Wind
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Understanding wind and breezes is the key factor for going cross country, ahead of understanding thermals. This understanding not only helps you in getting somewhere on your flight, it also helps in staying on the safe side. Being able to find and exploit thermals flawlessly has little use if you are unable to convert the gained altitude into distance. Altitude won't get you far when blocked by a strong upper wind for example, or when trying to pass a strong venturi. The positive side of having a lot of altitude in these cases, is that you have more time to enjoy your mistake all the way to the ground. Not finding a thermal could results in landing out. Failing to spot a lee side, could result in far worse events. Flying the Alps without understanding the various wind systems, especially breezes, and the way they shape the clouds such as seen in them image here, is like playing Russian roulette. It is bound to go wrong after a few lucky escapes.

Clouds indicating strong and turbulent wind.Wind could be your best friend or your worst enemy. It all depends on how much attention you give the relationship. Over the years, wind has become a good friend of mine. And although this bond has been under pressure quite a few times (and will be in the future), these unpleasant moments have deepened my understanding and appreciation of wind in general and local breezes in particular. When there are (momentarily) no decent thermals to be found for example, wind often allows me to make low saves. Either by soaring on the spot, waiting for the next thermal, or by soaring to a more promising location. It could mean the difference between flying an early out and return to Dormillouse, or landing out in Lambruisse after a few kilometres. Wind allows me to cover distance when thermals are not (yet) working confidently, considerably extending my opportunities and thus range. Wind facilitates top landing in tight spots. It creates convergence in places where thermals are unlikely to be found, allowing me to cross otherwise difficult terrain with ease. Wind could also create waves that allow me to do the same. Wind pushes the thermals, so that I can cover distance and gain altitude at the same time. Wind is a wonderful friend indeed.

Of course, even your best friends surprise you now and then. Wind could do nasty things such as induce collapses (occasionally just before landing when in a bad mood), mushroom your wing in mid air, send you over the top of a ridge, or drag you all over launch. But to be honest, these kind of quarrels are mostly due to piloting errors. Yes, this means it is your fault. If you listen to what the wind is trying to say to you, frictions are less likely to happen. So, let's have a look at how we can remain friends with a force that could become a formidable foe when neglected.

General wind system

There is no wind, only winds. First of all, wind direction and strength can vary throughout the region, especially when flying (long) distances. The distribution of low and high pressure areas determine the general wind characteristics. The famous French mistral for example, is caused by the air flowing from a high pressure area near the Atlantic coast to a low pressure area near the Mediterranean coast. The resulting strong (and usually dry) north-west wind over the south-east of France keeps paragliding pilots grounded. Sail planes love this wind though, as long it is not too strong. Wind direction and strength not only changes with location, but also with altitude. The resulting wind shear often creates turbulence that has impressive glider-deflating capabilities, often catching you at unexpected moments.

Breezes, local winds caused by thermal activity, can strengthen, weaken, or otherwise modify a general wind. The interplay between the various winds and breezes is one of the things that make free flight so interesting. Since we don't have an engine to pull us out of trouble, we need to rely on understanding wind even more than ordinary aircraft pilots. Being able to understand wind patterns and exploit them well, feels like putting the pieces of a giant puzzle together. It is all part of preparing and understanding your flight. But of course there will be quite a few days when you are stuck with pieces that do not seem to fit together at all, no matter how you turn them. When in doubt, get out! If I would be able to understand and predict all winds, I would be making loads of money selling wind instead of getting pushed around by it.

Wind direction and strength

Wind force and direction are leading parameters for planning your flight. A specific wind could make a trip easy, or (close to) impossible. A strong wind could turn an otherwise easy spot into a dangerously difficult one. Instruments such as an anemometer, GPS or 'balise météo' (broadcasting the maximum and average wind values observed every fifteen minutes or so, at 143,9875 MHz in the French Alps), can help you in assessing wind direction and strength. Be aware of their limitations though. These instruments might be very precise, but not necessarily very accurate. A GPS will tell the wind for where you are currently flying, not for where you like to go. And often you want to know the latter, when approaching a ridge from above for example.

Ridge soaring, but on which side of the ridge?The ridge in the image on the left is called Coupe and runs north-south. Imagine that you have been thermalling on the south side of Couard (the peak which terminates the north side of Coupe, right behind you) just a little while ago, but you are slowly losing the gained altitude as you fly south. Soon, you will have to decide whether to go left (east) or right (west) of the ridge. Your GPS indicates a west wind, so you pick the west side. Excellent! This is usually the right choice here. Unfortunately, not today. As you tumble from the sky, you realise that near the ground the Tartonne breeze (from the east) is much stronger than the west wind from Digne. A fool with a tool is still a fool. Your GPS did not see the wind shear just above the ridge, but with a little training you could have. And hopefully the information on this page is going to help you with that, assessing wind direction and strength without instruments.

But before doing so, let's have a look at a few more issues while we are above Coupe. Making the opposite error for example (choosing the east side, with west wind), would have resulted in a less dangerous situation. The rotor on the east side (west wind) will be more gentle than the one on the west side (east wind). East wind gently flows from the valley up the eastern slope and then plummets down the vertical top side on the west, creating a significant rotor. A west wind is pushed skywards by the near vertical cliff (creating the soar band and a bit of turbulence behind it), and then gently flows down the eastern slope. This slope has few obstacles and is mainly covered by bushes. The western slope however is full of trees, with an occasional rocky part creating thermals (i.e. turbulence). If you get to low here with east wind, the trees are likely to grab you. The green fields in the valley to the right probably being too far away for landing. If you are unable to estimate wind direction when flying above a ridge, then go for the safer side. The side which has little consequence if you were wrong about the wind. The side that has a gentle slope with little vegetation, or a valley to land in. Not the one that has lots of trees, large boulders, power lines, or a glacier. If the safe side turns out to be the wrong side, than you should be able to land safely and possibly able to re-launch from the other side, rather than crash and not being able to get to the other (right) side without climbing gear.

