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may still produce slope lift. Vertical wind shear is also
a consideration. High ridges may have little or no wind
along the lower slopes, but the upper parts of the ridge
may be in winds strong enough to produce slope lift
there.
Eddy
Lift Zone
Figure 9-23. Slope lift and eddy with near-vertical slope.
A
B
C
Figure 9-24. Airflow along different ridges.
The area of best lift varies with height. Below the ridge
crest, the best slope lift is found within a few hundred
feet next to the ridge, again depending on the slope and
wind strength. As mentioned, very steep ridges require
extra speed and caution, since eddies and turbulence
can form even on the upwind side. Above the ridge
crest, the best lift usually is found further upwind from
the ridge the higher one climbs. [Figure 9-22]
When the air is very stable, and the winds are sufficient
but not too strong, slope lift can be very smooth,
enabling safe soaring close to the terrain. If the air is
not stable, thermals may flow up the slope. Depending
on thermal strength and wind speed, the thermal may
rise well above the ridge top, or it may drift into the lee
downdraft and break apart. Downdrafts on the sides of
thermals can easily cancel the slope lift; hence, extra
speed and caution is required when the air is unstable,
especially below the ridge crest near the terrain. The
combination of unstable air and strong winds can make
slope soaring unpleasant or even dangerous for the
beginning glider pilot.
Moisture must be considered. If air rising in the slope
lift is moist and cools sufficiently, a so-called cap cloud
may form. The cloud may form above the ridge, and if
the air moistens more with time, the cloud will slowly
lower onto the ridge and down the upwind slope, limiting
the usable height of the slope lift. Since the updraft
forms the cloud, it is very easy to climb into the cap
cloud—obviously a dangerous situation. Under certain
conditions, a morning cap cloud may rise as the day
warms, then slowly lower again as the day cools.
WAVE SOARING WEATHER
Where there is wind and stable air, there is the likelihood
of waves in the atmosphere. Most of the waves
that occur throughout the atmosphere are of no use to
the glider pilot. However, often mountains or ridges
produce waves downstream, the most powerful of
which have lifted gliders to 49,000 feet. Indirect measurements
show waves extending to heights around
100,000 feet. If the winds aloft are strong and widespread
enough, mountain lee waves can extend the
length of the mountain range. Pilots have achieved
flights in mountain wave using three turn points of
over 2,000 km. Another type of wave useful to soaring
pilots is generated by thermals, which were discussed
in the previous section.
A common analogy to help visualize waves created by
mountains or ridges uses water flowing in a stream or
small river. A submerged rock will cause ripples
(waves) in the water downstream, which slowly
dampen out. This analogy is useful, but it is important
to realize that the atmosphere is far more complex,
with vertical shear of the wind and vertical variations
in the stability profile. Wind blowing over a mountain
will not always produce downstream waves.
Mountain wave lift is fundamentally different from
slope lift. Slope soaring occurs on the upwind side of a
ridge or mountain, while mountain wave soaring
occurs on the downwind side. (Mountain wave lift
sometimes tilts upwind with height. Therefore, at
times near the top of the wave, the glider pilot may be
almost directly over the mountain or ridge that has
produced the wave). The entire mountain wave system
is also more complex than the comparatively simple
slope soaring scenario.
MECHANISM FOR WAVE FORMATION
Waves form in stable air when a parcel is vertically
displaced and then oscillates up and down as it tries to
return to its original level, illustrated in Figure 9-26. In
the first frame, the dry parcel is at rest at its equilibrium
level. In the second frame, the parcel is displaced
upward along a DALR, at which point it is cooler than
the surrounding air. The parcel accelerates downward
toward its equilibrium level, but due to momentum, it
overshoots the level and keeps going down. The third
frame shows that the parcel is now warmer than the
surrounding air, and thus starts upward again. The
process continues with the motion damping out. The
number of oscillations depends on the initial parcel
displacement and the stability of the air. In the lower
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Glider Flying Handbook(119)
