From the first day in a glider, pilots have always been captivated at how remarkable it is to be able to climb in rising air in an aircraft that has no power. Ascending in a thermal the first time is an exhilarating experience, and most of us crave the lift from then on.
     Finding and using lift effectively is basic to all soaring, and although there are many reasons we lose people from our sport, the inability to climb and stay aloft is a major one, and more easily remedied than most.
     After a brief consideration of the mechanics of circling flight and the variometer, this discussion has been broken into the basic phases of thermaling: finding, entering, centering and optimizing lift.

Flying Skills
     Effective use of thermals requires the ability to fly in a true circle - constant bank angle and airspeed without surges, dives, spirals, or spins. Weight-shifting the glider with constant but prudent inside brake, using the outside brake to dampen and flatten the turn produces excellent controllable circles. This is a fundamental skill that must be almost automatic. A strong preference for one circling direction can be a handicap either when entering thermals already occupied by other gliders or by encouraging turns in the wrong direction.
     There is no single bank angle that's always suitable, shallow bank angles can keep the glider out of the best lift and slow the centering process, and steeper ones quickly increase your sink rate. A good tight circle in a small bullet thermal may only take 10 seconds to complete, while a wider circle in a big fat thermal accommodating many gliders may take as many as 16 seconds.
     You must be able to fly safely with other gliders. This requires that you develop the habit of seeing all nearby gliders (and keeping track of what they are doing), and that you know how to enter a thermal that other gliders have found (briefly, you start with a circle that's a bit too large and then tighten it to match the other glider's circles).

Instruments
     A variometer with audio feedback is essential. Without audio, a potentially dangerous amount of time must be spent looking at the variometer. A student trying to learn thermaling skills without a vario will be seriously handicapped.
     All varios exhibit an instrument lag, generally about 2 seconds. This arises in part from the instrument itself, and in part from the fact that variometers respond to height changes and it takes time for the glider to change height.

Searching for Lift
     Finding lift is like picking stocks: In a bull market everyone does pretty well. In typical markets, good stocks are to be found, but only those who make use of all the information available, who are systematic in their approach, and who are neither greedy or impatient consistently succeed.

When Cu Are Present
     Cumulus clouds are by far the best indication of lift. But often matters are not as simple as just flying under a cloud and finding a thermal there. Large and inviting clouds may arise from small thermals that can be hard to locate. Patience is essential, and the search for lift under cu needs to be systematic. The time and place to begin to understand the day's lift is right off launch. This should include noting whether lift is on the upwind or downwind side of the clouds, its location with respect to the sun, and its strength at various altitudes. It should be possible to relate lift to the appearance of clouds, to estimate the lifetime of clouds (this can also be done on the ground), to get a good feel for the size of the day's thermals, and to begin to develop a mental picture of the wind.

In the Blue
     Beginners and even experienced pilots are often needlessly unhappy about flying without any cu for guidance, but it's not all bad when the conditions are blue. No clouds mean no cloud shadows, so lift can be stronger and more widespread. No clouds mean no cloudbase so the height of the lift is controlled by the inversion, not the condensation level. But with no clouds, it is essential to pay a lot more attention to the many other thermal clues.

ABOUT THERMALS Thermals necessarily start out as a large, shallow area of heated air, bouyant in the cooler air that surrounds them, but unable to move because of the enormous drag that would be associated with any upward motion. This allows for significant heating of large volumes of air. A trigger of some sort is needed for the heated air to ascend. The trigger could be many things, but all effective triggers make it possible for air to begin ascending in a relatively narrow column somewhere within the pancake of warm air. Once this happens, there is no stopping the process. The entire area of unstable air starts to flow into the trigger area and a thermal is born. Our Thermal model (Figure 1) is circular and about 1,000 ft in diameter. It's surrounded by a region of sink, and the strength of the lift increases towards the center of the thermal. Experience suggests that this is a reasonable assumption. Thermals significantly smaller become harder to use, but in a paraglider, we are able to circle much tighter than hang gliders or sailplanes. Larger thermals are nice if you find them, but they still need to be centered. The diameter of thermals generally increases with increasing altitude and clouds are generally bigger than the thermals feeding them. Gliders are small in comparison to thermals; circles often aren't. Thermals are like the water that forms on the surface of a glass of ice water as it condenses. The drips finally gather and run down to the lowest point of the glass and drip off downward. Visualize the same thing happening with the heated air, only upside down. Picture this occuring on the landscape below while flying, you can see why holding the high ground is so important.

The Importance of Going for the High Ground
     Thermals favor high ground, and high can mean only a few hundred feet. Sometimes a small hill will make enough difference to justify your attention. There are good reasons for this: Air ascending a hillside continues to be heated, the angle of the hill may allow it to capture more radiation, sloping ground is drier, hills act as triggers, and if there is any surface wind there is an additional impetus for rising air.
     "As soon as you're off the high ground, start thinking about landing, because that's what you are going to be doing pretty soon."

