. This is because the warm air below can surge through the ruptured inversion into the colder unstable air above.
The Cause for Source Cbs.
Main things needed for to produce a ’source’ thunderstorm are a strong lapse rate, relatively moist air & strong heating of the terrain. If the dew point of a parcel of air is quite high it will produce a lower cloud base, because the temperature drop needed to reach condensation is less. That’s why dry air masses produce higher base days or even blue days.
However something else is happening to these parcels of wet air that makes them more dangerous. When the air turns to cloud as the dew point is reached the chemical reaction of condensation is heat producing. This gives this parcel of air even more energy than it got from just the Sun.
This energy boost also happens when this water vapour reaches the freezing level, giving the thermal/cloud an extra boost that can push it up over 10-12000m. What is dangerous about these extra boosts is the fact that as the air in the cloud rises faster it starts to suck air from below the cloud, known ‘cloud suck’. Once a cloud starts comes alive and starts to pull in huge amounts of air around it it no longer needs the power of the sun to start the next phases, hail, lightening, thunder & then strong rain. It’s this effect that allows Cb’s to roll on through the night, even when the sun is long gone.
The Risks.
The main danger in the Alps from Cbs is not as most pilots would think, getting sucked in. The flying media has reported on many stories of unsuspecting pilots being dragged up to ultra hypoxic levels. Some survived and some were less fortunate.
If a pilot finds him/herself in the pull of one of natures most powerful beasts, there is not a lot you can do about it expect perhaps start praying to God, Buddha or Allah as the outcome will pretty much be as simple as a roll of the dice. Poor flight planning has lead to this point and only luck will get you out. Simple.
Inside the Beast.
Pilots have reported climbs of 30m/s+ inside big clouds and even more impressive sink but scientists have suggested that in the core it could be nearer to 100m/s. This means a person could maintain altitude in freefall, without metres of sailcloth above them. Once the cloud has start to really get going it can enter the next phase, above the freezing level. The smaller rain droplets are frozen and fall out the back of the cloud if the cloud is active then they’ll be recirculated several times freezing and growing in size each time until they become too heavy for the amount of lift available. This will fall to the ground in the form of hail or rain. If a cloud is producing hail there is a huge chance that this up/down flow will cause a huge static electric charge. This is then discharged as lightening, which at 1,000,000degC burns the air and causes thunder, the surface of the Sun is only 6000degC.
Gust fronts in the mountains…
The main danger that I worry about when Cbs start to form is what is happening on the ground or in the valleys. As when the cloud reaches a critical point it will start to rain on the backside on the cloud. This falling wall of water is a physical object and will displace large amounts of air. Not such a problem in the flatlands but a big problems in the mountains. As the valleys systems act a channels and funnels for the 3-400m thick gust front. Simple rule is that big clouds in the Alps should always be treated with respect.
The parts of a thunderstorm.
At the front of the cloud the extra energy the chemical reactions has given to this system will draw in huge amount of air. This is where pilots tend to get into trouble and may have problems escaping the strong lift. As the cloud sucks air in from the front. This air will circulate up the front of the cloud, turn to rain then ice until it starts to descend as rain/hail on the backside. The strength of the up wind will determine how many times this air will circulate as the rain turns to hail, once the hail reaches such a size that the lift can’t support it it will finally fall to Earth. This falling wall of water is what will produce the gust front, a danger on the ground. At the top of the cloud the anvil head spreads out where the atmosphere actually starts to warm with height, this halts the progress and produces the classic spread out of ice crystals you see regularly at altitude. Only clouds in the tropics may challenge this strong inversion.
Some pilots have fancy instruments to work this out but some have more simple means. I always do this in comps so I know when to leave if the last turnpoints are close together. It works most of the time and you can even break it down into complicated legs so that each wind direction is accounted for. The following is just for a average long ish glide in.
Working this end glide to goal is simple enough so you don´t arrive at cloudbase having to spiral down, not good to spiral off 2000m+ when you are tired and busting for a piss.
Measure the distance from a good source that you expect to get one of the last thermals needed to make it. Say 10-15km out on a good day. Next you need to estimate your average glide, say 7:1.
So 1/glide x distance x 1000 = height needed over ground. Then add this to the height of the LZ.
This figure is the height needed to comfortably glide in from 10-15km at a 7:1 glide scope.
This also works if you want to add more kilometers to your flight by on the last leg pushing out to somewhere that might not be working anymore, like the launch site you started from so you can get a close to closing the route as possible.
