The Crux of Cross-Country Flying Often Lies in Correctly Answering the Question, Where

Thermalling (by Will Gadd)

The crux of cross-country flying often lies in correctly answering the question, “Where’s the next thermal?” If you could answer that question correctly even 90 percent of the time then life would be very, very good. I think it’s key for every XC pilot to develop his or her own system for understanding thermals, then continuously refine it. Only in this way will the pilot actually learn something with each “success” or “failure.” I often hear students in clinics I teach say, “Ah, I sort of knew that, but this simplifies things a lot.” That’s the goal: To have a simple, clear system that you can refine each season to produce better results. I broadly split my thermal- prediction model into two parts: ground-based thermal prediction ideas, and sky- based thermal clues. This article is my attempt to explain to myself and anyone who finds it interesting how thermals form on the ground and how to find them efficiently, part two will deal with the sky, part three with staying flying thermals.

Collectors

I call potential thermal generating areas, “collectors” because they collect the sun’s energy and release it as warm air or thermals, a process any successful XC pilot should be very interested in. I think the air in collectors tends to warm up as the sun heats the ground, first releasing relatively slowly and steadily (early morning mountain thermals are the best example of this), followed later in the day by more violent “sets” or cycles in much the same way waves hit a beach. Imagine small waves coming in continually, then a big set ripping through, followed by small waves again. If you find a good collector, you can often maintain in a zero over it and wait for a good set to go through; if you’re low, this may be your only chance.

Collectors are all about sun. If there’s no sun, then there’s probably not much air leaving the ground (cold fronts and other very unstable air masses are exceptions). When looking at any potential thermal collector, I first ask, “How long and at what angle has the sun been shining on the collector?” A perfect collector would be at right angles to the sun for hours. I first learned this lesson flying in the ’96 US nationals when all the top pilots flew to the sunny but lee side of the ridge and I went to the windward side where the sun was just starting to hit. I sunk out, they didn’t. At the time I thought this experience was bad luck; luck had nothing to do with it, the slopes simply hadn’t been in the sun long enough.

The next factor that determines how much the air heats up is the surface the sun is striking. For an excellent analysis of surface thermal theory, read Reichman’s Cross-Country Soaring. Basically, dry surfaces with a lot of trapped or sheltered air will produce the best thermals. Late-season cereal (wheat, oats, etc) crops are dry, hold a lot of still air, and consequently release some of the best thermals. Dry shrubbery also works well; rocky terrain with a lot of dead airspace between the rocks works well, but takes longer to heat up. Moist ground cover absorbs the sun’s energy and uses it to evaporate water, a cooling process that kills thermals.

Wind tends to destroy thermals by continuously mixing the air in potential collectors, preventing it from either reaching the temperature at which it will leave the ground or turning what could have been a decent thermal into a ragged mess, especially close to the ground. A large line of hedges or trees around a very dry but bushy field will often hold a nice still “pocket” of air. You can experience thermals on the ground by just walking around; sunny, dry spots protected from the wind will be warmer. As odd as it might sound, I’ve learned a lot by simply walking in the mountains and feeling the cool air in the pines, contrasted with the warm air on avalanche slopes or other treeless areas. The more protected and sunny a collection area is, the warmer it will be and the better chance you as a pilot will have of going up. This means that the best thermals are often found in sunny lee areas; this is no problem if you’re high and fly above them, but you have to make your own decisions about how much rotor you want to play with if you’re lower. This isn’t an article about safety.

Many pilots believe black pavement such as that found in big parking lots or roads will be a good thermal source; although pavement is black and absorbs tremendous amounts of energy, it often doesn’t work very well because there is nothing to “hold” the air in place; if you watch birds soaring above a parking lot or freeway, they will almost always be turning very small circles and not gaining much altitude. The thermals are frequent, sort of like grease popping off a skillet, but frequently unusable. Interestingly, a parking lot filled with cars generally works better than one without cars because the cars hold dead air nicely. A road can be good “wick,” but more on that below.

The aspect angle of the terrain is critical. For example, dry plowed fields almost always work better than dry flat fields. I think this is because the sides of the furrows tend to face into the sun like little solar collectors, while the actual furrows protect the warm pockets of from the wind and allow them to develop. If you’re mountain flying, then look for the slopes that have been at right angles to the sun the longest. Lee slopes often work better than windward slopes because the air in the lee is protected, but a windy slope in the sun will beat a shaded slope in the lee every time. Really massive, South-West facing slopes in the mountains may offer continual strong thermals from mid-day through early evening, but east and due- west facing slopes will only work in the morning and evening respectively.

The anti-collector is of course a lake. Cool, reflective, moist, often windy. You will almost never find a thermal that comes from a lake. That is not to say you won’t find thermals over lakes, but they aren’t coming from the lake itself very often. One exception may be very late in the day when the relatively warm water releases heat, but I’ve very seldom seen this happen in a strong enough manner to produce usable thermals. Long glides over lakes in the evening are often quite buoyant, but don’t count on “magic” air too often or you may be swimming.