It seems logical to fly the western side of Coupe in the afternoon, more logical than the eastern side at least. Most pilots therefore automatically choose the west side, even when the other side is the right side to fly. It does not happen that often, but it surprises them when it does. It all depends on the wind system, time of day and location. On some days, the northern part of Coupe needs to be flown on the east side, and the southern part on the west side. The tricky part being the transition zone between the two. Especially if you are flying on autopilot here and not aware of the winds of change. I have also flown the west side, while facing the cliff two hundred metres in front of me. This happens when east wind collides with the west breeze from Digne, creating convergence far away in front of the ridge. Soaring while facing the ridge is a bit of a weird feeling, since I am used to the more logical situation of soaring with the mountain in my back and the valley in front. Never mind logic when exceptions strike! Be aware of your brain possibly blocking when this happens, preventing you from reacting correctly to reality. Honing your observation skills, rather than relying on instruments or automatism, could help you to be ready for them. Assuming that “the wind always comes from that direction”, might be all right 99% of the time, but fatal on a day that it is not.

Every now and then I see pilots using an anemometer to find out whether conditions are flyable or not, often ignoring that they are not measuring the wind speed they would like to know. The wind above take-off (i.e. above their sampling point, the place where the glider will be when launched), is likely to be stronger than measured. Another common mistake is not realising that conditions further out could be a lot tougher than those measured on take-off. This happens for example, when a take-off is (a bit) shielded from the wind. This west facing take-off on Chalvet is behind a small summit to the north, about 50 m above take-off and covered with trees. In case of north wind, the take-off is in the lee of this summit and the trees. The wind flows around and over the them, creating a local (north-)west wind near the ground on take-off, as well as wind shear and turbulence higher up. Pilots are able to launch in what seems strong but manageable west wind, but run into much stronger (north wind) conditions as soon as they leave the lee area. If you are not prepared for those conditions, the ensuing pilot panic could get you into more trouble than you already are. If you need instruments to decide whether the conditions are flyable, then maybe you shouldn't fly in the first place.

Pilots discussing flight opportunities.The pilots to the right are discussing flight plans while waiting for the navette (shuttle bus) that will drive them to take-off. A quick glance at the sky behind these pilots tells us that today is not a day for flying a paraglider (click on the image if you missed the clue). Fortunately, the navette driver is a friendly and responsible guy who has been around in paragliding world since the beginning. He knows about flying conditions, but especially about pilots and their desires. I guess he has seen all possible pilot types by now, especially the risky gung-ho ones. He is more focussed on customer care than flying nowadays, providing customised feedback for his clients before driving them up to launch. Some pilots are hard of hearing when advice on a day like this does not suit their desires, but parking the navette in the garage is a sign even they usually understand. Human responsibility is a great facilitator when it comes to safety. Far more important than technological stuff.

This does not necessarily mean instruments are bad, just that in paragliding they are often a lousy substitute for observation skills. Drinking a pint of beer at the end of a rewarding flight, you can simply see how full your glass is. You don't need a level gauge to see that it is only one-third full, realising that you would like another one soon. A paraglider is more like a leaf in the wind than a glider, let alone an actual aircraft. The variations in the air are large compared to our flight envelope, resulting in a substantial noise in the measurements. Noise that could distract you from flying well. Does it really matter knowing that your ground speed is five kilometres per hour, rather than eight? Or is it more important to know that slightly to the east of where you are currently flying, you will be pushed by the breeze instead of being fought head-on by a strong wind?

We are very vulnerable to changing weather conditions, especially when strong. Often the difference between being in control and being controlled by the conditions, is just a minor tick on the scale of your instrument, but a major tick on your safety scale. Being aware of the conditions you are flying in, without needing to rely on instruments, should be a natural sense for any pilot. Only after you have developed this natural sense, you could use instruments to hone it if you wish. Being already aware of the conditions in a natural way, you can use instruments to squeeze even more out of your flight than you already do. Being unaware of the conditions, and using instruments to hide that fact, seems like a bad idea to me.

When flying, one often has to make split second decisions. Unfortunately, most instruments need a bit of time before they can supply the requested info. Mostly in order to filter out noise in the measured values. A vario that communicates every up and down movement would drive even the most dedicated instrument enthusiast crazy. Constantly being aware of what is going on around you, helps you in making quick and correct decisions. On the fly, without needing to think. Flying without instruments forces me to observe all events in the air as well as the ground that could help me in analysing the situation. It leaves no other way, other than to use my skills. Since I cannot fall back on instruments for telling me what the conditions are like, I am constantly aware of them. This approach has considerably raised my skills and understanding for all kinds of outdoor activities, but especially paragliding.

That does not mean it is easy. Finding out wind directions and strength during flight is surprisingly difficult in the beginning. The only wind you feel is the one created by the speed at which you are flying through the air. But after a while, you will be able to derive wind characteristics from other observations. As you slowly adjust to it, the sky becomes your natural habitat. The clues below could help you along during this voyage. They are far from exhaustive, but you will get the picture.

