Introduction to Glider Flying > Soaring Techniques > Thermal
Soaring flight, maintaining or gaining altitude rather than
slowly gliding downward, is the reason most glider pilots take
to the sky. After learning to stay aloft for two or more hours
at a time, the urge to set off cross country often overcomes
the soaring pilot. The goal is the same whether on a cross-country
or a local flight— to use available updrafts as efficiently
as possible. This involves finding and staying within the strongest
part of the updraft. This chapter covers the basic soaring techniques.
In the early 1920s, soaring pilots discovered
the ability to remain aloft using updrafts caused by wind deflected
by the very hillside from which they had launched. This allowed
time aloft to explore the air. Soon afterward, they discovered
thermals in the valleys adjacent to the hills. In the 1930s,
mountain waves, which were not yet well understood by meteorologists,
were discov-ered leading pilots to make the first high altitude
flights. Thermals are the most commonly used type of lift for
soaring flight, since they can occur over flat ter-rain and
in hilly country. Therefore, we will begin with thermal soaring
As a note, glider pilots refer to rising air
as lift. This is not the lift generated by the wings as was
discussed in chapter 3—Aerodynamics of Flight. The use
of this term may be unfortunate, but in reality it rarely causes
confusion when used in the context of updrafts. This chapter
refers to lift as the rising air within an updraft and sink
as the descending air in downdrafts.
When locating and utilizing thermals for soaring
flight, called thermalling, glider pilots must constantly be
aware of any nearby lift indicators. Successful ther-malling
requires several steps: locating the thermal, entering the thermal,
centering the thermal, and finally leaving the thermal. Keep
in mind that every thermal is unique in terms of size, shape,
In the last chapter, we learned that if the
air is moist enough and thermals rise high enough, cumulus clouds,
or Cu (pronounced ‘q’) form. Glider pilots seek
Cu in their developing stage, while the cloud is still being
built by a thermal underneath it. The base of the Cu should
be sharp and well defined. Clouds that have a fuzzy appearance
are likely well past their prime and will probably have little
lift left or even sink as the cloud dissipates. [Figure 10-1]
Figure 10-1. Photographs of (A) mature
cumulus likely producing good lift, and (B) dissipating cumulus.
Judging which clouds have the best chance for
a good thermal takes practice. On any given day, the lifetime
of an individual Cu can differ from previous days, so it becomes
important to observe their lifecycle on a par-ticular day. Agood
looking Cu may already be dissipat-ing by the time you reach
it. Soaring pilots refer to such Cu as rapid or quick cycling,
meaning they form, mature, and dissipate in a short time. The
lifetime of Cu often varies during a given day as well; quick
cycling Cu early in the day will often become well formed and
longer lived as the day develops.
Sometimes Cu cover enough of the sky that seeing
the cloud tops becomes difficult. Hence, glider pilots should
learn to read the bases of Cu. Generally, a dark area under
the cloud base indicates a deeper cloud; therefore, a higher
likelihood of a thermal underneath. Also, several thermals can
feed one cloud, and it is often well worth the deviation to
those darker areas under the cloud. At times, an otherwise flat
cloud base under an individual Cu has wisps or tendrils of cloud
hanging down from it, producing a particularly active area.
Cloud hanging below the general base of a Cu indicate that that
air is more moist, and hence more buoyant. Note the importance
of distinguishing fea-tures under Cu that indicate potential
lift from virga. Virga is rain or snow from the cloud base that
is not yet reaching the ground and often signals that the friendly
Cu has grown to cumulus congestus or thunderstorms. [Figure
10-2] Another indicator that one area of Cu may provide better
lift is a concave region under an otherwise flat cloud base.
This indicates air that is especially warm, and hence more buoyant,
which means stronger lift. This can cause problems for the unwary
pilot, since the lift near cloud base often dra-matically increases,
for instance from 400 to 1,000 (fpm). When trying to leave the
strong lift in the con-cave area under the cloud, pilots can
find themselves climbing rapidly with cloud all around—another
good reason to abide by required cloud clearances.
Figure 10-2. Photographs of (A) cumulus
congestus, (B) cumulonimbus (Cb), (C) virga.
