Mountain lee wave and ridge lift are another two major lift sources exploited by glider pilots. These are somewhat related, in that both are the result of air movement against a physical feature in the landscape like a steep ridge line, the side of a section of a mountain, or the whole mountain itself.
In the case of ridge lift, a wind blowing against the side of the ridge is forced upwards, creating lift that a glider pilot can exploit. This lift can extend to heights of around five times the height of the ridge, depending on the strength of the wind. By flying along the ridge line, the glider pilot can travel many kilometres, depending on the length of the ridge, at high speeds and without losing any height.
Mountain lee waves are a different effect, formed when atmospheric conditions are right and a significant depth of the atmosphere moves as a giant air mass over a large feature like a mountain. It’s a bit like water moving over submerged stones in a fast moving creek, when stable ripples form in the water downstream of the obstruction. Like in the creek, stable ripples form in the air mass downstream of the mountain. In effect, the whole air mass trips over the mountain and bounces off downstream. By flying along the wave front like a surfer on a wave at the beach, the glider pilot can climb in the wave while travelling along the wave front. These waves can be very stable and extend to great heights.
In Western Australia, mountains are rare and the ones we do have would be called hills in most parts of the world. However, what we do have is a very flat landscape so when our mountains happen, they have a significant effect. In the south-west of WA we have the Stirling Ranges National Park. These are a one kilometre high, isolated ridge of uplifted sedimentary deposits that push up from the flatlands of the Great Southern, about 200km south of Narrogin. In the right conditions the lee wave they produce can take a glider to over 10,000m.
Unfortunately, the Stirlings are just a little bit too far south to be part of our regular flying. However, NGC holds a ridge and wave camp at the Stirlings in October every year. This usually turns into a bit of a family holiday, with visits to Albany on the south coast and an opportunity to do some flights with the bushwalkers and campers who turn up to enjoy the National Park at this time of year.
Wave can also be produced when, due to the weather patterns, two different air masses are flowing over each other in different directions and at different speeds. When this happens, friction between the two layers can produce stable waves called shear waves. Glider pilots can locate and use these to maintain height over large distances in exactly the same way as lee waves.