Clouds

Clouds are excellent wind consultants, using their shape and drift for telling you wind direction and strength. A quick look at at the clouds will tell you which way they are drifting and how fast, and thus from which way the wind blows and how strong. For example, clouds drifting north indicate south wind. At higher altitudes, contrails and other clouds spread out by the wind are often denser at the windward side. A southbound plane creating a contrail with a the denser part on the western side for example, indicates west wind at that altitude.

A small glacier, with a cloud shadow showing wind direction.Distinguishing cloud drift becomes less clear when flying, since the observer is moving too. It is hard to tell which way the clouds are drifting when you are a little cloud yourself. Observing cloud shadows on the ground solves this problem and has the advantage of nearby references for more precisely estimating wind speed. A cloud shadow keeping up with somebody jogging below you, indicates a wind strength of about ten kilometres per hour. More if their movements are not well aligned. A cloud shadow taking ten seconds to cross the length of a professional soccer field, roughly indicates wind at trim speed. Unfortunately, cloud shadows become less pronounced with increasing altitude and cloud cover. Their contrast diminishes and their edges become blurred. They could even completely disappear when the sky is (densely) overcast.

Close to the earth, observing your ground speed is probably a better indicator than looking for cloud shadows. The slower your ground speed, the stronger the wind you are facing. The amount of counter steering that is required to keep your direction, tells you which side the wind is coming from. As does an impressive ground speed, i.e. tailwind. The error margin increases with altitude however, meaning you are probably better off looking for clues such as the cloud shapes and shadows when flying at altitude.

Fortunately, you don't have to descent in order to see cloud shadows when flying high in the Alps. Mountains rise up to greet you everywhere, bringing cloud shadows with them for a closer look. Tall, snow covered mountains such as seen in the image here, form an excellent high contrast screen for displaying the latest shadow movie. You can clearly see a cloud shadow on the glacier to the right of the ridge below me. In this case the shadow goes east, which confirms the wind direction shown by the shape of the clouds (more about that a bit further on). A click on the image will reveal that I am flying in the lee of Mont Pelvoux and Barre des Écrins (in the centre of the image, the glacier a bit to the right of them). I had 'seen' there was turbulence and sink slightly to the left of where I was flying. And my opinion had been confirmed by a pilot landing out in the valley below me earlier on, and by a sail plane passing me overhead at the current spot and losing lots of altitude trying to pass the col above the glacier. Having worked my way up from below, I knew there was some lee turbulence below me as well.

In all, this seems quite an illogical place to be flying given the current conditions. Probably even dangerous. Well, it certainly is a challenging place that demands respect. But if everything would be easy when flying, it wouldn't be as much fun for me I guess. The real reason for going here is that there were few other options left, except landing. Considering the already strong wind at this altitude, landing in the valley seemed more dangerous to me than continuing my flight. And besides that, I was on my way north and just getting into the mood. After passing the lake of Serre Ponçon, I had tried to go north-west via les Écrins. This is my preferred route when heading north, passing all the snowy summits. Looking down into the deep blue coloured crevasses, occasionally hearing the glaciers moan or seeing (and hearing!) an avalanche running down the slope. It's wild and impressive. It's nature as few other people will be experiencing in their life. Nature however decided otherwise and blocked my original intentions with a strong west wind near Mont Guillaume. So I headed east... and ran into a strong east wind over les Queyras. From there I followed a north-west route, hoping to run into the convergence of these two conflicting wind directions. I found it over the Durance valley, but it was just a bumpy mess that made me seasick rather than rise. Something that had never happened before, warning me that conditions were a bit more than just challenging. I worked my way further up north to Col du Lautaret, where I finally found a well structured encounter of the conflicting winds. The narrow east-west oriented valley channels the winds here. The only way for colliding winds is up, taking me along with them. A relaxing glide to Maurienne valley followed, without taking another thermal for the next half hour. A nice compensation for two hours hard work.

Clouds telling wind direction and strength.Cloud shapes are also a good indicator for wind strength and direction. It tends to be less precise than observing cloud shadows, since there are other factors than wind that shape a cloud, but has a great advantage when it comes to judging wind from a large distance. The imprecision is mainly due to interference with thermal activity, which created the cloud in the first place, so by clever observation you will be able to judge thermals and wind in one go. The clouds in the image on the right are skewed to the left, meaning the wind is coming from the right (south to south-east in this case). The more skewed the clouds, the stronger the wind or the weaker the thermals. The more straight, the weaker the wind or the stronger the thermals.

It's not really obvious from the image, but if you click on it you will see another cloud characteristic that might help you estimating wind. You will notice that the clouds in the other picture are larger and fatter. The reason for this difference is that the first image has been taken just before noon, when thermals were waking up, while the other has been taken two hours later when the thermals were working well. In addition to being skewed, you may also have noticed that the clouds are often thicker on the downwind side (the wind in the other image is coming from the right as well). The air (and thus the cloud) is moving with the wind, while the thermal source is fixed to the ground. The thermal moves with the wind as well of course, creating a skewed cloud, but at a given moment becomes disconnected from its source. The latter starts feeding feeding a new cloud, next to and often joined with the previously generated cloud. Its relatively young age means this new cloud is less tall than the preceding (dying) ones downwind. In addition to that, air has mass and thus inertia. When the thermal source stops heating the air, the latter still has vertical speed. Meaning the cloud will continue to grow for a while after having passed its thermal source on the ground, until the vertical momentum has been digested by friction and gravity. In very unstable conditions, a cloud that originated from a thermal, could start creating its own updraft and grow into a towering cumulus or Cb for example. Especially when breaking through an inversion in the process.