Passive Triggers (and wicks)

I believe thermals have some form of surface tension, and tend to track along the ground before releasing, sort of like oil up a wick. I call the point at which the thermal leaves the wick a Passive Trigger. The most PT is the top of a sharp peak; there will often be a cloud over it from 9:00 in the morning until sunset, even as the sun rotates from east to west. First the east facing slopes warm, wick up the hill and release, then the south-east facing slopes, then the south slopes, followed by the west-facing slopes at the end of the day. However, the thermal comes up the wick to the same passive trigger. Think about the “House thermals” at your local site; what’s really happening with each one as the sun rotates? If you’re high then you can fly straight to the wicking top of the peak, but if you’re low then you need to fly to the sunny side of the peak and then climb out. Ridges often work the same way, with convergences happening if both sides of the ridge release at the same time.

When mountain flying I look for PTs where I think bubbles might break their tension and lift off; ridges above protected slopes in the sun and places where a ridge forms a mini-summit for thermals to break off at (like water running down your arm and falling off at the elbow) seem to work best. Two or more ridges coming together are better than one, each ridge increases the chance that you’ve picked the right wick. If you’re bored, take a spoon and stick it into a glass pot of boiling water some time, it nicely illustrates how all this works.

Passive triggers can be very, very small when flatland flying. For example, a road on the downwind edge of a large, dry plowed field will often have a small ditch between the road and the field; this is a passive trigger for sure. Just the edge of a dry field against a more vegetated field may be enough to lift the air off; I almost invariably find my best thermals in downwind corners of large, dry fields, places with maybe a hedge or even simply grass instead of plowed dirt. A group of houses in the middle of a barren section or even a lone oil well breaking the monotony of flat ground will often wick thermals skyward. Some people believe strongly in powerlines as passive triggers, but I think the thermals found above powerlines generally have more to do with the terrain. The exception is that really big high-tension towers be wicking thermals skyward, but this is suspect. Thermalling over power lines does impose a bonus hazard as well.

Large rocks are often good wicks and passive triggers, as they tend to pierce the surface tension and also release “bullet-style” thermals, allowing larger pockets of air to also leave the ground. Finally, contrasts in surface temperature may affect lapse rates and also act as triggers. I often find thermals at the junction of two disparate surface types; miles of dry fields leading up to a large lake will often have a reliable thermal at the boundary between the two (if the wind is coming from the fields, this thermal will slope out over the lake). However, wet fields or lakes will often shut down all activity in their immediate area, especially on the downwind side. These surface temperature differences can be quite small, but thousands of examples have taught me that they matter.

Active Triggers:

Active Triggers are triggers that move. For example, a tractor harvesting dry wheat field will almost invariably be a thermal source. Cars driving back and forth on a road next to a big dry field will also act as triggers. Any type of motion, be it from people, farm equipment, cars, even other glider pilots landing, will often cause a collector to release. How many times have you landed in a likely field only to watch someone climb out above you?

I am starting to believe that cloud shadows will often act as active triggers also; I have flown enough sites now where the forward edge of a cloud shadow will produce dust devils as the shadow advances across the ground, something like a mini cold front lifting the warm air up. It’s a theory, but it does seem to work some of the time.

How to apply all of this:

On any given day thermals reach a certain height before stopping, a distance between the ground and cloudbase or the top of the usable climbs. I call anything below half this distance “low,” and anything above it “high.” For example, if cloudbase is 6,000 feet above ground level, then I think I’m high over 3,000 agl and low below this point. This article deals with making decisions while in the “low” zone. If you’re low, head for collectors that are in the sun and have been for a long time. Be very careful flying into cloud shadows; if you’re low, it’s very rare to climb up out of a cloud shadow. Connect the collectors with the potential wicks and triggers; sunny meadows below a sunny ridge in a light lee with puffy clouds directly above are perfect. If you’re on the shady side of a ridge then you’re in the wrong place and need to find some sun in a hurry. A big brown field with a small knoll on the downwind edge could be good, or a big dry grassy field that meets a busy Interstate. I try to fly over as many potential collector/wick/trigger combinations as possible. If I get even a consistent “zero” on my vario while low, I’ll stop and circle until a thermal “set” comes through. Of course, if you see a hawk going up like mad or a big dust devil spinning off the back of a tractor, well then things get simpler. I won’t mess with weak thermals if I’ve just topped out a climb and am starting a glide, there’s no point as they will probably end soon anyhow. I will stop for anything solid once I get into my “low” zone. It’s important to understand that the lift and sink generally balance each other out, especially in relatively small areas. If your climb rate is 1,000 fpm, expect at least 1000fpm+ sinking air when leaving the thermal. If the thermals are large, expect big areas of sink. If you’re in an area of violent sink, then somewhere close by is probably a violent thermal. You should ask, “where’s the collector, where’s the wick, where’s the trigger, attack!” Collectors also tend to draw air into them as they release; you will often notice an increase in your ground speed as your near a thermal. Your glider will also often pitch ahead by a few degrees as the air accelerates toward the thermal, and your heavier body lags. Older gliders will generally fall slightly behind you as they hit a strong thermal but be very pressurized (you can feel this in the brakes). Wind gusts or turbulence may cause an glider to fall back behind you as well, but the pressure will not be as high in the glider. This is a great way to tell if you’re entering a thermal or have just found a wind gust. If the glider is pressurized harder, then you’ve found a thermal. No pressure, no thermal. Newer (’99 and on) or higher performance gliders usually surge forward into a thermal, no matter how strong it is, but the feeling of increasing brake/glider pressure is the same.