After a thermal rises from the surface and
reaches the Convective Condensation Level (CCL), a cloud begins
to form. At first, only a few wisps form. Then the cloud grows
to a cauliflower shape. The initial wisps of Cu in an otherwise
blue (cloudless) sky indi-cate where an active thermal is beginning
to build a cloud. 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), diverting to an ini-tial wisp of
Cu is often worthwhile. On some days, when only a few thermals
are reaching the CCL, the initial wisps may be the only cloud
markers around. The trick is to get to the wisp when it first
forms, in order to catch the thermal underneath.
Lack of Cu does not necessarily mean lack of
thermals. If the air aloft is cool enough and the surface tempera-ture
warms sufficiently, thermals will form whether or not enough
moisture exists for cumulus formation. These blue or dry thermals,
as they are called, can be just as strong as their Cu-topped
counterparts. Glider pilots can find blue thermals, without
Cu markers, by gliding along until stumbling upon a thermal.
With any luck, other blue thermal indicators exist, making the
search less random.
One indicator of a thermal is another circling
glider. Often the glint of the sun on wings is all you will
see, so finding other gliders thermalling requires keeping a
good lookout, which glider pilots should be doing any-way. Circling
birds are also good indicators of thermal activity. Thermals
tend to transport various aerosols, such as dust, upward with
them. When a thermal rises to an inversion it disturbs the stable
air above it and spreads out horizontally, thus depositing some
of the aerosols at that level. Depending on the sun angle and
the pilot’s sunglasses, haze domes can indicate dry thermals.
If the air contains enough moisture, haze domes often form just
before the first wisp of Cu.
On blue, cloudless days, gliders and other
airborne indicators are not around to mark thermals. In such
cases, you must pay attention to clues on the ground. First,
think about your previous flight experiences. It is worth noting
where thermals have been found previ-ously since certain areas
tend to be consistent thermal sources. Remember that weather
is fickle, so there is never a guarantee that a thermal will
exist in the same place. In addition, if a thermal has recently
formed, it will take time for the sun to reheat the area before
the next thermal is triggered. Glider pilots new to a soaring
location should ask the local pilots about favored spots—doing
so might save the cost of a tow. Glider pilots talk about house
thermals, which are simply thermals that seem to form over and
over in the same spot or in the same area.
Stay alert for other indicators, as well. In
drier climates, dust devils mark thermals triggering from the
ground. In hilly or mountainous terrain, look for sun-facing
slopes. Unless the sun is directly overhead, the heating of
a sun-facing slope is more intense than that over adjacent flat
terrain because the sun’s radiation strikes the slope
at more nearly right angles.[Figure 10-3] Also, cooler air usually
pools in low-lying areas overnight; therefore, it takes longer
to heat up during the morning. Finally, slopes often tend to
be drier than surrounding lowlands, and hence tend to heat better.
Given the choice, it usually pays to look to the hills for thermals
Figure 10-3. Sun’s rays are concentrated
in a smaller area on a hillside than on adjacent flat ground.
Whether soaring over flat or hilly terrain,
some experts suggest taking a mental stroll through the landscape
to look for thermals. Imagine strolling along the ground where
warmer areas would be found. For instance, walking from shade
into an open field the air suddenly warms. A town surrounded
by green fields will likely heat more than the surrounding farmland.
Likewise, a yellowish harvested field will feel warmer than
an adja-cent wet field with lush green vegetation. Wet areas
tend to use the sun’s radiation to evaporate the mois-ture
rather than heat the ground. Thus, a field with a rocky outcrop
might produce better thermals. Rocky outcrops along a snowy
slope will heat much more effi-ciently than surrounding snowfields.
Though this tech-nique works better when at lower altitudes,
it can also be of use at higher altitudes in the sense of avoiding
cool-looking areas, such as a valley with many lakes.