Clouds drifting over a mountain.Identifying breeze direction tends to be more complicated than wind direction. The wind in the image on the left comes from the other side of the mountain (west in this case). It is a no-brainer, considering the shape of the clouds. The breeze however, is coming from the right. It is heading towards a range of high mountains at the end of the valley to the left, but a few hours ago it was coming from the opposite direction.

Global winds are created by low and high pressure areas, and thus quite predictable over a large area. Breezes however, are local winds that vary with the landscape and local conditions. You need to be able to read the landscape in order to actually 'see' the air flowing through it. The easiest way it to locate the biggest mountain(s) in the area, that's where the breezes are heading. The larger a mountain, the more sunbeams it catches and the more it heats the surrounding air. The air heated by the mountain rises, attracting relatively cooler air from the surrounding areas. This airflow is canalised by the valleys, creating valley breezes. I don't mean to be disrespectful, but those majestically sized mountains are nothing more than a giant vacuum cleaner during the day. Look for the biggest vacuum cleaner in the region and you will see the breeze. And if you are too low for spotting it, then look at the river flowing in the valley. Water flows down the valley, while wind usually flows up the valley.

Beware of exceptions though! A slight change in the weather (a depression moving in, a nearby Cb or downburst, changing cloud cover, et cetera), can switch the direction of a valley breeze. A valley breeze generally has two choices of direction, up or down the valley that is. This means that breeze direction can suddenly turn 180 degrees when an local weather phenomenon turns up uninvited. Not a pretty thought when landing in a tight spot with a strong breeze.

One breeze system can overrule another, messing up the local conditions you are used to. Thermal activities creates local depressions, which influence one another and might influence the global wind system as well when large enough. This means the wind could be very different locally from what is expected. Especially tall mountain ranges have a major influence, blocking or redirecting winds and breezes. For example, a south facing take-off on a low mountain range located to the north of much taller mountains, could have a southerly breeze in the morning when thermal activity is still low. Just as expected. Excellent conditions for an early start. But when the thermals generated by the taller mountains get the upper hand during the day, collecting cooler air from the region, our south facing take-off is suddenly in the lee. Even when south wind has been forecasted that day. A tricky situation if your launching a bit (too) late, when the breeze direction is changing or about to change. Similarly, the south facing take-off on Chalvet is in the lee when the western breeze chases the southern breeze around noon. Usually this happens when you are just about to launch, which makes the decision to move to the west launch even more difficult. Preferring to avoid packing their gear or to avoid the sweaty twelve hundred metres longs walk with a mushroomed glider and harness on the back, most pilots get away with flying rather than walking to the west side when the rotor strikes. Some did not.

Different local wind directionsThe ribbons in the image reveal a light wind from the right, which is the usual situation in the afternoon on the west facing Chalvet take-off. Except that it is early still morning (10h00), so this indicates west wind rather than the slow start of the regular breeze. A bit further south however, a cloud reveals an easterly breeze (coming from the left). This is precisely a day for thinking twice before launching from the south-east take-off at the end of the morning.

Conflicting winds like these often happen on days with west wind around St.-André-les-Alpes. The morning thermals create breezes coming from the east to south, cancelling the (gentle) west wind. In the afternoon however, as the sun starts heating the west facing slopes, the breezes turn west and reinforce the west wind. If you looked at the shape of the clouds during the day, this sudden increase in wind strength does not come as a surprise. If you counted on instruments for telling you wind direction and strength in time, you risk being surprised by an unpleasant situation that arrived surprisingly fast.

Conflicting winds could occur anywhere, any time. Not just on take-off or near noon. Always be aware of possible conflicts in the air, and ready to deal with them. Depending on the circumstances, conflicting winds could create convergence, lift, sink, wind shear, turbulence, or all at the same time. You can see another example by clicking on the image. Try to determine wind direction from this example before reading on. The clouds show that the wind is not that strong and comes from the left (north). The big cloud on the left is the most obvious clue, but the smaller clouds are slightly skewed to the right as well. You probably got that part right by now. But did you get the other part too? Lower down, the breeze is from the south and stronger than the north wind. You probably didn't see that in the image. If you did, I would like to know how! If the wind higher up differs from the breeze below, wind shear is lurking somewhere to get you. It would be nice to know where, in order not to be surprised by it. But how can you tell that the breeze is south today when there are no tell tale clouds at the lower altitudes? Well, that's where other observations come in. I launched near Courbons in a south breeze and drifted with it to Saint-Geniez, where pilots where flying on the southern slope. Apparently they were no longer bothered by the north wind that I had seen at nine thirty in the morning when passing nearby Sisteron in my car. Furthermore, all the birds I observed were soaring on the southern sides of the hills. Clouds may be my major source of information for telling wind direction and strength, but not the only ones as you can see. There are other clues that merit some attention as well, especially on a cloudless days. So, let's have look at them.

