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Turbulence associated with lee waves deserves respect. Low- level turbulence can range from unpleasant to dangerous. Glider pilots refer to any turbulence under the smooth wave flow above as “rotor.” The nature of rotor turbulence varies from location to location as well as with different weather regimes. At times, rotor turbulence is widespread and fairly uniform; that is, it is equally rough everywhere below the smooth wave flow. At other times, uniformly moderate turbulence is found, with severe turbulence under wave crests. On occasion, no discernable turbulence is noted except for moderate or severe turbulence within a small-scale rotor under the wave crest. Typically, the worst turbulence is found on the leading edge of the primary rotor. Unfortunately, the type and intensity of rotor turbulence are difficult to predict. However, the general rule of thumb is that higher amplitude lee waves tend to have stronger rotor turbulence. 

Clouds associated with the mountain wave system are also indicated in Figure 9-25 . A cap cloud flowing over the mountain tends to dissipate as the air forced down the mountain slope warms and dries. The first (or primary) wave crest features a roll or rotor cloud with one or more lenticulars (or lennies, using glider slang) above. Wave harmonics farther downstream (secondary, tertiary, etc.) may also have lenticulars and/or rotor clouds. If the wave reaches high enough altitudes, lenticulars may form at cirrus levels as well. 

It is important to note that the presence of clouds depends on the amount of moisture at various levels. The entire mountain 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 lenticulars above, as in Figure 9-26A . On other days, only mid-level or upper- level lenticulars are seen with no rotor clouds beneath them. When low and mid levels are very moist, a deep rotor cloud may form, with lenticulars right on top of the rotor cloud, with no clear air between the two cloud forms. 

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In wet climates, the somewhat more moist air can advect (meaning to convey horizontally by advection) in, such 

that the gap between the cap cloud and primary rotor closes completely, stranding the glider on top of the clouds, as in Figure 9-26B . 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. Windspeed should be at least 15 to 20 knots at mountaintop level with increasing winds above. The wind direction should be within about 30° 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 windspeed with height, but an actual decrease in windspeed 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 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 