Wind also has important influences not only
on thermal structure, but on thermal location as well. Strong
winds at the surface and aloft often break up thermals, mak-ing
them turbulent and difficult or impossible to work at all. Strong
shear can break thermals apart and effec-tively cap their height
even though the local sounding indicates that thermals should
extend to higher levels. On the other hand, as discussed in
chapter 9—Soaring Weather, moderately strong winds without
too much wind shear will sometimes organize thermals into longstreets,
a joyous sight when they lie along a cross-oun-try course line.
Figure 10-4. Photograph of cloud streets.
In lighter wind conditions, consideration of
thermal drift is still important, and search patterns should
become “slanted.” 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, typical
thermal strength on that day, and dis-tance below cloud base
(the lower the glider, the fur-ther upwind the gliders needs
to be). Add to this the fact that wind speed does not always
increase at a con-stant rate with height, and/or the possibility
that wind direction also can change dramatically with height,
and the task can be challenging.
Wind speed and direction at cloud base can
be esti-mated by watching the cloud shadows on the ground. With
all the variables, it is sometimes difficult to esti-mate exactly
where a thermal should be. Pay attention to where thermals appear
to be located in relation to clouds on a given day, and use
this as the search crite-ria for other clouds on that day. If
approaching Cu from the downwind side, expect heavy sink near
the cloud. Head for the darkest, best defined part of the cloud
base, then continue directly into the wind. Depending on the
distance below cloud base, just about the time of passing upwind
of the cloud, fly right into the lift form-ing the cloud. If
approaching the cloud from a cross-wind direction (for instance,
heading north with westerly winds), try to estimate the thermal
location from others encountered that day. If only reduced sink
is found, there may be lift nearby, so a short leg upwind or
downwind may locate the thermal.
Of course, thermals drift with the wind on
blue days as well, and similar techniques are required to locate
ther-mals using airborne or ground-based markers. For instance,
if heading toward a circling glider but at a thousand feet lower,
estimate how much the thermal is tilted in the wind and head
for the most likely spot upwind of the circling glider. [Figure
10-5] When in need of a thermal, pilots might consider searching
on a line upwind or downwind once abeam the circling glider.
This may or may not work; if the thermal is a bubble rather
than a column, the pilot may be below the bubble. It is easy
to waste height while searching in sink near one spot, rather
than leaving and searching for a new thermal. Remember that
a house thermal will likely be downwind of its typical spot
on a windy day. Only practice and experience enable glider pilots
to consistently find good thermals.
Figure 10-5. Thermal tilt in shear
that (a) does not change with height, and that (b) increases
As a note, cool stable air can also drift with
the wind. Avoid areas downwind of known stable air, such as
large lakes or large irrigated regions. On a day with Cu, stable
areas can be indicated by a big blue hole in an otherwise Cu-filled
sky. If the area is broad enough, a detour upwind of the stabilizing
feature might be in order. [Figure 10-6]
Figure 10-6. Blue hole in a field of
cumulus downwind of a lake.
When the sky is full of Cu, occasional gliders
are marking thermals, and dust devils move across the landscape,
the sky becomes glider pilot heaven. If glid-ing in the upper
part of the height band, it is best to focus on the Cu, and
make choices based on the best clouds. Sometimes lower altitudes
will cause glider pilots to go out of synch with the cloud.
In that circum-stance, use the Cu to find areas that appear
generally active, but then start focusing more on ground-based
indicators, like dust devils, a hillside with sunshine on it,
or a circling bird. When down low, accept weaker climbs. Often
the day cycles again, and hard work is rewarded.
When searching for lift, use the best speed
to fly, that is, best L/D speed plus corrections for sink and
any wind. This technique allows glider pilots to cover the most
amount of ground with the available altitude.
Once a thermal has been located, enter it properly,
so as not to lose it right away. The first indicator of a nearby
thermal is often, oddly enough, increased sink. Next a positive
G-force will be felt, which may be subtle or obvious depending
on the thermal strength. The “seat-of- the-pants”
indication of lift is the quickest, and is far faster than any
variometer, which has a small lag. Speed should have been increased
in the sink adjacent to the thermal, hence as the positive G-force
increases, reduce speed to between L/D and minimum sink. Note
the trend of the variometer needle (should be an upswing) or
the audio variometer going from the drone to excited beeping,
and at the right time in the antici-pated lift, begin the turn.