This works great in comps when the task setter may set a very long final glide from the mountains, then to a shady side across the valley, then to a turnpoint in the valley then finally into goal. Seeing as the shady side would be slow it would be worth calculating the height needed around the whole thing and take that height before working the whole way around. If you´ve been lucky with avoiding nasty sink you should arrive fast and only a couple of hunded metres over the goal line. There is a saftey margin built in due to the 400m radius´ around each turnpoint. This means that you fly 800m less per turnpoint. This can even be worked out in the calculation but you may find yourself landing short, very frustrating but depends how much risk you want to take.
Have fun and try it out yourself even on you local hills on a good day. From the hill to the pub then back to LZ.
KF
You’ve probably heard this before but air behaves very similar to water. So if you watch a fast river, flowing over rocks you’d see two things. A slight acceleration as the water goes over & around and the water tumbling on the back side of the obstacle. Airflows in the mountains behave the exact same way.
As a low-level valley wind is forced up a slope in it’s path it will create potential lift and smooth air on the windward side. This could be soarable with some nice ‘Luv’ side (pron. Loof) thermals. On the back side of this slope you can expect to find that the wind has accelerated, changed direction & will tumble. This is the Lee.
An important thing to note about this falling rotor is that it will pull down large amounts of air from above it. So the bad effects of this Lee will not only be felt locally but higher up as well. In this area of ‘Lee’ there will be mainly sink, rotor and should be avoided as hanging around here will flush pilots to the ground pretty quickly.
The more pronounced the tumble, the higher the sink will extend above the mountain. So stronger winds or sharp change in vertical elevation will greaten this effect. This is particularly bad when flying over steep passes or cols where a strong valley wind is tumbling over. Normally the areas are blue if this effect has set up. The PaĂź Thurn in the Pinzgau is a classic example.
If you are unsure about the effects lower down in the valley you should ask yourself a few questions.
1. Where are you in relation to heat low?(see last months article)
2. Your location in Alps?
3. What time of year?
4. Time of day?
5. Strength of day?
6. How big are the clouds growing?
With these answers in mind pilots should be able to gauge the strength, depth and direction of the lower valley wind. If pilots are smart, by the first 3 questions it should be easy to work out the direction of the lower flow or failing that asking some local pilots what they think could be a good idea too.
It’s possible to use this low level wind to your advantage if you understand what’s happening out there. So it’s also important to know the depth of the valley wind. You can then work out how far you can soar up on a good Luvside.
Also it will help locate ’safer’ Leeside climbs. Which although are in the Lee, if the heat source happens to be protected by the mountain that is acting as a block to the flow, rather than just causing rotor. The wind strength will determine how far this rotor could extend around/along the mountain.
So you’ve located a climb and you’re above the valley wind again. Now if you’ve done your flight plan correctly you’re flying on a light wind day. Then hopefully the associated mechanical turbulence will be minimal. However even if the Meteo wind is light there will be lee effects on the bigger mountains too. This could effect the amount of sink found around thermals and the direction. A rule of thumb also in the bigger mountains is to try to work the windside first when winds are over 12-15kmh, if at all possible. It’s also possible with stronger Meteo winds that there may be rotor on the Lee side of thermals as they also cause a direction change of the wind, causing a tumble. A good rule is that the Lee be avoided. As there will nearly always be strong sink together with hard edged, twisted strong thermal. This goes double when low in the valley wind searching for a way out.
In principal there are two different types of Lee, there are quite easy to spot the theoretical differences. One is protected and one the less comfortable is not. By learning the difference pilots will fly safe and confident without unexpectedly find themselves in an uncomfortable position.
KF
Over the years I’ve been experimenting flying accelerated for long periods of time between climbs, whether it’s thermic or rough. Obviously if I feel I’m in danger of losing the front edge I’ll step off the gas, stabilize the wing then slowly apply gas again.
Flying in World Cups and FAI cat.1&2 comps where most pilots are giving pretty much as much gas as they can unless climbing I noticed that some have 2 distinct techniques for holding the angle of attack positive.
The Obvious Stuff.
When we push on the bar the A’s and B’s are pulled down by different amounts this results in the wing accelerating but as the angle of attack is decreased the glider is more prone to taking a slap. So to stop a front stall or worse an high speed asymetric we need to keep the angle of attack positve.
The Valic method.
The Valic Brothers are reknown for flying full speed between thermals in some pretty turbulent air and I’ve rarely seen them take big whacks, although I’m sure they do from time to time. They can’t see the air either or get it right the whole time after all.