Moving objects

Anything moved by the wind comes in handy for determining its direction and strength. So far, I have used smoke, flags, grass, bushes, trees, wildfires, balloons, leaves, plastic bags, newspapers, kites, fellow gliders, sailplanes, and dust, to mention a few. The ones that have a large surface in relation to their weight, such as flags, smoke, empty plastic bags, and leaved top branches, are the most useful ones. The flags on the Citadelle of Sisteron for example, come in quite handy when trying to determine which side (north or south) to choose upon approaching Baume. Or for assessing the chances of Saint-Geniez being flyable without having to drive fifteen kilometres (and fifteen back if it is not). Flying the Swiss Alps sometimes seems a bit easier than flying the French Alps, because of the abundance of flags. Every chalet in the Swiss mountains seems to have one, either the national or the canton version. Some of those flags remind us of territorial events that happened ages ago, but are still smouldering. There are isolated spots in the canton of Bern that belong to Wallis for example, and their inhabitants are not afraid to show this. It probably means a lot to them, but even more to us paragliding pilots. Flags, no matter their purpose, serves as excellent wind socks. Let's focus on vegetation however, since it is the most widely spread windsock throughout the Alps. Even in Switzerland.

Leaves telling wind direction.Vegetation bends with the wind, pointing to the direction the wind is going. The more inclined, the stronger the wind. But beware that the bending angle depends on the flexibility and height of the vegetation as well. Trees tend to move less than tall grass, for example. Pine trees move less than foliaged trees. A larch looks like a pine tree, but its branches move more easily with the wind. And when you do see pine trees moving their branches, then the wind is likely to be strong, possibly dangerous to fly in. Don't mistake a pine tree for a larch tree when judging wind. The reverse mistake is safer, if you are not that much into trees.

Leaves usually have differently coloured sides, the side facing the sun being darker than the backside. As the wind plays with the leaves, these alternately expose their sides, resulting in a flickering that is more pronounced at the windward side of the tree. This flickering is easier to observe than the bending caused by the wind, especially from larger distances. As the wind grows stronger, the bright side of the leaves is more or less continuously exposed. The tree or bush now has a light and dark side, being the windward and lee side respectively, often signalling wind direction and strength from hundreds of metres or even further away. Some leaves, such as seen in the image, have an almost silver like backside that strongly contrasts with the sun facing dark side. This high contrast allows wind observation for a specific spot, long before you get there. This could help you in staying away from nasty places such as rotors.

Be aware though that if there is a prevailing wind such as a valley breeze, then the leaves are likely to stay in their regular position after the wind has died or changed direction. The leaves in the image show a wind from the left, which is the usual valley breeze from the south here. Today however, there is a mistral tendency. The wind is coming from the right, despite the leaves saying the opposite. This strong but temporary north-west wind is not able to convince those leaves to turn in just a few days. Wrong wind indications like this occur often enough to merit another example. Flying over the Entremont valley one day, on my way to Annecy during one of those lovely flights from Grand Bornand at the end of the morning, I could see the leaves of the trees near the river below me indicating the usual breeze from the west. This conflicted with the drift and ground speed I had observed while thermalling above Roc des Tours a little earlier, in a light morning breeze from the south-east. This earlier observation made me prefer the usual morning side of Lachat (east), rather than being tricked by the leaves into the lee on the western side. Always be aware of nature trying to pull your leg now and then. She doesn't mean to hurt you. She just wants to keep you humble and attentive, and on the safe side.

Pine trees have more difficulty in airing their emotion and don't bend as easily with the wind as their foliaged friends. But even if they don't seem moved that much, the wind could make its presence felt over time by subtle changes. To compare a pine tree with that tricky bush that told us the wrong wind, click on the latter. Try to determine the prevailing wind direction before reading on. Being the most flexible part, the ends of the branches point skywards rather than sideways. The ends of the branches are lighter and more easily moved by the wind than their thicker part near the trunk of the tree. The ends are climbing up an invisible 'wind wall'. But as their length and weight grows, the wind can no longer support them and gravity slowly pulls them into their originally intended horizontal position. In the meantime, newly grown buds have taken over the leading role from the old branch ends and start their skyward growth. Until these new enthusiasts are overtaken by their offspring as well. And so on, until the trees dies and falls over. But even fallen trees could still be useful for revealing the prevailing wind. A tree that is about to fall, is an easy pray for a wind gust. Since the prevailing wind occurs more often than the other wind directions, a dead tree is likely to have been toppled by a wind gust from the former rather than the latter. If you see various dead trees point the same way, they are probably pointing to where the prevailing wind is going. As always, mind the exceptions. Autumn storms could create a wind pattern that has little to do with the prevailing wind.

If there is no wind and you are waiting for the breeze to install itself, a quick look at details such branch ends can tell you whether you are on the windward or lee side. Telling you to wait or move on, respectively. In addition to the branches, the top of the tree is skewed to the right here. The tree is taller than the surrounding vegetation and its top is continuously exposed to the wind, hence a good indicator for finding out the prevailing wind direction. In this case, both branch ends and tree top tell us the same wind direction. A comforting confirmation. It also tell us that a large tree top rising above forest canopy, could be just as helpful for identifying wind as a solitary tree in a field.

