Introduction
Many glider pilots enjoy searching for lift to gain altitude. Staying aloft often involves finding and staying within the
strongest part of any updrafts. This chapter covers some basic soaring techniques that assist with that task.
In the early 1920s, soaring pilots discovered how to remain aloft using updrafts deflected by the hillsides they used for
launch. Soon afterward, they discovered thermals in the valleys adjacent to the hills. In the 1930s, the discovery of mountain
waves, which were not yet well understood by meteorologists, allowed glider pilots to make the first high-altitude flights.
Since thermals occur over both flat terrain and hilly country, they remain the most-used type of lift for glider flights today.
As a note, glider pilots refer to rising air as lift, which differs from the lift generated by the wings. This chapter refers to
lift as the rising air within an updraft and sink as the descending air in downdrafts.
Thermal Soaring
Successful thermalling requires several steps: locating the thermal, entering the thermal, centering within the thermal, and
exiting the thermal.
When locating thermals, glider pilots look for nearby lift indicators. In sufficiently moist air and if and thermals rise high
enough, cumulus clouds (Cu) (pronounced “cue”) form. Glider pilots look for Cu in the developing stage, while the cloud
builds from the thermal underneath it. The base of a developing Cu appears sharp and well defined. Dissipating Cu have a
fuzzy appearance and little lift or sink underneath. [Figure 10-1]
Figure 10-1. Photographs of (A) mature cumulus, which can produce good lift, and (B) dissipating cumulus.
The lifetime of Cu often varies during a given day as Cu that develop early in the day may change into well-formed and
longer-lived clouds. A promising Cu in the distance may also start dissipating, and glider pilots refer to such Cu as rapid or
quick cycling, which means the Cu forms, matures, and dissipates in a short time.
Sometimes Cu cover much of the sky, which makes seeing the cloud tops difficult; however, the cloud bases also indicate
the presence and strength of thermals. Generally, a dark area under the cloud base indicates a deeper cloud and a higher
likelihood of a thermal underneath that spot. Since several thermals can feed one cloud, darker areas under the cloud
Chapter 10: Soaring Techniques
may indicate a stronger thermal. At times, an otherwise flat cloud base under an individual Cu has wisps or tendrils of
cloud hanging down, which indicate a particularly active area and the presence of warm rising air. Note the importance of
distinguishing features under Cu that differentiate potential lift from virga. Virga consists of precipitation in the form of
rain, snow, or ice crystals that descend from the cloud base and evaporate before striking the ground. Virga may signal that
the Cu has reached towering cumulus or thunderstorm status. [Figure 10-2]
Figure 10-2. Photographs of (A) towering cumulus, (B) cumulonimbus (Cb), and (C) virga.
Lift near a cloud base often increases dramatically in a concave region under an otherwise flat cloud created by especially
warm air. When trying to leave the strong lift in the concave area under the cloud, glider pilots can find themselves
climbing rapidly with cloud all around. Adherence by a glider to the minimum distance below a cloud as listed in 14 of
the Code of Federal Regulations (14 CFR) part 91, section 91.155, normally prevents an accidental climb into a cloud.
In addition, maintaining the minimum separation gives a glider pilot the opportunity to avoid an aircraft operating under
instrument flight rules in, near, or emerging from a cloud.
As any thermal rises from the surface and reaches the convective condensation level (CCL), a cloud begins to form. At
first, only a few wisps form. The initial wisps of Cu in an otherwise blue (cloudless) sky indicate where an active thermal
has begun building a cloud that will grow to a familiar cauliflower shape. When crossing a blue hole (a region anywhere
from a few miles to several dozen miles of cloud-free sky in an otherwise Cu-filled sky), getting to an initial wisp of Cu
can provide lift from the thermal underneath. On some days and depending on the moisture in the air, these wisps undergo
no further development and provide the only indication of thermals.
Lack of Cu does not necessarily mean lack of thermals. If the air aloft is cool enough and the surface temperature warms
sufficiently, thermals form even without enough moisture for cumulus formation. These dry, or “blue thermals” can be just
as strong as their Cu-topped counterparts. Glider pilots sometimes find these thermals by chance while gliding. However,
other indicators may exist and can make the search for thermals less random.
Other Indicators of Thermals
Another circling glider may indicate the presence of a thermal. Circling birds may also indicate thermal activity. Thermals
tend to transport various aerosols, such as dust, upward with them. When a thermal reaches an inversion, it disturbs the
stable air above it, spreads out horizontally, and deposits some of the aerosols at that level. Depending on the sun angle
(and the pilot’s type of sunglasses), haze domes may indicate dry thermals. If the air contains enough moisture, haze domes
often form just before the first wisps of Cu.