If everything has gone per-fectly, the glider will roll into
a coordinated turn, at just the right bank angle, at just the
right speed, and be per-fectly centered perfectly. In reality,
it rarely works that well.
Before going further, what vital step was left
out of the above scenario? CLEAR BEFORE TURNING! The var-iometer
is hypnotic upon entering lift, especially at somewhat low altitudes.
This is exactly where pilots forget that basic primary step
before any turn—looking around first. An audio variometer
helps avoid this.
To help decide which way to turn, determine
which wing is trying to be lifted. For instance, when entering
the thermal and the glider is gently banking to the right, CLEAR
LEFT, then turn left. A glider on its own tends to fly away
from thermals. [Figure 10-7] As the glider flies into the first
thermal, but slightly off center, the stronger lift in the center
of the thermal banks the glider right, away from the thermal.
It then encounters the next thermal with the right wing toward
the center and is banked away from lift to the left, and so
on. Avoid letting thermals bank the glider even slightly. Sometimes
the thermal-induced bank is subtle, so be light on the controls
and sensitive to the air activity. At other times there is no
indication on one wing or another. In this case, take a guess,
CLEAR, then turn. As a note, new soaring pilots often get in
the habit of turning in a favorite direction, to the extreme
of not being able to fly reasonable circles in the other direc-tion.
If this happens, make an effort to thermal in the other direction
half the time—being proficient in either direction is
important, especially when thermalling with traffic.
Figure 10-7. Effect of glider being
allowed to bank on its own when encountering thermals.
Optimum climb is achieved when proper bank
angle and speed are used after entering a thermal. The shal-lowest
possible bank angle at minimum sink speed is ideal. Thermal
size and associated turbulence usually do not allow this. Large-size,
smooth, and well-behaved thermals can be the exception in some
parts of the country. Consider first the bank angle. The glider’s
sink rate increases as the bank angle increases. However, the
sink rate begins to increase more rapidly beyond about a 45?bank
angle. Thus, a 40?to a 30? bank angle may increase the sink
rate less thanthe gain achieved from circling in the stronger
lift near the center of the thermal. As with everything else,
this takes practice, and the exact bank angle used will depend
on the typical thermal, or even a specific ther-mal, on a given
day. Normally bank angles in excess of 50?are not needed, but
exceptions always exist. It maybe necessary, for instance, to
use banks of 60?or so to stay in the best lift. Thermals tend
to be smaller at lower levels and expand in size as they rise
higher. Therefore, a steeper bank angle is required at lower
altitudes, and shallower bank angles can often be used while
climbing higher. Remain flexible with techniques throughout
If turbulence is light and the thermal is well-formed,
use the minimum sink speed for the given bank angle. This should
optimize the climb because the glider’s sink rate is at
its lowest, and the turn radius is smaller. As an example, for
a 30?bank angle, letting the speed increase from 45 to 50 knots
increases the diameter of the circle by about 100 feet. In some
instances, this can make the difference between climbing or
not. Some gliders can be safely flown several knots below mini-mum
sink speed. Even though the turn radius is smaller, the increased
sink rate may offset any gain achieved by being closer to strong
lift near the thermal center.
There are two other reasons to avoid thermalling
speeds that are too slow: the risk of a stall and lack of controllability.
Distractions while thermalling can increase the risk of an inadvertent
stall and include, but are not limited to: studying the cloud
above or the ground below (for wind drift, etc.), quickly changing
bank angles without remaining coordinated while cen-tering,
thermal turbulence, or other gliders in the ther-mal. Stall
recovery should be second nature, so that if the signs of an
imminent stall appear while thermalling, recovery is instinctive.
Depending on the stall charac-teristics of the particular glider
or in turbulent thermals, a spin entry is always possible. Glider
pilots should carefully monitor speed and nose attitude at lower
alti-tudes. Regardless of altitude, when in a thermal with other
gliders below, maintain increased awareness of speed control
and avoid any stall/spin scenario. Controllability is a second,
though related, reason for using a thermalling speed greater
than minimum sink. The bank angle may justify a slow speed,
but turbu-lence in the thermal may make it difficult or impossible
to maintain the desired quick responsiveness, espe-cially in
aileron control, in order to properly remain in the best lift.