Finally, remember that the wind slopes thermals; if you’re relatively low and coming into a collector then it won’t matter much, but the higher you are the more downwind of their source you’ll need to be to intercept the column.

The system above may be largely wrong, but it’s the best one I’ve developed yet. Each year it seems to get a bit better, and each year I look back and think, “Oops, was I ever wrong about that!” I try to honestly look at each flight and think, “What worked? What didn’t?” Why did I sink out and someone else succeed? Good pilots create their own thermal luck remarkably consistently. So good luck developing your own system, that’s the one that matters! Thermals and Clouds This article is part two in a three-part series. Part one covered how thermals form and release from the ground; this article covers the relationship between thermals and clouds. The final article in this series will cover thermal flying techniques.) First, this article focuses on dealing with our best visible thermal indicators, clouds. There are dozens of books written on lapse rates, instability and the like, so the ideas presented here are more field rules for flying clouds and other sky-based clues rather than a meteorology text, please forgive the gross simplifications I make. The basis for understanding what’s going on in the sky comes from watching it; reading books (or articles like this one!) helps, but you need to have your own on- board sky-interpretation system to fly well. Every good pilot I know has spent literally thousands of hours looking at the sky and trying to figure out what’s going on up there. I have spent many blown-out days lying on my back watching the sky swirl over me, and these days are some of the most valuable time I’ve ever put into flying. Are the clouds being blown to bits? Do they remain relatively constant over set points or form over a set point and then drift off downwind decaying as they move? Do they cycle evenly, starting as thin whispies and then forming into ever-more solid masses before decaying, or do some pop up very quickly and then disperse slowly? Do they have hard, flat bottoms or a rounded, mushy appearance? Each answer to these questions provides a wealth of knowledge about the thermals that are generating these clouds. Clouds are infinitely variable, but I believe they do have patterns that can be learned by watching them. The big concept here is that clouds cycle based on their attached thermals. As a warm air mass rises it eventually reaches an altitude where its moisture condenses out. This process continues only while the cloud is being fed by a thermal (condensation “pumps” basically act the same as thermals, so I’ll treat them the same here for simplicity). At some point the collector or pool of warm air on the ground is exhausted, but the cloud is still being fed by a “bubble” rising above the ground. Eventually no more rising air feeds the cloud and it starts to decay; at this point there is no more lift under it. This is why many of the best-looking clouds often provide no lift when you fly under them; while pretty, they are at the end of their useful cycle. As clouds decay they will in fact usually produce sinking air, which is annoying if you’ve flown to one expecting an elevator ride back to base. What’s more useful is to connect with the rising air under clouds that are still forming. So how do you tell ‘em apart? The simplest cloud game is to try and predict whether a cloud is forming or decaying; before doing this in flight, I like to play the cloud prediction game while mowing the lawn, driving, or looking out the office window. Pick one cloud and make a snap decision: is it forming or decaying? Then carefully track that particular cloud through the rest of its cycle; if you think it’s forming, it will grow in size (either vertically or horizontally or both) while becoming ever-more resistant to light (more suspended water means going from whispies to small “clumps” of moisture to solid white to gray). If it’s decaying then it will become ever lighter and slowly fragment into smaller pieces, How long does this process take? Two minutes? Ten? Twenty? Or does it just continue to develop into a monster cumulus savage-your-gliderus? I can seldom make good predictions based on just one look at a cloud, but after watching it for a couple of minutes I can usually tell which direction it’s heading. I believe that it’s absolutely basic to learn the life cycles of clouds if you want to fly XC; this is the aerial equivalent of knowing how to read. Michael Champlain, one of the better XC pilots I’ve met, taught me a good trick to help understand what clouds are doing while you’re flying. He recommended taking a series of mental snapshots of the sky as I climbed in a thermal. With every circle I look downwind and take a quick “picture” of what all the clouds in my predicted flight direction look like; a long climb may allow for 30 or more good snapshots, and with minimal practice I have learned to memorize which clouds are forming and which decaying based on these snapshots. Over the course of a few climbs my snapshots also give me good clues on how long the clouds are lasting, information which then tells me which ones may still be forming after I glide to them. If the cloud cycles are lasting 30 minutes then I can glide for 10 or 15 minutes and still arrive at a growing cloud with plenty of time to catch a ride. Generally, the more distance between clouds the longer they will last (a larger volume of air is feeding into a single cloud), and the higher cloud base is. If you go on glide toward a cloud that has been forming for 30 minutes and arrive low, the odds are slim that you will find lift no matter how beautiful the cloud over your head is. Many pilots make the mistake of climbing to base, then looking around and heading for whatever cloud looks “best,” regardless of where it is in its life cycle. If you arrive at a cloud after it’s useful lift cycle then it’s worse than gliding into a pure blue hole as there will be sink under it, plus the ground may be shaded, a double hit to your odds of staying in the air. But if you near the top of your climb and see whispies start to pop within gliding distance and head out on glide toward them, then the odds are much better that you will find useful lift. OK, you’re on glide toward a fine forming cloud, but where will you connect with the lift? Again, observing the cloud cycles will tell you. If the wind is stronger aloft than on the ground, the clouds will be forming at their upwind edges and decaying at their downwind edges. This tells you that the thermal will be sloped at some angle from upwind of the cloud to it. If you have a GPS or learn to read your groundspeed even while fairly high, you can figure out how strong the wind gradient is and therefore how much the thermal slopes. As a rule of wing, I visualize thermals in wind gradients of 10 MPH or less as sloping at up to 20° , 20 or less MPH at 30° and so on. Also realize that the gradient will often not be linear; there are many days where you will encounter some sort of strong gradient at a particular altitude; the thermals here will often become disorganized, but if you can fight through this barrier then you may continue on to base. Remember this altitude and anticipate doing battle to get through it instead of getting discouraged and heading off.

