Figure 10-29.  Deep shade is where colder air goes down. Figure 10-28.  The crest is where rising air starts back down. Thermal source can be found where drainages area along the  ridge meet. Where the ridge or peaks are numerous and slope  down into a valley, thermal sources may be found. Canyons  or large bowls hold areas of warm air. Ridges may form cloud streets above the ridge. A glider pilot  should try to climb in a thermal to reach these streets. Fast  cruising speeds can be found under these streets. The glider  pilot must stay upwind to stay in this type of lift near the ridge. Since gliders tend to seek the same conditions for lift,  thermals and areas of ridge lift are areas for special diligence  for avoiding other aircraft. To some extent, for the same  reasons that gliders seek rising air, other low flying airplanes  and helicopters will plan to use that area for flights, so be  aware for all aircraft. A portable VHF radio and sharing  a common channel for traffic calls is an enhancement for  safety, but nothing replaces a good visual scan at all times  looking for other aircraft.  Density altitude is increasing as the glider climbs. At 10,000  feet mean sea level (MSL), a pilot is required to have  approximately 40 to 45 percent more room to maneuver a  glider. The air is less dense by 2 percent per one thousand  feet of altitude gained.  [Figure 10-30] Almost all high-altitude flights are made using mountain lee  waves. As covered in Chapter 9, Soaring Weather, lee wave  systems can contain tremendous turbulence in the rotor, while  the wave flow itself is usually unbelievably smooth. In more  recent years, the use of lee waves for cross-country soaring  has led to flights exceeding 1,500 miles, with average speeds  of over 100 mph.  [Figure 10-31] can be found on the shady or dark side of the ridge. This  is true when the sun angle is low or late in the afternoon.  [Figure 10-29] Wave Soaring Figure 10-27.  Expect turbulence and sink in the downwind of any  hill. 10-16