Using sufficient speed will ensure that the pilot, and not the
thermal turbulence, is controlling the glider.
Soaring pilots’ opinions differ regarding
how long to wait after encountering lift and before rolling
into the thermal. Some pilots advocate flying straight until
the lift has peaked. Then, they start turning, hopefully back
into stronger lift. It is imperative not to wait too long after
the first indication that the thermal is decreasing for this
maneuver. Other pilots favor rolling into the thermal before
lift peaks, thus avoiding the possibility of losing the thermal
by waiting too long. Turning into the lift too quickly will
cause the glider to fly back out into sink. There is no one
right way; the choice depends on personal preference and the
conditions on a given day. Timing is everything and practice
Usually upon entering a thermal, the glider
is in lift for part of the circle and sink for the other part.
It is rare to roll into a thermal and immediately be perfectly
cen-tered. The goal of centering the thermal is to determine
where the best lift is and move the glider into it for the most
consistent climb. One centering technique is known as the “270?correction.”
[Figure10-8] In this case, the pilot rolls into a thermal and
almost immedi-ately encounters sink, an indication of turning
the wrong way. Complete a 270turn, straighten out for a few
seconds, and if lift is encountered again, turn back into it
in the same direction. Avoid reversing the direc-tion of turn.
The distance flown while reversing turns is more than seems
possible and can lead away from the lift completely. [Figure
Figure 10-8. The 270?centering correction.
Figure 10-9. Possible loss of thermal
while trying to reverse directions of circle.
Often stronger lift exists on one side of the
thermal than on the other, or perhaps the thermal is small enough
that lift exists on one side and sink on the other, thereby
preventing a climb. There are several techniques and variations
to centering. One method involves paying close attention to
where the thermal is strongest, for instance, toward the northeast
or toward some featureon the ground. To help judge this, note
what is under the high wing when in the best lift. On the next
turn, adjust the circle by either straightening or shallowing
the turn toward the stronger lift. Anticipate things a bit and
begin rolling out about 30?before actually heading towards the
strongest part. This allows rolling back toward the strongest
part of the thermal rather than fly-ing through the strongest
lift and again turning away from the thermal center. Gusts within
the thermal can cause airspeed indicator variations; therefore,
avoid “chasing the ASI.” Paying attention to the
nose attitude helps pilots keep their focus outside the cockpit.
How long a glider remains shallow or straight depends on the
size of the thermal. [Figure 10-10]
Figure 10-10. Centering by shifting
the circle turn toward stronger lift.
Other variations include the following. [Figure
Figure 10-11. Other centering corrections.
1. Shallow the turn slightly (by maybe 5?????? ?????) when encountering
the weaker lift, then as stronger lift is encountered again
(feel the posi-tive g, variometer swings up, audio variometer
starts to beep) resume the original bank angle. If shallowing
the turn too much, it is possible to fly completely away from
2. Straighten or shallow the turn for a few
seconds 60?after encountering the weakest lifts or worst sink
indicated by the variometer. This allows for the lag in the
variometer since the actual worst sink occurred a couple of
seconds earlier than indicated. Resume the original bank angle.
3. Straighten or shallow the turn for a few
seconds when the stronger seat-of-the-pants surge is felt. Then
resume the original bank. Verify with the variometer trend (needle
or audio). For the new glider pilots, it is best to become profi-cient
using one of the above methods first, and thenexperiment with
other methods. As an additional note, thermals often deviate
markedly from the conceptual model of concentric gradients of
lift increasing evenly toward the center. For instance, it sometimes
feels as if two (or more) nearby thermal centers exist, making
centering difficult. Glider pilots must be willing to constantly
adjust, and re-center the thermal to maintain the best climb.
In addition to helping pilots locate lift,
other gliders can help pilots center a thermal as well. If a
nearby glider seems to be climbing better, adjust the turn to
fly within the same circle. Similarly, if a bird is soaring
close by, it is usually worth turning toward the soaring bird.
