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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, making
them turbulent and difficult or impossible to work
at all. Strong shear can break thermals apart and effectively
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 long
Sun's Rays
Sun's Rays
Small Area -
Strong Heating
Large Area -
Weaker Heating
Figure 10-3. Sun’s rays are concentrated in a smaller area on
a hillside than on adjacent flat ground.
Figure 10-4. Photograph of cloud streets.
10-4
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.
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]
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 gliding
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 circumstance,
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 “seatof-
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 anticipated
lift, begin the turn. If everything has gone perfectly,
the glider will roll into a coordinated turn, at just
the right bank angle, at just the right speed, and be perfectly
centered perfectly. In reality, it rarely works that
well.
Figure 10-5. Thermal tilt in shear that (a) does not change with height, and that (b) increases with height.
Lake
Figure 10-6. Blue hole in a field of cumulus downwind of a
lake.
10-5
Before going further, what vital step was left out of the
above scenario? CLEAR BEFORE TURNING! The variometer
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
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Glider Flying Handbook(137)