Glider pilots talk about a house thermal or thermals that seem to form over and over in the same spot or in the same area.
Glider pilots new to a soaring location should ask the local pilots about favored spots—doing so might make additional
tows unnecessary. While some thermals may arise from consistent sources, no one can guarantee that a thermal currently
exists where one existed before. In addition, if a thermal has recently formed, it takes time for the sun to reheat the area
before another thermal might form at the same location.
On days without airborne indicators to mark thermals, pilots can look for clues on the ground. For example, in drier
climates, dust devils often mark thermals triggering from the ground. At certain times of the day in hilly or mountainous
terrain, the sun’s radiation may strike a particular slope at or near a right angle. [Figure 10-3] A sun facing slope receives
more energy per unit of area and can warm the surrounding air more effectively. Darker ground or surface features heat
quicker than grass covered fields. Huge black asphalt parking lots can produce strong thermals. A large tilled black soil
field can be a good source of lift if the pilot can find the associated narrow plume of rising air. Thermals often form where
hills exist since slopes tend to be drier and heat better than surrounding lowlands. Finally, cooler air usually pools in low-
lying areas overnight and takes longer to warm during the morning. Pilots might avoid searching for thermals from those
areas early in the day.
Small area—strong heating
Sun’s rays
Sun’s rays
Large area—weaker heating
Figure 10-3. Sun’ s rays are concentrated in a smaller area on a hillside than on adjacent flat ground.
Other subtle ground markers exist. An open field surrounded by shade usually results in a space where the air suddenly
warms. A town surrounded by green fields results in a rise in temperature since the town surfaces absorb more heat than the
surrounding farmland. Likewise, a yellowish harvested field gets warmer than an adjacent wet field with green vegetation.
Wet areas tend to use the sun’s radiation to evaporate the moisture rather than heat the ground. A field with a rocky outcrop
might produce better thermals since rocks can’t hold moisture. Rocky outcrops along a snowy slope heat much more
efficiently than surrounding snowfields. While searching for smaller features works at lower altitudes, pilots can use their
knowledge of ground heating when at higher altitudes by avoiding areas such as a valley with many lakes.
Wind
Wind influences thermal structure and location. Strong shear can break thermals apart and effectively cap their height.
Strong winds at the surface and aloft often break up thermals, making them turbulent and difficult or impossible to use.
On the other hand, as discussed in Chapter 9, Glider Flight and Weather, moderately strong winds without too much wind
shear sometimes organize thermals into long streets, which can provide lift when they lie along a cross-country course
line. [Figure 10-4]
Figure 10-4. Photograph of cloud streets.
If suspecting the presence of waves associated with a thermal above the cloud, the pilot can climb in the thermal near
a cloud base and then head toward the upwind side of the Cu. Often, only weak lift exists in smooth air upwind of the
cloud. Once above cloud base and upwind of the Cu, the pilot should find climb rates of a few hundred fpm in the thermal
wave. The glider can climb flying back and forth upwind of an individual Cu, or by flying along cloud streets if they exist.
When thermal waves exist without clouds, the pilot should climb to the top of the thermal and penetrate upwind in search
of smooth, weak lift. Without visual clues, thermal waves prove difficult to work. Sometimes the pilot stumbles upon a
thermal wave by chance.
In light wind conditions, pilots should use a slanted search pattern. For instance, in Cu-filled skies, glider pilots need to
search upwind of the cloud to find a thermal. How far upwind depends on the strength of the wind, thermal strength on
that day, and distance below cloud base (the lower the glider, the further upwind the glider needs to be). The pilot should
consider the fact that windspeed does not always increase at a constant rate with height and the possibility that wind
direction can change dramatically with height. [Figure 10-5] Pilots can estimate wind direction and speed at a cloud base
by watching cloud shadows on the ground.
Wind
Wind
Wind
Wind
Sink
Sink
Figure 10-5. Thermal tilt in shear that (A) does not change with height, and that (B) increases with height.
Pilots can find where thermals appear in relation to clouds on a given day and use those encounters to determine how to
search that day. If approaching Cu from the downwind side, heavy sink may occur near the cloud. From that position, the
pilot should head for the darkest, best-defined part of the cloud base, then continue directly into the wind. Depending on
the distance below cloud base, the pilot should find the lift forming the cloud about the time the glider passes upwind of
the cloud. If approaching the cloud from a crosswind direction (for instance, heading north with westerly winds), the pilot
can also estimate the thermal location from previous encounters that day. If only encountering reduced sink, lift may exist
nearby, and a short leg upwind or downwind may locate the thermal.