Some of the most frustrating XC days come when the winds are slower aloft than they are on the ground; I have found this situation surprisingly often and could never understand how to find thermals until I realized that the clouds were forming on their “downwind” edges and dissipating on their upwind edges! The more moisture-laden areas of the cloud will be on their downwind edges; in this situation you will actually connect with the thermal downwind of the cloud. The shape and texture of finished clouds also offer a wealth of information. Clouds taller than they are wide generally mean stronger thermals and may lead to over-development later in the day (don’t get me started on instability…). Puffy, closely-spaced clouds that cycle relatively quickly but never attain flat or “hard” bottoms generally don’t have very good lift under them; however, the light lift will be easy to find, just fly downwind and you’ll probably blunder into something. Because these clouds cycle so rapidly it’s almost impossible to time your arrival under one that’s developing. However, they often form up in general areas, and these areas will offer better chances of staying in the air. On humid days the sky will be absolutely filled with evenly spaced clouds; unfortunately, only a few of these clouds will be active while the vast majority are slowly and irritatingly decaying. On dryer days the few clouds that are in the sky will most likely be active, but make sure to get there while they are still in their active cycle. Finally, flat cloud bases indicate well-formed thermals feeding continuously. Rounded, puffy bases usually indicate less well- formed feeder thermals and weaker lift. On days with larger clouds, pay careful attention to what part of the base is highest; the best lift will almost always be feeding to the highest part of the cloud. As you climb to base, keep looking around, you may be able to get higher under a different portion of the cloud than you climbed to it under. This is especially common when flying the border between moist and relatively dry air masses; I have seen clouds that are stepped up to 4,000 feet on the Texas dry line. In addition to understanding what kind of clouds to fly under, most people want to know what kind of clouds to avoid It’s often difficult to tell what your particular cloud is doing as you climb because the cloud tends to block your side- view of it; however, if you’re taking mental snapshots with each circle then you should have a good idea of what’s going on with the other clouds. It’s possible that you are thermalling up under the one giant cu-nimb in the sky, but it’s rare. If the sky is starting to over-develop all around you then it’s probably time to get out of the air regardless of what’s happening over your head. Even large clouds can cycle regularly; some days with cumulus clouds up to five or ten miles across are fine to fly on, but as soon as the clouds start growing much higher than they are wide I usually find myself either running for a much better portion of the sky or landing. After I land and my glider is secure I like to really watch what happens to the clouds I was worried about; did they cycle harmlessly, or are they continuing to blow up? If they did over-develop, how long did it take from the point I called my flight off to when the first gust front hit the ground? I have occasionally been frustrated that I landed early, but the few times I’ve pushed and stayed in the air too long were truly terrifying. The more I fly, the more conservative I become. If the clouds in the sky start “spiking” radically and look like fists on a day when the forecast is for thunderstorms then land immediately. Observing the sky intensely while flying isn’t just about finding the next good climb; it is the basis for safe flying. This leads me to the broader part of this article: In general, clouds form in related patterns. These patterns may be due to any combination of literally thousands of factors (again, it’s worth understanding the meteorology, buy the book), but these areas of instability are where you want to be flying to connect with the lift. I’ve blundered off into large blue areas only to hit the dirt enough to believe this. It’s almost always worth flying the clouds around the edge of a blue hole rather than jamming straight through it, no matter how much more direct the blue line looks. Sail plane pilots have the luxury of making huge transitions across sky features up to a hundred miles apart, we generally don’t. Most pilots dream of getting under cloud streets and flying straight until dark; while this does happen occasionally, I’ve found it more useful to treat streets as linked but individual clouds. If the street is set up with flat, hard bottoms and is maintaining good color (dense but not decaying and not over-developing as you fly along it, then stuff the bar and fly as fast as your understanding of speed to fly theory allows. But keep looking ahead and analyzing what is going on; sooner or later the clouds will end, and you need to be paying attention to what’s happening in front of you as well as to either side. I’ve often found it’s better to treat large gaps in streets as blue holes and jump sideways to another street if the gap in front of you is wider than the lateral jump by a significant margin. Many “blue days” actually offer some very good sky-based clues. For starters, even if clouds don’t form at the top of thermals, “haze domes” often will. These are areas where the light refracts differently through the air due to more moisture, dust or just a different air mass. I’ve seen haze domes most frequently when flying relatively stable blue days in Mexico and the desert southwest; often the haze domes are marked simply by areas of the sky that are less blue. Haze domes are also often the precursors to proper clouds—in the morning you might just get haze domes at an inversion level, but they still mark lift and often are the first areas to pop through an inversion and become clouds. Blue days will often still form dust devils or swirling thermal cores; if you can see hay, fine dust or other debris in the air then that’s a sign of a thermal core as well. Flying strategies: The classic model of thermal formation suggests one rising cylinder of air feeding one cloud. In reality, I picture the thermals feeding into clouds as trees, with many small thermal “roots” feeding into larger ones until they reach the trunk and lead to the cloud. The higher you are above the ground, the farther apart the “trunks” are and the closer to the clouds you have to fly to truly intercept a large thermal. Anyone who has flown competitions will have seen gliders climbing relatively close together but in different cores before joining and continuing to base. Gliders that are low can take advantage of the smaller “root thermals,” not just the trunks. If you’re in the “low” zone, meaning below half way below cloud base, then you will most likely find relatively small cores. Sailplanes have a relatively hard time taking advantage of these lower-altitude thermals, but we can core up in very small circles, following the individual roots until they expand and join with other thermals. If you’re below half the distance between the ground and the cloud then you can pretty much forget intercepting a large core that connects to the cloud; however, most clouds are fed by multiple smaller cores that join together, so searching over good collectors and triggers upwind of clouds is a good strategy (remember to know the day’s gradients for which way the thermals will slope—the thermals may be “downwind” of the clouds on days with an inverted gradient). I usually try to connect the collectors and triggers to the clouds they are feeding; this is also useful for predicting where the cloud is in its life cycle. For example, clouds that form over mountain ranges are generally flushed downwind. Once they are flushed past their thermal sources there may still be lift under the cloud as the thermal “bubble” continues to feed it, but you need to arrive relatively high to climb in this bubble no matter how great the cloud looks. The higher the cloudbase, the longer your glide to the next climb will be (unless you have the good luck to be flying under a street of some kind). Reichmann predicts that the distance between clouds is approximately two and a half times their distance above the ground. If the base is 5,000 feet above the ground then the distance between thermal “trunks” is likely to be 12,500 feet (the distance between the “roots” will likely be somewhat less). Even if your glider only goes at 5:1 then you should have a reasonably good chance of intercepting a thermal before intercepting the ground! Theoretically, it’s very rare to glide all the way from base to the ground without hitting lift. In reality, I have done it often, particularly on blue days, but usually in retrospect I went gliding off into a large blue hole or down a sink street and should have turned 90 degrees after sinking more than half the distance between base and the ground to find lift. In the flats I think lift generally forms in lines and so does sink; even on blue days, the next logical place to look for a thermal is above a good collector/trigger downwind of your last climb. In the mountains the thermals and clouds generally form above ranges which may or may not be oriented with your planned flight or wind direction. If you are crossing anything except very narrow mountain valleys on very high-base days then you need to base your decisions less on what the clouds are doing and more on the ground-based tactics covered in the previous article. If you are crossing small gaps while flying along a range then it’s often reasonable to use the clouds to plan your next climb, especially in the American West where base can exceed our FAA-imposed limit of 18,000 feet regularly. Most of our ranges in North America run roughly north- south, while the wind predominantly blows from west to east. One good trick for crossing the valleys between ranges is to climb to base, then drift over the gap with a cloud. This is slow, but XC flying is often more about staying in the air than speed. I’ve used this trick several times at King Mountain and other sites to beat gliders with far better glide ratios. The cloud will eventually start seriously decaying, so it’s better to leave it before this point or you will have to deal with sinking air.