Trees shaped by wind.Not being surround by other objects that could disturb the airflow, solitary trees are a excellent wind socks. The stronger and more regular the prevailing wind, the more pronounced its effects on vegetation. The tree in the image is skewed to the left, telling us that the prevailing wind comes from the right (west in this case). Wind induced asymmetric shapes such as these, are clearly visible from far away. Be aware though that a similar imbalance could have been created by the sun as well. Always try to find other clues that confirm your current one. A clue is just a guess. Multiple clues that lead to the same conclusion, approach truth. Unless they are all wrong of course. But the more they agree, the less likely this is to happen. In this case (click on the image), nearby vegetation has a similar asymmetric shape. The strongest confirmation was made by the wind itself, blowing around fifty kilometres per hour. During the night, the wind strength approached seventy to eighty kilometres per hour and then calmed down. Hiking up in the morning in order to take-off again from a mountain ridge to the north, the air was calm. It seemed perfect weather for the last leg home. A few minutes before reaching the ridge, I began to feel and hear the wind that had turned rather than calmed. I had been sleeping in the lee of the mountain. There was a very strong and gusty wind (about 80 to 100 km.h) at the other side. Well, trees can't be right all the time. Especially if they are in the lee.

Still, I usually trust trees more than my own wind sense for an unknown location. They spent all their life in the same spot, while I am just passing through. They are far more familiar with the local weather conditions than I will ever be. And they are kind enough to show this to the careful observer. Even the tougher ones will be influenced, given time. Pine trees for example, are less subject to immediate wind than foliaged trees. Yet, over time they too have to bend in case of a prevailing wind. The skewed pine tree in the image speaks for itself, but often the effects are more subtle. Have a look at the branch ends on the right side of the tree (the effect is most visible halfway up the tree). These are slightly bend upwards, while on the left (lee side) the branches and their ends are more level and straight. I agree that it is not as evident as in our previous example, but tiny details can be just as important as large hints that are hard to overlook.

As a final note and warning, remember that prevailing wind does not necessarily mean actual wind. Find other clues before committing your life to the wind perceived from the shape of a tree, or any other vegetation for that matter.

Surfaces

Areas of homogeneous vegetation often show distinctive wave patterns when caressed (or beaten!) by the wind. The most common ones in the Alps are grass, wheat or corn fields, but similar observations can be made from large dusty surfaces such as quarries and barren fields. The latter come in handy at the end of summer, when all fields have been harvested, sending dust clouds in the air when touched by wind or dust devils. The more you move south and the more the season progresses, the more dust from these spots is likely to help you.

A field by itself may be homogeneous and air flow friendly, but surrounding objects and other artefacts create turbulence that cause an uneven distribution of the wind in time and location. This organised chaos becomes visible as wind waves rolling across the field, or curved patterns in case of a thermal. The larger or the further away the thermal, the less curved these patterns become. The hue of the moving vegetation differs from those untouched by the wind, comparable to the flickering seen with our bush above. In this case the difference shows up as blots that move across the field, their speed and direction telling tell you where the wind is heading and how fast. And when you see a wind pattern in a field while there is hardly any wind to speak of, you are probably looking at (the start of) a thermal. And if you are this low to spot this difference, you are probably very happy to see the thermal.

Lake surfaces are very homogeneous areas, water being very flexible and looking for equilibrium all the the time. Wind over the lake is well organised due to the (near) absence of obstacles on the water and creates well established surface patterns as it disturbs the water's equilibrium. Not all of these patterns are easy to read or even useful to us, but combined with obstacles that disturb the regular wind over the lake, we have an excellent watery windsock. Be aware though that changes on the surface could also be caused by changes underwater, instead of the wind.

Ripples on a lake showing breeze direction.The airflow at the edge of a lake is often disturbed by the land-water transition. Vegetation or constructions near the shore, such as reed, trees, bushes, dikes, dams, and houses, partly shield the lake from the wind. The water in this lee has a smoother surface than the downwind section. The lake in the image tells us that there is a (moderate) wind blowing from right to left near the ground. This is the usual direction here in the afternoon, as well as the most logical one, since there are some huge mountains further up the valley that create the this breeze. Two hours earlier however, while doing a morning tour of the valley, the breeze was from the left. Which is also quite logical. The pattern on the lake's surface helps us to determine whether the breeze has already made the usual daily switch in the morning.

These kind of wind induced water patterns are often visible from far away. The image here has been taken with a zoom lens, but the pattern was clearly visible to the naked eyes while flying 1200 m above it. I have used water windsocks with success from kilometres distance, often aided by the two surfaces reflecting the sun in a different manner (the rippled downwind part being brighter, since it reflects more light into different directions than the smoother part). It is much easier and faster than trying to spot a regular windsock, no matter how big.

The rougher the water surface, the stronger the wind. Waves topped with a bit of foam, indicate winds near or surpassing trim speed. If the wind on take-off is all right, but you see these kind of waves on the lake in the valley below, think twice before launching. The opposite is also true. Pilots on the south Chalvet take-off in the morning often wander whether the air is already capable of carrying them for an early start, but no one volunteers to be the wind dummy that risks sinking out. Which is a bit of a pity, since they risk waiting too long rather than sinking. They risk missing out on the already good conditions, or getting into the lee from the west breeze if they really wait too long. Even sadder, there is often no need for wind dummies here. As long as the lake's surface is smooth, you risk a sledge ride to the landing field. As the breeze starts waking up, ripples appear at the far end of the lake (near the bridge). When the rippled surface has extended all the way to the northern shore (i.e. the landing field), the air should be more than ready to carry you.

Objects on the water such as boats, driftwood, algae, and pollution, drift with the wind until stopped by the shore, anchor line, or other barrier. The downwind side of a lake often collects debris and other pollution, far more at least then the upwind side. If there happen to be boats anchored on a lake, then their bows tend to face the wind. The boats drift with the wind, until stopped by their anchor line. And since anchor lines are usually (beware of clumsy captains!) tied to the bow, the bow points into the wind. The owner is probably unaware of this, but his anchored multi-million yacht is nothing more than a giant windsock to us pilots. It is probably a sickness or a mental defect rather than jealousy, but I see windsocks everywhere when flying.