Thermals also drift with the wind on cloudless days, and similar techniques can locate thermals using airborne or ground-
based markers. For instance, if heading toward a circling glider but at a thousand feet lower, the pilot can estimate how
much the thermal tilts in the wind and head for the most likely spot upwind of the circling glider. When in need of a
thermal, pilots might consider searching on a line stretching upwind to downwind once under or over the circling glider.
This may or may not work; if the thermal is a bubble rather than a column, the pilot may miss the bubble. The pilot in sink
should limit the search if not finding lift. Searching for a thermal in sink near one spot, rather than leaving and searching
for a new thermal can consume a lot of altitude.
Cool, stable air can also drift with the wind. Pilots should avoid areas downwind of known stable air, such as large lakes
or large irrigated regions. [Figure 10-6] On a day with Cu, a big blue hole in an otherwise Cu-filled sky indicates a stable
area. If the area is broad enough, a detour upwind of the stabilizing feature might conserve a lot of altitude.
Lake
Wind
Figure 10-6. Blue hole in a field of cumulus downwind of a lake.
The Big Picture
With a sky full of Cu and if gliding in the upper part of the strongest lift, the pilot should focus on the Cu, and make choices
based on the best clouds. At lower altitudes glider pilots may find it difficult to associate thermals with clouds above. If
that happens, the pilot should use the Cu to find areas that appear generally active and then focus more on ground-based
indicators, like dust devils, a hillside with sunshine on it, or a circling bird. When down low, the pilot can accept weak
climbs. Often the thermal cycles again and rewards patience.
When searching for lift using the best speed to fly, L/D~MAX speed plus corrections for sink and any wind, allows a glider
pilot to cover the most ground for a given altitude loss. See Chapter 5, Glider Performance, to review shifting of the glider
polar for winds and sink.
Entering a Thermal
Oddly enough, increased sink often indicates a nearby thermal. Next, the pilot feels a subtle or obvious positive G- force,
depending on the thermal strength. The “seat-of-the-pants” indication of lift occurs faster than shown by any variometer,
which lags slightly. The pilot should reduce speed to between L/D and minimum sink and note the trend of the variometer
needle (should be an upswing) or the audio variometer going from the drone to more rapid beeping. At the right time in
the anticipated lift and in a perfect scenario, the pilot rolls into a coordinated turn at just the right bank angle and speed to
center within the thermal.
Before going further, what vital step was left out of the above scenario? VISUALLY CLEAR THE AIRSPACE BEFORE
TURNING! Pilots sometimes forget that basic primary step before any turn as the variometer attracts a lot of pilot visual
attention upon entering lift. Low-altitude flight and a glider without an audio variometer increase the likelihood of this
omission.
To help decide which way to turn, the pilot determines which wing seems to lift when entering the thermal. For instance,
if the glider gently banks right when entering the thermal, the pilot should CLEAR LEFT, then turn left. A glider on its
own tends to fly away from thermals. [Figure 10-7] Pilots who maintain a light touch on the controls can sense proximity
to a thermal and avoid letting thermals continue to bank the glider away from the thermal. If no thermal-induced banking
occurs, the pilot decides arbitrarily which way to turn. Note that new soaring pilots often turn in a favored direction. This
could cause pilot inability to fly reasonable circles in the other direction. If this happens, the pilot should make a conscious
effort to thermal in each direction half the time to improve proficiency and as preparation for thermalling in traffic.
Figure 10-7. Effect of glider being allowed to bank on its own when encountering thermals.
As a glider encounters lift on one side, the wing in the stronger lift will start to rise, and some gliders will tend to slip
laterally as indicated by the yaw string. The glider pilot should apply aileron pressure to bring the climbing wing down
not just to level the wings, but to bank further to begin the turn into the lifting columns of air. If the rising air on one wing
results in a sideslip, a turn toward the head of the yaw string should also bring the glider into the rising air.
Inside a Thermal
Optimum climb occurs when the pilot uses proper bank angle and speed after entering a thermal. Under ideal conditions,
pilots use the shallowest possible bank angle at minimum sink speed. However, thermal size and associated turbulence
usually do not favor this combination.
Bank Angle
The glider’s sink rate increases as the bank angle increases, and the sink rate begins to increase more rapidly beyond about
a 45° bank angle. A 40° compared to a 30° bank angle may allow for circling in the stronger lift near the center of the
thermal. Maximizing thermal lift takes practice. The optimized bank angle depends on the size and strength of the thermal.