Don’t get too aggravated if you can’t get to base, I generally only get there on days with well-organized climbs leading into flat-bottomed, dense clouds. On more humid days with poor lapse rates (oops, slipping into tech talk here), there may be plenty of clouds but no way in hell to get them. Do note how high you got in your climb before it disintegrated, and roughly how far below base you were. If your first climb of the day ended at 6,000 feet and base looked to be at about 8,000, then expect that the top of your next few climbs may be at a similar altitude unless the clouds start looking better or moving higher. Cloud base usually moves higher throughout the day, and climbs generally improve until late afternoon. If the clouds go to 10,000 feet and start looking really solid, then you might expect to climb higher and closer to the clouds. The best way to truly understand the sky is to study it with near religious fervor. Read the books and understand the meteorology of any given day, then correlate what was predicted to what actually happened on your flight. If you can’t get into the air due to earthly responsibilities you can still learn a tremendous amount about flying. This will help you immeasurably when it comes time to make decisions while under your glider. My next article will deal with flying your glider in thermals and putting everything in these last two articles together. Happy flights! Thermalling Technique

My favorite part of flying is undoubtedly thermalling; in fact, thermalling may be my favorite thing to do in life. There’s nothing like hooking a sharp-edged, positive ripper of a thermal and riding it upward for a couple of miles. My least favorite part of flying is also thermalling; those days when everyone else goes up flying straight and you hit the deck like a dropped park bench - repeatedly. On those days you’re glad you landed alone so no one else can hear you scream. The following is my latest “thermalling system.” I hope it helps you develop yours.

Thermal Theory

A little more thermal theory is useful to understand how to fly them. I believe thermals close to the ground are often quite small and relatively violent. As they rise they tend to smooth out and expand. Pressure also tends to influence thermal formation; high-pressure days tend to produce smaller, sharp-edged, “punchy” thermals. Lower-pressure days can produce very strong thermals obviously, but they tend to have mellower edges and be larger in size.