Triggering
     Thermals do not require triggering - unstable systems have fluctuations that sooner or later become triggers themselves. But triggering features such as buildings, local hot spots (rock faces, antennas, fires, factories, tractors in a field, etc.) will usually beat random fluctuations to the draw. Once the concept of a large surface-bound pancake of unstable air is accepted (see About Thermals inset), the search for thermals reduces to the search for surface features that might act as triggers.

Local Convergence
     When air moves from a flat (low friction) region to a region with features that slow the airflow, there is horizontal convergence and vertical motion. This can act as a trigger. Some examples: Wooded areas, the edge of a lake, towns (which are also local hot spots and so doubly effective when there is surface wind), and of course any kind of higher ground.

Arial Clues
     I never second-guess birds, either in finding lift, or in centering it. Birds have about the same sink rate as gliders and a much lower wing loading. Even without all the extra practice they get, they are going to be hard to beat.
     Other gliders are obvious indicators of lift, but it is worth keeping in mind that misery loves company, so it is essential to assess the climb of a circling glider or a gaggle before running off to join them - it's discouraging to leave a weak thermal and join a gaggle that's not climbing at all. And since safety requires constant awareness of all nearby aircraft, it really is inexcusable to stay in weak lift while a nearby glider climbs well.

Figure 1: Thermal Model. This diagram depicts the speed of the rising air across the thermal. There is an annular region of sink. The area of lift is 1000 ft in diameter, with a core of 200 ft. The lift increases linearly from -2 kt (approx -200 ft/min) to +8 kt (approx +800 ft/min) Note that the vertical axis is speed, not height.

Entering Thermals
Timing the Turn
     Any thermal worth climbing in will usually impart enough vertical acceleration to the glider both to indicate lift and help timing the turn.

Figure 2: Timing the Turn. Getting the timing right always helps, even if things don't go quite as well as they have for Pilot C. A) Too soon. Even with the correct turn direction, Pilot A gets to fly in sink. B) Too late. Pilot B also flies in sink, but at least he knows where he has come from. C) Just right. Pilot C is almost centered in the first circle. Note that he started his turn as the lift began to decrease.

This acceleration, which is usually obvious without instrument feedback, is the single best indication that useful lift is present, and that the time to start the turn is fast approaching. It suffers no instrument lag and is proportional to the strength of the lift.
     Timing the turn is important - making corrections for mistimed turns takes time, and even more time is needed to make up the altitude lost circling in bad air. Figure 2 makes it clear why correct timing matters.
     Anxiety when lox, excitement when high, and perhaps an element of wishful thinking, make it easy to start the turn too soon - this is a very common error. There is no sure way of knowing the optimal time to turn, but for the thermal model the rule of thumb of counting to three after getting a good indication of lift is sound. A better approach is to rely on the variometer: with a 2-secong lag, the glider is somewhat ahead of the vario, so starting the turn at the first indication of decreasing lift will usually position the glider nicely.

Entering on a Diameter

Figure 3: Which Way To Turn. The further the entry courseline is from a diameter, the greater the cost of a turn the wrong way. A) No need to worry - both ways are wrong, and there is no way to distinguish. B) Right turn is favored, but unless the right wing lifts prior to the turn, the pilot does not know this. C) The sufficiently lucky or skillful pilot can perfectly tome the turn, get the direction right, and be centered almost immediately. In reality, the very best pilots can do this perhaps once in ten thermals.

     Entering a thermal on a diameter (Figure 3a) guarantees that the turn will be the wrong way. It is important to remember this as the vario promptly heads south, more or less as soon as the turn is started in what had been 800 ft/min lift. In all three case - turning too soon, too late, or at the perfect time - a correction will be needed.

Entering on a Chord
     It's more likely that the thermal will be entered on a chord (Figure 3b,c), and in this case there is a right and a wrong way to turn. The good news is that there is some chance that the glider, birds, or even debris will provide a clue.
     The chance of turning the wrong way when entering a thermal on a chord is 50%. Add to this the certainty of turning the wrong way when entering on a diameter, and it is clear that in the absence of additional clues, it's likely that the turn into the thermal will be in the wrong direction. It helps to know that geometry and probability, not caprice, account for this.

Climbing in Thermals
     There are many ways to center a thermal. What follows is a basic technique that works well and is easy to learn. Plan to practice and master this basic technique, then experiment with other centering methods.