Normally, the pilot does not use bank angles exceeding 50°, but exceptions exist. A pilot might use a bank of 60° to stay in
the best lift. Thermals tend to be smaller at lower levels and expand in size as they rise higher. Therefore, a pilot generally
uses a steeper bank angle to remain in lift at lower altitudes.
Thermalling illustrates the importance of understanding and training in steep turns (as previously discussed in Chapter
7: Launch & Recovery Procedures & Flight Maneuvers.) A steep turn during thermalling causes the outer wing to travel
faster than the inner wing and results in an overbanking tendency. While the pilot holds aileron pressure against the turn
to balance the lift between the wings, this pressure causes additional drag on the lowered wing and could result in a skid
and lead to a spin entry. Applying slight top rudder pressure and slipping gently during a steep turn can compensate for the
increased drag of the inside wing and result in better overall climb performance. Glider pilots should consult their flight
instructors regarding the proper technique for steep turns during thermalling.
Speed
Flying at minimum sink speed for the G-load on the glider optimizes the climb in a well-formed thermal and light turbulence.
Flying in thermals either above or below this speed will degrade glider performance. As discussed in Chapter 3, decreasing
the airspeed in a turn will decrease the radius of the turn. The examples in this paragraph assume a minimum sink speed
of 60 mph. At a 30° bank angle, decreasing speed from 60 to 40 mph decreases the radius of the circle by almost 250 feet.
While this reduced radius may place the glider closer to strong lift near the thermal center, glider performance will suffer
from the increased rate of descent while flying at an airspeed below the minimum sink speed. An increased bank angle to
achieve a smaller diameter circle may provide better optimization. For example, while maintaining a minimum sink speed
of 60 mph, increasing the bank angle from 30 to 45 degrees will decrease the turn radius by over 175 feet. Increasing the
bank angle from 30 to 60 degrees will decrease the turn radius by over 275 feet. While some gliders can safely fly several
knots below minimum sink speed to reduce the turn radius, the increased sink rate at lower speeds may offset any gain
achieved.
Pilots should avoid thermalling speeds well below minimum sink speed due to the increased risk of a stall and the lack
of controllability. Distractions while thermalling may include studying the cloud above or the ground below, quickly
changing bank angles without remaining coordinated, thermal turbulence, or abrupt maneuvers to avoid other gliders in the
thermal. The pilot may allow the airspeed to decay to the stall speed or may increase the bank angle thereby increasing the
load factor and the stall speed. This increases the risk of an inadvertent stall. While thermalling, any stall recovery should
occur instinctively and without delay. Without prompt corrective action a spin entry could occur, depending on the stall
characteristics of the glider or if in turbulent thermals. Additionally, at airspeeds well below minimum sink speed, a lack
of airflow over the wings and control surfaces may make the glider less controllable during stall recovery or during any
abrupt maneuvers. Glider pilots should carefully monitor speed and nose attitude at lower altitudes. Using sufficient speed
ensures that the pilot, and not the thermal turbulence, controls the glider.
Centering
Pilot opinions differ regarding how long to wait before rolling into a thermal after encountering lift. Some pilots advocate
flying straight until the lift has peaked. Then, they start turning back into stronger lift. If using this technique, pilots should
not wait too long after the first indication of decreasing lift. Other pilots favor rolling into the thermal before lift peaks,
thus avoiding the possibility of losing the thermal. However, turning into the lift too quickly causes the glider to fly back
out into sink. The choice depends on personal preference and the conditions on a given day.
Often upon entering a thermal, the glider experiences lift for part of the circle and sink for the other part. The pilot should
determine where the best lift exists and move the glider into it for the most consistent climb. If the pilot turns in the wrong
direction and almost immediately encounters sink, a 270° correction may correct the error. [Figure 10-8] The pilot should
complete a 270° turn, straighten out for a few seconds, and if encountering lift again, turn back into it in the same direction.
The pilot should avoid reversing the direction of turn since that procedure takes more time, covers more distance, and may
lead away from the lift completely. [Figure 10-9]
Figure 10-8. The 270° centering correction.
Figure 10-9. Possible loss of thermal while trying to reverse directions of circle.
If stronger lift exists on one side of the thermal, the pilot can use one of several centering techniques. One method involves
noting the current position with the best lift: for instance, toward the Northeast or toward some feature on the ground that
the pilot can see under the high wing when at the current point of best lift. [ Figure 10-10] On the next turn, the pilot can