So what are these boys doing to hold their protos open going full chat. The following method should work just as well with lower end gliders as it does with skinny weapons, in fact you’ll probably have an easier time on a lower aspect ratio’d wing.
From what I can see and understand, although this is not the method I’d use for reasons I’ll go into later.
So you’ve left the climb and want to push on to the next, putting the brakes to the keepers you slowly apply the accelerator until you reach your best speed to fly(this will be another article coming soon). The theory of this method now relys on your ability to feel the air through your leg muscles. As you feel the glider surge forward you use the speed system like the brakes and come off the gas slightly (how much depends on the surge) so the wing comes back. Holding it there until the glider stabilizes.
Why not use the brakes? If you are using over half speed it’s unwise to use large amounts of brake aswell as this will cause the glider to yaw about and could collapse or spin if you’re unlucky. Once stabilized or you feel the glider get pushed back you apply more gas. So in effect you are using the accelerator like the brake lines.
I’m not a huge fan of this method as it requires a lot of feeling through very strong muscles in you legs and usually the stronger the muscle the less feed back they will allow. Also if you gas peddle is stiff you’ll get pretty tired pumping your legs backwards and forwards the whole time. Plus if the glider surges asymetrically it’ll be difficult to compensate for this in time and it could get messy very quickly but as I said before this boys seem to have this technique wired.
The Steigler method.
Stefan Steigle, ex-World Champion in 1995, uses the same method as myself. Still holding the brakes at the keepers and slowly/smoothly apply the gas. The big difference is that unless I feel the nose is fragile I’ll hold the gas through the turbulence but I’ll feel what the glider is doing through the last row of lines, C’s or D’s depending on which wing I’m on.
If the glider surges forward I’ll quickly apply the right amount of pressure through the rear lines, thus holding the gliders AOA positive. The big advantage of the this method I’ve found is that I can hold the wing thru pretty rough air and not come off the gas unless it starts getting silly.
It’s a little less complicated than the legs method as the wrist is pretty weak link and thus easier to feel if the glider is surging asymetrically or not. If it does then I’ll apply the C’s accordingly.
This short article is not designed to enable pilots to senselessly hurtle through rough air. If you are close to terrain or in very turbulent air please treat the gas with respect. Once you master when and how much you can give and more importantly what your particular glider can take before going tits up will enable you to fly faster and therefore further, confidently and competently.
If you have any questions regarding this subject please leave a comment for me.
Have fun and please be careful out there as collapses at speed can surprise even the most routine of pilots.
The following 8 guidelines are general rules or tips for planning an XC route anywhere in the Alps. This assumes that there is a light enough wind at altitude so that pilots can fly in all directions. This also means that the dynamic turbulent created by wind will be minimal and more importantly manageable.OK so you’ve checked the forecast and it’s looking excellent. The wind is light, there are wispy cumuli building from 10 O’clock onwards at over 3000m with no noticeable drift. The forecast gives a low chance of thunderstorms even in the bigger mountains. So it would seem that all the elements are in place for a big day out.
As all our elements are there the day quality should be as good as it gets. This however probably means that lower down later in the day the valley flow will be strong, remember that the heat low will be drawing in vast amounts of air throughout the day.
So you are now on launch and there is only one thing left to do if you haven’t already planned your route in advance. We are going to assume that you’ve never flown in this particular area before so this will rule out having to make route choice based on quirks in local conditions. This is a far simpler way of planning….
RECAP.
Good luck.
KF
How to Assault the Ahorn Spitz, 2968m.
Over the many years guiding pilots around the valley I have to say that still one of my favourite & awe-inspiring flights is making it up the Ahorn Spitz. This flight is both impressive and very accessible for lower airtime pilots.
When is it possible?
On a good thermic day between early May - early September the valley wind is noticable from late morning onwards. Depending on the time of year it will normally peak at between 17-1900hrs.
As a rule I try to avoid landing in the vallley at this time. The strongest time time of year is June & July. Generally speaking the better the day the stronger the valley. Some factors can affect it’s strength, stronger meteo winds help support it.
Usually it is around 1-1200m thick & very bouyant as it has had to travel many kms over baking fields to reach us.It is not uncommon for it to peak at the almost our trim speed in some accelertaed areas. Thunderstorms forming else where can produce a very strong valley wind & should be treated with respect. There are a few exit points for this flight around the valley, however, the following article was from our house mountain the Penken.
