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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] 

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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] 

## Wave Soaring 

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] 

