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wave system can form in completely dry conditions
with no clouds at all. If only lower level moisture
exists, only a cap cloud and rotor clouds may be seen
with no lennies above as in Figure 9-28(A). On other
days, only mid-level or upper-level lennies are seen
with no rotor clouds beneath them. When low and mid
levels are very moist, a deep rotor cloud may form,
with lennies right on top of the rotor cloud, with no
clear air between the two cloud forms. In wet climates,
the somewhat more moist air can advect in, such that
the gap between the cap cloud and primary rotor closes
completely, stranding the glider on top of the clouds
(B). Caution is required when soaring above clouds in
very moist conditions.
Suitable terrain is required for mountain wave soaring.
Even relatively low ridges of 1,000 feet or less vertical
relief can produce lee waves. Wave amplitude depends
partly on topography shape and size. The shape of
the lee slope, rather than the upwind slope is important.
Very shallow lee slopes are not conducive to
producing waves of sufficient amplitude to support a
glider. A resonance exists between the topography
width and lee wavelength that is difficult to predict.
One particular mountain height, width, and lee slope
is not optimum under all weather conditions.
Different wind and stability profiles favor different
topography profiles. Hence, there is no substitute for
experience at a particular soaring site when predicting
wave-soaring conditions. Uniform height of the
mountaintops along the range is also conducive to
better-organized waves.
The weather requirements for wave soaring include
sufficient wind and a proper stability profile. Wind
speed should be at least 15 to 20 knots at mountaintop
level with increasing winds above. The wind direction
should be within about 30° of perpendicular to the
ridge or mountain range. The requirement of a stable
layer near mountaintop level is more qualitative. A
sounding showing a DALR, or nearly so, near the
mountaintop would not likely produce lee waves even
with adequate winds. A well-defined inversion at or
near the mountaintop with less stable air above is best.
Weaker lee waves can form without much increase in
wind speed with height, but an actual decrease in wind
speed with height usually caps the wave at that level.
When winds decrease dramatically with height, for
instance, from 30 to 10 knots over two or three thousand
feet, turbulence is common at the top of the wave.
On some occasions, the flow at mountain level may be
sufficient for wave, but then begins to decrease with
altitude just above the mountain, leading to a phenomenon
called “rotor streaming.” In this case, the air
A
B
Figure 9-28. Small Foehn Gap under most conditions.
9-23
downstream of the mountain breaks up and becomes
turbulent, similar to rotor, with no lee waves above.
Lee waves experience diurnal effects, especially in
the spring, summer, and fall. Height of the topography
also influences diurnal effects. For smaller topography,
as morning leads to afternoon, and the air becomes
unstable to heights exceeding the wave-producing
topography, lee waves tend to disappear. On occasion,
the lee wave still exists but more height is needed to
reach the smooth wave lift. Toward evening as thermals
again die down and the air stabilizes, lee waves
may again form. During the cooler season, when the
air remains stable all day, lee waves are often present
all day, as long as the winds aloft continue. The daytime
dissipation of lee waves is not as notable for large
mountains. For instance, during the 1950s Sierra Wave
Project, it was found that the wave amplitude reached
a maximum in mid- to late afternoon, when convective
heating was a maximum. Rotor turbulence also
increased dramatically at that time.
Topography upwind of the wave-producing range can
also create problems, as illustrated in Figure 9-29. In
the first case (A), referred to as destructive interference,
the wavelength of the wave from the first range
is out of phase with the distance between the ranges.
Lee waves do not form downwind of the second range
despite winds and stability aloft being favorable. In the
second case (B), referred to as constructive interference,
the ranges are in phase, and the lee wave from
the second range has a larger amplitude than it might
otherwise.
Isolated small hills or conical mountains do not form
“classic” lee waves. In some cases, they do form waves
emanating at angle to the wind flow similar to water
　
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