Along with the thrill of soaring with a hawk or eagle, it usually
leads to a better climb. Collision avoidance is of primary importance
when thermalling with other gliders. The first rule calls for
all gliders in a particular thermal to circle in the same direction.
The first glider in a thermal establishes the direction of turn
and all other gliders joining the ther-mal should turn in the
same direction. Ideally, two glid-ers in a thermal at the same
height or nearly so should position themselves across from each
other so they can best maintain visual contact. [Figure 10-12]
When entering a thermal, strive to do so in a way that will
not interfere with gliders already in the thermal, and above
all, in a manner that will not cause a hazard to other gliders.
An example, of a dangerous entry, is pulling up to bleed off
excess speed in the middle of a crowded thermal. A far safer
technique is to bleed off speed before reaching the thermal
and joining the thermal at a “normal” thermalling
speed. Collision avoidance, not optimum aerodynamic efficiency,
is the priority when thermalling with other gliders. Announcing
to the otherglider(s) on the radio when entering the thermal
enhances collision avoidance. [Figure 10-12]
Figure 10-12. Proper positioning with
two gliders at the same altitude. Numbers represent each glider’s
position at that time.
Different types of gliders in the same thermal
may have different minimum sink speeds, and it may be difficult
to remain directly across from another glider in a ther-mal.
Avoid putting yourself in a situation where you cannot see the
other glider, or the other glider cannot see you. Radio communication
is helpful. Too much talking clogs the frequency, and may make
it impossi-ble for a pilot to broadcast an important message.
Do not fly directly above or below another glider in a ther-mal
since differences in performance, or even minor changes in speed
can lead to larger than expected alti-tude changes. If you lose
sight of another glider in a thermal and cannot establish position
via a radio call, leave the thermal. After 10 or 20 seconds,
come back around to rejoin the thermal, hopefully with better
traf- fic positioning. It cannot be stressed enough that colli-sion
avoidance when thermalling is a priority! Mid-air collisions
can sometimes be survived but only with a great deal of luck.
Unsafe thermalling practices not only endangers your own safety
but that of your fellow
glider pilots. [Figure 10-13]
Figure 10-13. When thermalling, avoid
flying in another glider’s blind spot, or directly above
or below another glider.
Leaving a thermal properly can also save you
some altitude. While circling, scan the full 360?of sky with
each thermalling turn. This first allows the pilot to con-tinually
check for other traffic in the vicinity. Second, it helps the
pilot analyze the sky in all directions in order to decide where
to go for the next climb. It is better to decide where to go
next while still in lift rather than losing altitude in sink
after leaving a thermal. Exactly when to leave depends on the
goals for the climb— whether the desire is to maximize
altitude for a long glide or leave when lift weakens in order
to maximize time on a cross-country flight. In either case,
be ready to increase speed to penetrate the sink often found
on the edge of the thermal, and leave the thermal in a man-ner
that will not hinder or endanger other gliders.
The preceding pages describe techniques for
locating thermals, as well as entering, centering, and leaving
thermals. Exceptions to normal or typical thermals are numerous.
For instance, instead of stronger sink at the edge of a thermal,
weak lift sometimes continues for a distance after leaving a
thermal. Glider pilots should be quick to adapt to whatever
the air has to offer at the time. Just as the mechanics of simply
flying the glider become second nature with practice, so do
thermalling techniques. Expect to land early because anticipated
lift was not there on occasion—it is part of the learning
curve. If thermal waves are suspected, climb in the thermal
near cloud base, then head toward the upwind side of the Cu.
Often, only very weak lift, barely enough to climb at all, is
found in smooth air upwind of the cloud. Once above cloud base
and upwind of the Cu, climb rates of a few hundred fpm can be
found. Climbs can be made by flying back and forth upwind of
an individ-ual Cu, or by flying along cloud streets if they
exist. If no clouds are present, but waves are suspected, climb
to the top of the thermal and penetrate upwind in search of
smooth, weak lift. Without visual clues, thermal waves are more
difficult to work. Thermal waves are most often stumbled upon
as a pleasant surprise when their presence is furthest from
the pilot’s mind.