The day’s lapse rate also influences thermal strength; a hot day with a very strong lapse rate will produce stronger thermals. Think of a very warm piece of air rising out of a collector on a day with a strong difference in air temperatures between the ground and say 5,000 feet above it. A thermal will rise quite quickly in this situation. An inversion is the opposite, and not surprisingly thermals usually stop or at least slow down at inversions.

The above factors (and hundreds more but this is a start) give each day its thermal “profile.” If you launch on a clear blue day (indicating high pressure) with a good lapse rate (you checked the day’s soundings), then you might expect sharp- edged, strong thermals. If, however, the sky is filled with soft cumulus and looks somewhat hazy due to moisture, then you might expect softer thermals. The first thermal of the day provides some good clues about what’s happening; if it rips you upward and all you have to do to stay in it all the way to base is turn a bit then you’re off to a good start. If it’s small and difficult to stay in then ends abruptly 1000 feet later and you can’t take it any higher, then you know the day will be more difficult. I take a mental note of three important characteristics with each thermal I use during the day. What is my average climb rate? Not the spikes, but the true climb rate as expressed by a 20-second average? How high do I get before it totally falls apart, and are there any altitudes that seem tricky to keep climbing through? And finally, what are the size and drift of the circles I’m making?

The climb rate tells you what to expect as the day progresses; climb rates tend to improve until late in the day, and thermal size also tends to increase as the day wears on (sink too unfortunately). If you’re getting solid 600fpm climbs, then it’s probably not worth stopping in 100fpm on a glide unless you’re low (anything going up when you’re low is great). The peak thermal altitude is also useful; if you are getting to 6,000 feet AGL consistently but a strong thermal suddenly “stops” at 4,000 AGL then you’ve probably lost it and should search for it. However, if the thermal stops at 5,800 feet then it’s most likely done and time to go on glide. Remember that the peak altitude of the thermals should increase as the day progresses. On good days in Texas it’s not uncommon to see thermals in the morning only reach 4000 AGL, then 6000 AGL at noon, 10,000 at 2:00 p.m. and 14,000 at 5:00 p.m. This progression is generally less in the mountains but still observable.

Finally, the size and drift of your circles at various altitudes also tells you what to expect on the next climb and information on wind speeds aloft. This tells you what angle your thermal will be flowing from a collector so you can intersect that line I (note-very strong thermals will have no problem pushing the wind around them like a bridge abutment in the river).

Coordinated Circles, not Swings

OK, so you’re flying along and your vario starts beeping with the good noises. What to do? First, did your glider surge forward or fall back behind you just before the beeps? If it went behind you then you’re probably dealing with a “gust.” Wait and see if the beeping continues or goes back to sink. If it’s a thermal and the beeping increases, turn. I don’t worry too much about which direction; if one side of the glider is noticeably more pressurized or higher above you, then lean meaningfully in that direction and pull on the brake smoothly. How much pull ? Higher pressures in your glider indicate a stronger thermal, meaning you can pull harder you can. However, the most common mistake in thermalling is to pull too aggressively on the inside brake. When you pull too hard on the inside brake your body tends to swing to the outside of your turn in a small wing-over. Then your body swings back under the glider, you lose the turn and fly straight out of the thermal. Many pilots then crank another wild-ass turn to try and get back into the thermal; I flew this way for about five years before getting it figured out. What you want to do is fly in a “coordinated” banked turn. This is like riding a bicycle; you and the bike are at the correct bank angle for your speed and the sharpness of the turn. One of the most common problems pilots have is maintaining a consistent circle while thermalling; I expect you know what I mean… The correct technique is to start a turn with a smooth, controlled lean and simultaneous progressive inside brake application. The glider will bank up, your body will follow it, and due to centrifugal force you will continue to stay outside the glider’s circle and smoothly ride the thermal up. Jerking the brake instead of applying smooth increasing pressure will just swing you to the outside of the glider--then you’ll swing back under it, repeat. The glider will also remain over your head in a true coordinated turn; if it falls behind you, reduce brake. If it threatens to surge in front of you, apply a quick correction while maintaining your lean and turn. If you can’t figure out what I mean, pull on one brake sharply and release it; you’ll swing out from your glider then back under it, usually with an oscillation or two as a bonus. Then try leaning hard for a second or two then go back to neutral lean; you’ll swing out to the side of your glider then back under it, but not as much. Now smoothly lean, pull gently and progressively on the brake and hold it; you’ll enter a gentle spiral dive or circle, same thing. This is what you want.

Airspeed and bank angle are directly related; the higher the bank angle, the more airspeed you need to keep the turn coordinated (think of a spiral dive). The lower the bank angle, the less airspeed you’ll feel on your face. Thermals are seldom perfectly consistent; this means you will have to continually adjust your brake and lean to maintain a coordinated turn. If your airspeed starts decreasing and the glider levels out, lean a little more, let up on the outside brake a little bit, and increase your airspeed and bank angle. If your air speed increases suddenly, lean a little less, pull a bit more on the outside brake, and maintain your bank angle. If you can learn how to thermal in a coordinated bank then you are well on your way to thermalling efficiently.

