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. In wet climates, the somewhat more moist air can advect  (meaning to convey horizontally by advection) in, such  Figure 9-26.  Small Foehn Gap under most conditions. 9-18 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  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. 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.  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. 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. A B