The Initial Correction
     Only rarely will there be constant lift in the first circle. Far more often the vario will at some point (not necessarily immediately on entering the turn) indicate decreasing lift, or even sink,

Figure 4: The Initial Correction. With the vario heading down for more than half the initial circle it's time for the correction: As the glider comes around to a heading 90° to the entry heading, roll out, then roll back in again, moving the circle in the direction of the lift. This only works if the entry into the turn is reasonably well timed.

and will continue to do so for a good fraction of the circle. When this happens, and when the turn has been timed correctly, the correction can be made by waiting until the glider has come around to a right angle to the entry direction, rolling out, then rolling back into the turn again. This will move the glider closer the center of the lift as shown in figure 4. The skill lies in knowing the distance to fly before rolling back into the turn again. A good starting point is to increase the distance flown after rolling out in proportion to the time spent in poor lift. It is better to err on the side of too small a correction, since it is easy to repeat the maneuver the next time around.

Centering

     It is helpful to translate seconds of vario lag to a fraction of a typical circle. I'm assuming the a circle takes about 16 seconds and that the vario lag is 2 seconds - 1/8 of a circle. You should check your vario's lag.
     When your glider is established in a circle but not yet centered, the correction is simple: 1/4 of a turn after the actual point of worst lift, smoothly roll out to level, then return to the original bank angle. Allowing 1/8 of a turn for vario lag, the correction should be applied 1/8 of a turn after the indicated worst point of lift.
     Thie may or may not center the thermal, but it will move the glider towards the center of the lift.

Figure 5: Centering. 45° after the worst vario reading is the time to rollout. This is easy to judge since the bisector of the low wing and the forward axis of your glider is 45° from the worst vario heading. At the worst vario heading, find a landmark on this bisector.

The correction should be repeated during each circle until a more-or-less steady climb is indicated.
     It is easy to establish the heading on which to roll out: At the indicated point of worst lift look for a feature on the ground aligned with the bisector of the low wing and your forward direction (45° from straight ahead), then roll out heading toward that feature. This procedure is shown in Figure 5.
     There is no need to wait until the second time around the circle to do this. The initial correction can (and should) also be made with the aid of the variometer and exactly the same considerations apply: 1/8 of the circle following the point of indicated worst lift, roll out for about a second. If everything has gone according to plan, the glider will be on a heading 270° from its entry heading, it will be accelerating vertically ("surge"), and the vario will start to head up. Roll back into your turn again. Repeat as necessary. Small errors in the timing are not important - provided they are less than about 20 degrees, the circle will still be shifted in the right direction.

Optimizing

Figure 6: Optimizing the Climb. Glider A is neither centered or optimized. Glider B with the same size circle is centered and climbing better. Glider C has a smaller, centered circle and is able to use better air. Glider D is in even better air, but the bank angle is now so steep (spiral) that sink rate is now an issue.

     Once centered, it remains to optimize and to keep centered. Both require constant attention and work. Figure 6 illustrates the potential for optimizing the lift. For reference, Glider A is shown neither centered nor optimized. Glider B is centered, but the circle diameter keeps the glider in slower rising air. Glider C, with a smaller circle, may be able to climb better, since only a relatively small increase in sink rate is associated with the smaller circle in stronger lift. Glider D is likely to find out that the increased sink rate (spiral dive) cancels the advantage of being in better air. As the glider climbs, the diameter of the thermal typically increases, and the circle should be adjusted accordingly.

Conclusion
     It's really not very difficult to find, center and optimize lift if a few simple rules are followed. When they are, and with a little practice, it is possible to stay aloft as long as there is lift, and to fly as far as the lift will allow.
     It is also possible to fly for thousands of hours, and hundreds of cross country miles, and still be learning, so great are the challenges and opportunities of soaring.

• Failure to fly round circles (constant airspeed and bank angle).
• Circling on any blip from the vario, without a good indication of real lift.
• Circling on the first indication of lift, rather than trying to time the first turn properly.
• Circling in huge 15-degree banked turns while steeply banked gliders climb better.
• Failure to make a small correction with each circle until a thermal is centered.
• Failure to spot birds, developing cumulus clouds, and climbing gliders.
• Circling in 200 ft/min (2 knots) lift while nearby gliders climb at 500 ft/min (5 knots).
• Chasing every circling glider, without watching to see if they're actually climbing.
• Trying to get the last 200' of climb from a thermal whose strength has dropped by half.
• Staring at instruments instead of looking around and watching for indicators.
• Settling for weak climbs on a day where strong thermals are available.
• Blaming the glider for the pilot's shortcomings; assuming that only expensive gliders can climb well.
• Trying to learn thermaling with no vario or an inadequate altimeter (no audio).
• Assuming that expensive instruments are the key to successful thermaling.
• Failure to master the basics of thermaling before trying special techniques.
• Failure to experiment with other thermaling techniques once the basics are mastered.