The Assault…
This flight first requires a relatively thermic day which has pulled enough air in from the flatlands. The first step is launching on the Penken(picture 1). The valley wind blows directly up this launch site. British pilots will feel right at home on strong days as it’s possible to play with the glider sitting overhead in the 20+kmh wind.
Once away from the hill it’s a straight glide along the ridge to the east(picture 2).It’s possible to pick up a little altitude on the way along. Looking towards the south you’ll locate the Ahorn Spitz, it’s 3 westerly spurs plus the main soarable face(picture 3).
If the valley wind is strong enough to soar it’s also strong enough to create some pretty impressive sink/rotor on the lee sides of the lower spurs of the Penken mountain. These should be avoided if you want to remain safe & high enough to connect easily with the ridge lift on the main face.
If high when you connect with the Ahorn ridge you can head straight towards the house thermal at the cable car pylons to the left of the Wiesenhof guesthouse(picture 4), however, sometimes you’re happy when you come in at the height of the house. If low here turn left and stay on the steepest part of the slope. Soar like you would a hill in the UK. Ideal for low airtimers you want to perfect their technique while being able to wait for cycles as they come through.
If the wind & thermals are good pilots should quickly find themselves topping out above the Wiesenhof, (picture 5). Now is a good time to move to the 2nd ridge just below the Filzen start place. Expect to lose very little on the glide as the steep cliffs to the right of the picture produce good lift. I very often use these to give myself plenty of clearance for searching for a thermal on the west facing spur.
At around 1700m and slightly to the left is a huge scar, very visible in Picture 6. When condtions are a little tricky and stable this scar regularly produces strong enough lift to get through the inversion, (if one is present).
It’s important to stay windward to the valley flow at all times, generally in the afternoon low in the shadows is a bad idea as you could easily find yourself being rotored in the lee. This is all pretty simple until you reach around 22-2300m as pilots will then find themselves in the meteo wind. According to the wind there are different routes to stay safe in the high alpine.
On this day there was a moderate north-westerly. Which enabled easy soaring up the main face(picture 8), this doesn’t work with another direction and can become dangerous with an easterly. This is were the local knowledge is King.
Depending on the meteo wind direction there are some obvious bowls and into wind triggers to the north of the peak. In a northerly pushing round the corner to take advantage of the dynamic lift there is never a bad idea, (picture 9). This is a great part of the flight as you can almost touch the cross on the summit. If you’ve chosen the right side of the peak with the wind blowing up it then getting above the summit is just a matter of making soaring beats or circles.
Picture 10 shows us level with the Ahorn Spitz, I flew away from the mountain to give it a sense of scale against the surroundings. Also taking photos in active air close to big rocks is not always advisable.
Picture 11 shows us over the Ahorn Spitz at an altitude of 3200m, base this day was around 3400m and thermals ranged from 1-5m/s. Once high and away from the terrain it’s time to sit back, relax and take in the views. On a clear day the Pinzgau valley to the north east is clearly visible as are the Dolomites to the south.
Picture 12 shows the Stilluptal and it’s 3000+m peaks that meet up at the end with the main ridge of the Alps and the border with Italy.
Although soaring up in the valley wind is pretty simple the last 700m or so of the assault can be technical. It does required previous knowledge of higher level winds and a little alpine awareness. Please ensure that you understand the conditions and remain safe at all times if you attempt this flight alone. Pilots who join one of our courses will of course be pre armed with the necessary info and will stand a much better chance of completing the flight safely.
You can comment on this article if you wish or if you wish to see these pictures in a larger size then click here
See the movie here
An Introduction…
I’ve been living in the Alps since 1995 and one question that is often asked by our clients flying here is ‘what is the foehn’. In my early years of flying I read the excellent ‘Touching Cloudbase’ but they only glanced on this phenomenon. Over the past years I’ve experienced the Foehn from the air and more safely from the ground. So I thought I’d pen a short article about it, hoping that pilots outside the Alps will learn a little about how to keep themselves safer in the mountains.
Several times I’ve landed early due to an expected foehn and still see pilots hacking themselves off the hill with little knowledge of what’s in store for them if this wind suddenly breaks thru. A lot of it is monkey see-monkey do, they’ve just seen someone land so it must be safe to fly. Little do they know that 30mins can mean a sedate glide turns into a full SIV programme with the pilot becoming just a spectator with little or no control over what’s going to happen. Luck takes over.
The Run up to the Foehn…
The Foehn is caused basically when a strong, humid air mass flows perpendicular to a mountain range, in our case the Alps.
