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strikes have caused little more than mild shock and
cosmetic damage. On the other extreme, a composite
training glider in Great Britain suffered a strike that
caused complete destruction of one wing; fortunately,
both pilots parachuted to safety. In that case, the glider
was two or three miles from the thunderstorm. Finally,
ground launching, especially with a metal cable, anywhere
near a thunderstorm should be avoided.
Severe thunderstorms can sometimes spawn tornadoes,
which are rapidly spinning vortices, generally a few
hundred to a few thousand feet across. Winds can
exceed 200 mph. Tornadoes that do not reach the
ground are called funnel clouds. By definition, tornadoes
form from severe thunderstorms. Obviously, they
should be avoided on the ground or in the air.
WEATHER FOR SLOPE SOARING
Slope or ridge soaring refers to using updrafts produced
by the mechanical lifting of air as it encounters
the upwind slope of a hill, ridge, or mountain. Slope
soaring requires two ingredients: elevated terrain and
wind.
Slope lift is the easiest lift source to visualize. When it
encounters topography, wind is deflected either horizontally,
vertically, or in some combination of the two.
Not all topography produces good slope lift.
Individual or isolated hills do not produce slope lift
because the wind tends to deflect around the hill,
rather than over it. A somewhat broader hill with a
windward face at least a mile or so long, might produce
some slope lift, but the lift will be confined to a
small area. The best ridges for slope soaring are at least
a few miles long.
Slope lift can extend to a maximum of two or three
times the ridge height. However, the pilot may only be
able to climb to ridge height. As a general rule, the
higher the ridge above the adjacent valley, the higher
the glider pilot can climb. Ridges only one or two hundred
feet high can produce slope lift. The problem with
very low ridges is maintaining safe maneuvering altitude,
as well as sufficient altitude to land safely in the
adjacent valley. Practically speaking, 500 to 1,000 feet
above the adjacent valley is a minimum ridge height.
[Figure 9-22]
In addition to a ridge being long and high enough, the
windward slope needs to be steep enough as well. An
ideal slope is on the order of 1 to 4. Shallower slopes
do not create a vertical wind component strong enough
to compensate for the glider’s sink rate. Very steep,
almost vertical slopes, on the other hand, may not be
Best Lift
Lift Zone
Figure 9-22. Slope soaring.
9-19
ideal either. Such slopes create slope lift, but can produce
turbulent eddies along the lower slope or anywhere
close to the ridge itself. In such cases, only the
upper part of the slope may produce updrafts, although
steeper slopes do allow a quick escape to the adjacent
valley. [Figure 9-23]
A ridge upstream can block the wind flow, so that no
low-level flow occurs upwind of an otherwise promising
ridge, and hence no updraft. Additionally, if
lee waves are produced by an upstream ridge or
mountain, slope lift can be enhanced or destroyed,
depending on the wavelength of the lee waves.
Locally, the downdraft from a thermal just upwind
of the ridge can cancel the slope lift for a short distance.
The bottom line: never assume slope lift is
present. Always have an alternative.
Just as the flow is deflected upward on the windward
side of a ridge, it is deflected downward on the lee side
of a ridge.[Figure 9-24] This downdraft can be alarmingly
strong—up to 2,000 fpm or more near a steep
ridge with strong winds (A). Even in moderate winds,
the downdraft near a ridge can be strong enough to
make penetration of the upwind side of the ridge
impossible. Flat-topped ridges also offer little refuge,
since sink and turbulence can combine to make an
upwind penetration impossible (B). Finally, an uneven
upwind slope, with ledges or “steps,” require extra
caution since small-scale eddies along with turbulence
and sink can form there (C).
Three-dimensional effects are important as well. For
instance, a ridge with cusps or bowls may produce better
lift in upwind-facing bowls if the wind is at an angle
from the ridge. However, sink may be encountered on
the lee side of the bowl. If crossing ridges in windy
conditions, always plan for heavy sink on the lee side
and make sure an alternative is available. [Figure 9-25]
Depending on the slope, wind speed should be 10-15
knots and blowing nearly perpendicular to the ridge.
Wind directions up to 30° or 40° from perpendicular
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Glider Flying Handbook(118)