If there is a boat sailing on the lake, a look at its sail and course should give you a fair amount of information about the wind as well. Remember that a sail is nothing more than a vertical wing. Having quite a bit of experience with wind ourselves, this knowledge could help you in judging the wind from a sailing boat.

Course and ground speed

One of the easiest ways to determine wind strength and direction, is to look at your ground speed and course deviation. Unless your exactly heading up- or downwind, your wing faces a direction different from your course in order to compensate for crosswind. If you need to steer to the left for maintaining your course, than the wind (partially) comes from the left. Similar for a correction to the right. If there is no correction needed, then the wind is either absent, or exactly face or tail. A quick glance at your ground speed will tell you which one.

Crosswind estimation based on course deviation.The more your wing faces away from its course, the stronger the crosswind. I will spare you the (simple) maths and just give you the figures. The graph shows the crosswinds you can handle with an ordinary DHV2 cross-country wing at minimum, trim and maximum speed. I have left out the different glide ratios for each speed in order not to complicate the issue. Consider it homework, there is a bit of a surprising result waiting for you. No surprises in the graph here, just general confirmation of what we already know. The stronger the crosswind, the more you have to correct your heading. But there is something interesting happening with strong crosswinds that you may have missed so far. For light to medium crosswinds, the relation with the required correction is approximately linear. An increase in crosswind requires a proportional increase in correction, resulting in a proportionally degraded performance. From about 45 degrees on however, the crosswind starts to get the upper hand. You will make little progress over ground if you need to correct more than 60 degrees, wasting nearly all energy on correcting the drift. The stronger the crosswind, the less efficient your flight path becomes. The graph shows that this inefficiency is manageable in light to medium crosswinds, but disastrous in strong crosswinds. As a rule of thumb, long before I made this graph, I try to avoid transitions with require more than 45 degrees crosswind correction. At this point I still have a large speed margin in order to cope with eventually increasing wind (when sinking into a venturi for example), while still making progress without loosing too much altitude. At 60 degrees correction the glide ratio is terrible (but not yet catastrophic), while the safety margin has shrunk to within a few kilometres per hour of the glider's maximum speed. Which means an incident is just a gust away. Stepping on the speed bar in order to make a transition in strong crosswind, does not enlarge your chance of success (which you should have found out from your homework given above).

To determine whether there is a wind component from the front or the back, check your ground speed. Telling ground speed without a GPS is easier than it looks. It just requires getting used to the various speeds of your glider, especially those at which you will be flying frequently such as trim speed or half accelerated. You won't be able to tell exactly how fast you are flying, but you will be able to tell whether you are flying slower or faster than usual. Being able to tell how much the wind is affecting your usual speed, you will be able to identify the amount of headwind or tailwind. It is not about absolute speed, but all about relative speed. After a while you will notice small (one to two km/h) deviations from your expected speed. When flying a new glider for example, I might notice that the glider is 'considerably' faster than my current glider. At least that's what it feels and looks like. On paper however, this difference is usually no more than two or three kilometres per hour. By flying a lot with the same glider, you will get a (near perfect) feel for ground speed solely based on the trim speed of your glider. Seeing a ground speed that differ from what your feeling expects, will then give you a good indication of how the wind affects your flight.

Beware when flying a different glider though! This could confuse your senses, with possibly nasty consequences. Coming in for a top landing one day on the summit of Grand Coyer, one of my preferred picnic spots for top landing, I noticed that I had a tailwind slightly below trim speed. Perfect for a top landing here, since the summit is steep. With weaker winds, I risk overshooting the summit on approach. Flying down the rocky slope just above the ground, while trying to land. After turning 180 degrees into the wind in order to land, I found out that wind was a few kilometres stronger than expected. This is precisely the difference in trim speed between my regular glider that I was flying at that moment, and that of the more performing XC3 that I had flown the ten preceding flights. Ten flights was enough to adapt my senses to the XC3. After switching back to my old glider however, I forgot to switch my senses back as well. There is only a minor speed difference between the two wings, but more than enough to confuse my senses and turn an otherwise harmless top landing into an almost disastrous one. Stepping on the speed bar got me out of my negligence here. Some say that a GPS might have prevented this negligence by giving me more precise speed information. But would you really prefer looking at your GPS display when flying at about 70 km/h towards a steep rocky slope twenty metres in front of you (looking earlier does not help because of the wind strength increasing due to the compression zone near the summit), rather than looking at where you are going? Even when your approach leaves room for error, a quick glance at your GPS might distract you just long enough to turn too late (i.e. in the rotor) or crash into the rocks due to sudden sudden sink or turbulence.

Birds and other animals

Animals, especially birds, can tell you the way the wind blows. Large mammals such as cows for example, tend to turn their behinds into the wind when it is raining in order to shield their heads from the rain. They could do the same when it is not raining, but still windy. There are probably lots of other similar clues, but being a sort of bird myself, I have more affinity with birds than other animals. Birds like to land into the wind for example, just like us. And since they do not like their feathers getting ruffled, they also tend to face the wind when resting.