Centering: The mental map

OK, so your vario is beeping like mad; how long do you wait before turning? If the day’s thermals are small and you’re low, start turning immediately after you’re sure you’ve hit something (not just a gust). Rules of thumb about waiting two seconds etc. are meaningless in my experience. You’ve found lift, initiate a smooth banked turn and see what happens. If you climb really well for a quarter circle and then start sinking, open your circle up a little bit in the direction you found the best lift then tighten as the lift increases; notice the pressure in your wing and how your butt feels in the seat, not just the vario beeping, these are critical clues. Listen to the noise in your ears as well; with practice, you can actually hear the different air flows as you fly through lift or sink; if you can’t hear the air then get a new helmet. At some point in your circle everything will add up to the best lift as defined by your vario, wing pressure and lift under your butt. If you’re flying a coordinated 360 then it’s relatively easy to develop a mental map of where the best lift is in each 360; don’t worry about the ground, but where you encounter the best lift within each circle. Try to develop a “mental map” of what’s happening in each 360.

To fly toward better lift, maintain a coordinated turn, just reduce the bank slightly as you come back around the 360 and move the center of your circle over a little bit toward where you got the best lift. NEVER STOP CIRCLING. Once in the best lift, tighten the circle up slightly while maintaining a coordinated turn. Perhaps you get solid lift for half the turn, general sink for half the turn. Move the circle in the direction of the best lift again. Now you get solid lift for three quarters of the turn and less lift for one quarter. Move it again. Now you’re climbing solidly for the full revolution of your turn at +400 fpm average, but one portion of your circle is going up at +600 and another at only +200. If you weren’t in a coordinated turn, and most pilots aren’t, this would probably be due to the oscillations inherent in thermalling in an uncoordinated turn and you would not have a clue what’s actually going on. But you know to thermal in a coordinated manner, so you move your circle toward the +600 and eventually lock in a perfect 1000fpm climb all the way to base. Irregular thermals may give irregular “instantaneous” readings on your vario, so focus on getting the best average climb rate that you can. Hang gliders and sailplanes can use all kinds of funky ovals and figure-eights to get better average climbs, but I have found paragliders climb best flying coordinated, continuously adjusted circles (or straight if the thermal is big enough!).

Circle Size and Bank Angle

I find I thermal with 30-45 or more degrees of bank on days with small, strong thermals, 15 to 30 on lower pressure days and almost flat on days with light, wide thermals. The extremes of bank angles come in dust devils (almost vertical) versus flying straight and flat while climbing like mad under a big cloud; somewhere between these two extremes is the correct angle for your thermal on that day. Every glider responds differently to brake force and the amount of lean; what works for one pilot on his glider usually has little to nothing to do with yours. However, every glider will circle in a coordinated manner, and the feeling is unmistakable once you get it.

Here are a few scenarios to help pick bank angles for thermalling. Say you’re flying along in -600 fpm and suddenly you’re screaming up at +800. You turn, then go down at -400,so you move your circle toward the +800 but can’t lock it in despite continually re-centering your circle. You probably need a higher bank angle and smaller circle. If you’re very low in a small thermal, you may only be able to get half a turn in. Do your best to just improve how much of each circle you spend in lift, you’ll lock it eventually as you climb. Another scenario: you’re flying along in -600 when your sink rate starts to decrease smoothly to zero sink, then +200, then +300. I would keep flying straight until the lift starts to decrease, then initiate a relatively gentle bank and center on the best average climb rate. A relatively gradual, consistent rise in your climb rate is a sign of a large thermal. Often you can find very strong cores in large thermals that will offer much higher rates of climb, but in general the larger the thermal, the less bank angle the better to maximize your climb rate. Some bank angle is usually good; a glider won’t turn in a coordinated circle without it, but you can fly in a coordinated turn with equal brake using lean; watch a good pilot fly and you can tell he or she is often controlling the glider primarily with lean and modest adjustments to the outside brake. There is no correct number of pounds to pull on your brakes while thermalling or distance to pull them down (1/4 brake is meaningless across a range of gliders), but there is a correct amount of brake to pull and lean to maintain a coordinated turn. It’s like riding a bike; no one can tell you how to do it, but you stay upright when it works. I generally thermal with roughly twice the amount of brake pressure on the inside brake than the outside, and adjust my turn primarily with lean and the outside brake. You will probably do it differently, but know a good coordinated turn when you hit one.

Don’t change directions when thermalling, especially when low. There are three good reasons for this; First, changing directions messes your coordinated turn up and you have to fly straight for some time between turns which usually takes you away from the lift (all directions but one lead away from the lift…). Second, you lose your mental “map” of where the best part of your circle was. Third, the direction change will cause your vario to beep in all kinds of interesting but non-helpful ways. It is almost always better to simply move your circle over toward the better lift than try to switch directions and fly toward it.

If you’re having a hard time maintaining a coordinated turn, try flying a bit faster; use more lean and less inside and outside brake. Many pilots try to fly a perfectly flat circle; in truly massive lift this works well, and your glider may have its best sink rate with a fair amount of brake on. However, I find flying a bit faster with a mild bank often enables me to lock in the thermal’s best lift. Don’t confuse what works well while ridge soaring with what works best thermalling, it’s a very different game.

