rotor clouds or lenticulars. If no clouds exist, special attention  is needed to judge wind drift by finding nearby ground  references. It may be necessary to increase speed with altitude  to maintain position in the best lift. Often the wind is strong,  but not quite strong enough for the glider to remain stationary  over the ground so that the glider slowly moves upwind out of  the best lift. If this occurs, turn slightly from a direct upwind  heading, drift slowly downwind into better lift, and turn back  into the wind before drifting too far.  [Figure 10-36] Often, the wave lift is not perfectly stationary over the ground  since small changes in windspeed and/or stability can alter the  wavelength of the lee wave within minutes. If lift begins to  decrease while climbing in the wave, one of these things has  occurred: the glider is nearing the top of the wave, the glider  has moved out of the best lift, or the wavelength of the lee  wave has changed. In any case, it is time to explore the area  for better lift, and it is best to search upwind first. Searching  upwind first allows the pilot to drift downwind back into the  up part of the wave if he or she is wrong. Searching downwind  first can make it difficult or impossible to contact the lift  again if sink on the downside of the wave is encountered. In  addition, caution is needed to avoid exceeding the glider’s  maneuvering speed or rough-air redline, since a penetration  from the down side of the wave may put the glider back in  the rotor.  [Figure 10-37] If the winds are moderate (20 to 40 knots), and the wave  extends along the ridge or mountain range for a few miles, it  is best to fly back and forth along the wave lift while crabbing  into the wind. This technique is similar to slope soaring,  using the rotor cloud or lenticular as a reference. All turns  should be into the wind to avoid being on the down side of  the wave or back into the rotor. Once again, it is easy to drift  downwind into sink while climbing higher, and searching  for better lift should be done upwind first. When making  an upwind turn to change course 180°, remember that the  heading change will be less, depending on the strength of  the wind. Note the crab angle needed to stay in lift on the  first leg, and assume that same crab angle after completing  the upwind turn. This prevents the glider from drifting too  far downwind upon completing the upwind turn. With no  cloud, ground references are used to maintain the proper crab  angle, and avoid drifting downwind out of the lift. While  climbing higher into stronger winds, it may become possible  to transition from crabbing back and forth to a stationary  upwind heading.  [Figure 10-38] Weaker winds (15 to 20 knots) sometimes require different  techniques. Lee waves from smaller ridges can form in  relatively weak winds of approximately 15 knots. Wave lift  from larger mountains rapidly decreases when climbing to a  height where winds aloft diminish. As long as the lift area is  big enough, use a technique similar to that used in moderate  winds. Near the wave top, there sometimes remains only  a small area that still provides lift. In order to attain the  maximum height, fly shorter figure 8 patterns within the  remaining lift. If the area of lift is so small that consistent  climb is not possible, fly a series of circles with an occasional  leg into the wind to avoid drifting too far downwind. Another  possibility is an oval-shaped pattern—fly straight into the wind  in lift and, as it diminishes, fly a quick 360° turn to reposition.  These last two techniques do not work as well in moderate  winds, and not at all in strong winds since it is too easy to  be downwind of the lift and into heavy sink.  [Figure 10-39] 10-21 Figure 10-37.  Search upwind first to avoid sink behind the wave  crest or the rotor. Figure 10-36.  Catching a thermal by flying upwind away from  the slope lift. Wind Search upwind first Turbulent rotor Strong wind Glider speed less  than windspeed Glider speed  equals windspeed Glider speed greater than windspeed Wave lift Wave crest Wave sink