Sometimes it is not the birds, but a derivative that attracts my attention. It is hard to see the insects that are taken up in a thermal for example, but you can easily see the the swifts that feed on them. On a day that was too windy too fly, I was walking on the plateau of Calern. Sitting on the edge of the plateau, I was wondering how to get to the Haute Montets take-off a few kilometres away, at the other side of the wide valley. Swifts occasionally buzzed by within a metre of me, but most were hunting high in the sky. Struggling back from Audibergue to Gourdon a few weeks later on a similar windy day, I remembered the thermal I had seen because of the swifts. The swifts were having lunch elsewhere that day, but the thermal was on rendez-vous. It took me much higher than expected, higher than anything else I had encountered for the last hour. It got me across to the windward side of Montets in a straight line, against the considerable breeze.

The higher the desired speed , the less wing surface you need. That is why you see all those moving parts in the wing when you look out of an aircraft that is preparing to land. The aircraft needs a much lower airspeed during landing than during flight. The wing's surface is increased by cleverly extending various parts of the wing, allowing lower airspeed. Unfortunately, paraglider pilots are stuck with a wing whose surface is fixed. This surface is a compromise between various flight characteristics, such as performance, safety, and our ability to launch an land at running speed at the maximum. Birds on the other hand, can adjust their wing surface to their need and flight conditions, resulting in a much larger flight envelope than for paragliders. In light wind or wanting to fly slow, a bird stretches its wings as far out as possible in order to get the most lift. When wanting to fly slow or when dealing with strong wind, the bird's wings are (partly) retracted by bending them in the horizontal plane. This reduces surface, increasing the birds speed. Choucas are a delight to observe in strong winds, at a windy summit for example. If the wind is far too strong for flying a paraglider, you can still see the them playing around happily using a much reduced wing surface. Seeing the seagulls tucking in their wings when you are soaring at the coast, means it is time to land (soon).

An often overlooked bird factor is their course deviation and ground speed, precisely the same issues as discussed in the previous section. Birds too have to correct for crosswind and have less or more ground speed than usual when faced with headwind or tailwind respectively. The bigger they are, the more easily this can be seen. There are lots of griffon vultures around in the southern French Alps. These impressively large cross-country masters could teach you a lot, if you are willing to pay attention to their free lessons. Deriving wind strength and direction from their airspeed and course deviation is just one of them. The Meunier - Tournon passage for example, is usually a windy one. In strong wind conditions, it is often unclear whether the passage can be done or not. It could take a long time to battle your way up to col de Diable, being under the impression that the passage can be done, only to find out there that the wind is really too strong and dangerous to continue. If there happen to be griffon vultures passing low in the valley however, flying much slower than usual and some even turning back, this tells you that another passage is preferable for heading north, long before you even get near Tournon.

Similar to birds, fellow paragliding pilots are another handy way of deriving wind directions. Especially when they are close to the ground, when landing out for example. And since paragliders are a lot bigger than birds, they are visible from far away. I clearly remember heading north to La Meije one windy day, seeing a paraglider land a few kilometres below me, barely penetrating a strong north wind blowing down (not up, mind the exceptions!) the valley below. I was not bothered that much by north wind at my altitude, but the observation confirmed my decision to go around the east side of La Meije and then on to Galibier, instead of flying to les Deux Alpes via the southern side.

Sailplanes are even more effective here. Their size makes them easy to observe from kilometres away and they are more spread out throughout the Alps than vultures or paragliders. Being white and having a large aspect ratio, they clearly show their crosswind correction when flying above or below you in the distance.

Pollution

Seeing pollution as a friend, as something valuable, might shock you. I know this is completely politically incorrect attitude, but since I can't do anything about other than I am already doing, let's just profit from it. As a rule, I try to avoid polluting nature, even though nature sometimes seems to do just the opposite. Frequently spitting millions of tons of sulphur dioxide, carbon dioxide, and hydrogen fluoride into the air, seems very environmental unfriendly to me. Yet nobody seems to be ordering volcanoes to shut up, except for some crazy politicians that believe they can achieve just that. Lots of people however (including most of the politicians), tell me that I should not drive my car to the mountains. Can you hear the volcanoes roar with laughter?

Late spring and early summer are excellent times for going cross-country, but unfortunately not the best of season for those suffering from hay fever. It is the season when most of the vegetation is polluting the air with pollen. Most of us have learned to live with that and don't show hay fever symptoms, and all of us can profit from it. Pine trees for example, release huge clouds of pollen, colouring the thermals yellow as they rise. I have observed these coloured thermals from kilometres away, often like coil shaped vortexes rather than bubbles. Dust and smoke are similar indicators.

On a larger scale, you can derive the wind direction from characteristics of the air. Air from different regions has different characteristics such as smell, temperature and hue. Air flowing over a large homogeneous area such as a sea, takes on the characteristics of that area. The air coming from la Provence for example (low altitude, lavender fields, arid, hot), differs from the air coming from the north-east (high mountains, humid, meadows) or the south (sea, very humid, cold in summer, warm in winter). Sometimes the smell is nasty. Flying from St.-André to Bleine for example, you will start smelling the incinerator near Malamaire at some point. By drawing a line from your current smelly position to the incinerator, you obtain the wind direction.

Next to smell and dust, colour and hue of the sky are important air characteristics as well. Continental dry air blown in by a north-west wind, gives a clear blue sky in the southern Alps (mistral conditions). Air blown in from the Mediterranean sea in the south, gives a more milky blue sky because of its humidity. By looking at the colour difference in the sky, you can actually see where the north wind stops and the south wind commences.

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2011-01-12