What do to do when you lose the lift

First, know if you’re at the top of the thermal or not. If every thermal so far has ended at 6,000 AGL and you’re at 5,700 then forget about it and go on glide. But if you’re climbing well at 3,000 AGL and lose the thermal then it’s time to go into search mode. If there’s any wind at all, the thermal is probably either directly down or upwind of you. The first thing to do is expand the size of your circle and pay attention to your mental map. If you were climbing at +200 fpm and then start sinking at -600 on the upwind portion of the 360, open the circle up back downwind. If the sink improves to -400 and then -200, move it even more downwind. If nothing good happens, try moving upwind; again, an improvement in sink is as as relevant as finding more lift, work toward the area of lesser sink. Also pay attention to your groundspeed; it will generally increase as you follow the air flowing into a thermal, but decrease if you’re bucking the wind flowing into a thermal by flying away from it (remember that thermals, especially when low, pull or entrain air into them). If I’m low on windy days I tend to fall out the upwind edge of the thermal. If I’m high on a windy day I tend to fall out the downwind edge of the thermal. I have no idea why, but that’s how it works.

I’ve seldom encountered thermals that are smooth cylinders from the ground to base; the trick is to follow your vario, wing and seat pressure up in the best lift with continual gentle adjustments to your coordinated circle.

More Clues for Better Thermalling

If the outside of your wing loses pressure suddenly and ruffles or takes a mild collapse, you’ve just found a relative difference in lift. Perhaps you’re in +600 and your outside wing just hit some +50; you want to move your circle away from the area you just took the turbulence in and toward the better lift. If you’re thermalling in a gaggle and see someone take an outside wing deflation ahead of you in the circle, then it’s probably worth tightening your circle away from that area and then opening it slightly to fly toward the better lift, tightening the circle as you encounter better lift. Most pilots tend to fly the “pattern” in a thermal rather than really watching the climb rates of the other gliders; if everyone climbs better in one half of their circle than the other, move your circle toward the better lift; you’ll climb above the other gliders quite quickly using this tactic. If someone is out-climbing you off to one side then move your circle to them; there’s no heroism in climbing slowly by yourself.

If you see the glider in front of you in a gaggle start climbing like mad, you may want to start tightening your circle immediately so you are in a higher bank angle as you hit the rising air and can “grab” more of it; again, fly the thermal, not the other pilots.

Look for pollen, plastic bags, bugs and other debris in your thermal. Birds in general and Swifts in particular will almost always be in the best part of a thermal; follow them immediately. Swifts and other small birds seem to eat the bugs that are drawn into thermals; if you see a group of them swarming upward, jump in with them even if doing so requires a short glide. Because thermals are pulling air into them, trash often automatically centers itself in a thermal; I’ve climbed thousands of feet in the company of newspapers or other debris.

Some days produce thermals that seem to want to spit you out; most of the time I’ve found that this is due to flying with too large a circle. Think of a spout of water shooting upward; if you stick your wing into the center and keep your circle within the column, you’ll go up. But find the edge and you’ll lose pressure on the outside of your wing. This creates drag, you lose your bank angle and tend to get “pulled” out to the side. Try flying with your vario turned off; Chris Mueller and many other top pilots often fly long distances without their varios ! I don’t want to get too esoteric here, but how your glider feels in lift becomes clear if you focus on the clues. Turning your vario off forces you to pay attention to what’s really happening with your glider in different currents of air. I’ve learned a lot in the last year by playing this game, especially in gaggles where I can watch other gliders.

The smoothest air is often right in the core of a strong thermal, and your glider will be more pressurized and stable if you are flying a higher bank angle; if I’m climbing quite fast, I know that the edge of the thermal is likely to be quite turbulent. I’ve never flown away from a very strong thermal as I know I’ll hit turbulence doing so; the best thing you can do is lock into the core and take it to base.

The most extreme variations between sink and lift tend to be below five hundred feet off the deck; you’re flying along in 600 down and suddenly you’re ripping at 1000 up, then falling out of the sky again. However, the best true average climb rates tend to be higher in the thermal until it cools to the point where it won’t give you any more lift. I often will see spikes of over 1500 fpm low to the ground on days where I can’t get more than 600fpm climbs on the 20-second averager. A thermal’s real climb rate is what you can get out of it on the averager, not the “spikes.” I often hear pilots say, “Dude, I got 2000 fpm today!” They are almost invariably referring to the lift spikes and not their true rate of climb. The only place in the world I’ve seen true 2000fpm climbs is the Owens Valley in July, but crank a hard uncoordinated turn and you can easily create your own 1000+fpm “thermal” as your vario swings up and beeps happily; this is a lie, but many pilots will believe it and keep creating their own thermals with wild turns where there is nothing.

Finally, all of the above writing is just my own theory based off sailplane books, conversations with other pilots and personal experience. What really matters is your own theory; question it and refine it continuously for best results. If someone out-climbs you in a thermal it may be due to their glider, but it’s much more likely that they did something you didn’t. Don’t curse yourself as they ascend faster. Instead, try to figure out why. Are they using larger circles or smaller? Did they move their circle into better lift and you didn’t follow? I don’t believe anybody is born a better pilot than someone else, but some pilots do think about what they are doing and try to do better. I look forward to trying to do better this season, and wish everyone the best of luck! And, in the end, the best pilot is the one having the most fun.