Usually the classic pattern of weather over the Northern Alps goes like this, after a cold front passes thru the airmass becomes unstable. A high pressure with dry air moves in, cloudbase rises, thermals are good and the wind is light and usually from the N,NW in the beginning, slowly backing. As the High moves east across the Alps and the wind slowly switches W. Conditions around this time are still good gets more stable as the warmer air comes from the Southwest, slowing the thermals down. With the next Low over the W, NW of Europe a southerly stream will reach the Alps after it has crossed the Mediterranean and gained a lot of humidity. Once the wind switches to it generally gets stronger with the approaching low. A dry, warm turbulent wind blows on the North side of the Alps, Foehn storms can easily exceed 130kmh. Definitely unflyable!
Sometimes this wind is just a laminar south wind and is soarable if not too strong, usually the air is unstable and thermals help slow the wind down. Also with an unstable airmass the wind is allowed to go over the Alpine Divide or Main ridge unhindered.
However if the air is too stable after the high pressure, there could be inversions in the higher level, then this wind is squeezed and accelerated over the main ridge of the Alps, around 3-4000m high. As it tumbles it creates a massive rotor which can extend for 20-30km. Even more dangerous is if lower inversions break and this rotor is allowed to run through the valleys. This wind then rotors again as the terrain changes direction. This wind will persist until the front arrives restoring a more equal pressure on the north and south side of the alps, cloudbase will drop & the wind in the valleys will change to a strong damp gust front and after the front pass thru a High will usually build again.
The same happens on the south side when the north wind is strong from a low to the North. The Northern Alps are damp with a low base and in the south the North wind howls over the Alps and makes for very turbulent flying.
Why is the Foehn warm?
The reason why the Foehn is dry is easy to explain it’s due to most of the moisture being deposited on the Luv, windward side of the ‘Alpine Divide’ as rain. The reason why it’s warmer at the same altitude in the Lee than the Luv is a little more complex. It’s to do with the difference in Saturated and Dry Adiabatic Rates. Physics tells us that dry air will lower by 1deg/100m climbed. Saturated air ie. cloud will only lower by 0.65deg/100m.
Upon reaching the Alps the air is forced to rise, the humid air saturating fairly early. At the height of the mountain tops the air mass will have lost most of its humidity through precipitation. The sinking on the other side of the mountains and associated warming takes place with the dry-adiabatic lapse rate. At 1000m on the south side the tempertaure is 21degrees but at the same altitude on the north side it’s around 27degrees. The air has warmed up during descent at a rate higher than the cooling during ascent and reaches the plains as a very dry, very warm and very strong wind - the Foehn.
The Foehn is not good news in the winter either as this warm dry airflow eats snow like you wouldn’t believe. Partly due to the tempertaure but mainly due to the airs ability to absorb moisture. Like a dry sponge. The native Americans used to called this wind the ‘Chinook’, or snow eater.
So what does it means for Paraglider pilots?
The worrying thing about the Foehn it that it’s not the wind strength itself, long ago I used to think that if it goes over 30kmh that’s fine my glider goes 50kmh so with speed bar and ears and I’ll go forward no problem. Experience has shown me otherwise. The air in the Foehn is not a laminar air flow as is normal in the Alps but a gusty, twisting rotor that can produce some serious sink, lift and extreme turbulence. It’s a little like flying in out-of-phase wave. Only the mountains which are starting this wave are over 3000m high. It feels like you are always flying in the Lee, which of course you are. It’s almost impossible to accelerate the wing in these conditions, infact without extensive use of the brakes the glider probably won’t stay open long. Saving grace of the Foehn is that it’s mostly short lived and relatively easy for the forecasters to predict. Plus we have several ways of telling if the Foehn is ‘in the post’, mountain stations and near by lakes.
Final Word.
The Foehn can break thru very quickly, a pressure difference of just 4-5Hpa between the North and the south sides of the Alps is enough if the other conditions are in place to produce the squeeze and rotor. Sometimes the wind can even be felt in the valley before it’s felt on launch or between two inversions lower down, these are not ideal situations. Even a slight shift in the wind direction in the high level or the arrival of a nearby front can produce massive rotor in the valleys. So if there is Foehn in the forecast please be careful and if in doubt don’t fly as it’s just not worth it.
Here is some great time lapse of the foehn as it gets ‘Stau’d’ on the main Alpine divide
Here is a short video explaining the foehn
Here is a short computer simulation of the foehn, all be it this is what happens by North Foehn but interesting anyhow.