received. A thorough preflight and route planning needs to  be accomplished. This planning also includes ridge selection  based on the current winds. The horizontal distance from the  ridge varies with height above the ridge, since the best lift  zone, or optimum lift zones (OLZ) tilts upwind with height  above the ridge. These zones, or OLZ, vary but usually are  slightly off the top of the ridge, with a slight angle into the  prevailing wind. The bottom of the OLZ may be slightly down  from the top line under normal conditions. These OLZ vary  with the size and terrain makeup of the ridge.  [Figure 10-14] Exceptions to normal or typical thermals are numerous. For  instance, instead of stronger sink at the edge of a thermal,  weak lift sometimes continues for a distance after leaving a  thermal. Glider pilots should be quick to adapt to whatever the  air has to offer at the time. The mechanics of simply flying the  glider become second nature with practice, as do thermaling  techniques. Expect to land early because anticipated lift was  not there on occasion—it is part of the learning curve. If thermal waves are suspected, climb in the thermal near  cloud base, then head toward the upwind side of the Cu.  Often, only very weak lift, barely enough to climb at all, is  found in smooth air upwind of the cloud. Once above cloud  base and upwind of the Cu, climb rates of a few hundred fpm  can be found. Climbs can be made by flying back and forth  upwind of an individual Cu, or by flying along cloud streets if  they exist. If no clouds are present, but waves are suspected,  climb to the top of the thermal and penetrate upwind in search  of smooth, weak lift. Without visual clues, thermal waves  are more difficult to work. Thermal waves are most often  stumbled upon as a pleasant surprise. Efficient slope soaring (also called ridge soaring or ridge  running) is fairly easy; simply fly in the updraft along the  upwind side of the ridge. Although the appearance may seem  simple, it is very complicated and can be very hazardous for  the untrained glider pilot. Ridge soaring can also be very  demanding on the glider and the pilot. Even though it is  easy to fly, there are many situations in which a glider pilot  can be exposed to hazards if proper training has not been  Surface winds of 15–20 knots that are perpendicular to the  ridge are ideal. Wind flow within 45° of the perpendicular  line also provides adequate lift. Winds less than 10 knots  have also produced adequate ridge soaring dependent on  the terrain, but with 10 knots of wind or less, pilots should  avoid flying low over any ridge due to the possibility of  encountering sink. Local ridge pilots know about of these  conditions and the need for good preflight planning and  training is required.  [Figure 10-15] Even though the idea is simple, traps exist for both new and  expert glider pilots. Obtain instruction when first learning to  ridge soar/slope soar. Avoid approaching from the upwind  side perpendicularly to the ridge. Instead, approach the ridge  •  Airflow mirrors a hill or ridge shape. Imagine a flow of  water around the ridge instead of air. However, air is  thinner and can be compressed as in a “venturi effect”  and can be “squeezed” and accelerated, especially  along the ridge.  [Figure 10-16] •  Ridges that have an irregular profile are hazardous.  The more complicated the ridge is, the more erratic  the airflow may become.  [Figure 10-17] Ridge/Slope Soaring Atypical Thermals Figure 10-14.  Optimum lift zone of a ridge. Figure 10-13.  When thermaling, avoid flying in another glider’s  blind spot, or directly above or below another glider. Traps  $_{W}$ind  Optimum lift zones (OLZ) 10-10