When the wind angle shifts more than 30 degrees off from perpendicular, the ridge lift becomes less reliable. As a result, we strongly recommend that beginners “stick to the formula”; wind speed 15-25 knots at ridge top, within 30 degrees of perpendicular for the section you are flying. In these conditions, you can generally expect the well-flown ridges to have “robust” ridge lift. This is to say that the ridge lift should be consistent and reliable; few factors can suppress or significantly affect the ridge band.
However, when the wind is outside the formula, this is not to say that the lift shuts off. Instead, there are many more complications that can affect how the ridge band works. Certain sections may work very nicely, while other sections are completely flat. The high part of the band may allow you to float at a moderate speed, whereas the low band may not be working.
With experience, it becomes possible to anticipate these factors. That said, it requires flying on many days, progressively exploring the ridge, and building up your margins. Intermediates may explore these days in the comfort and safety of the local ridge and experts may take advantage of these days going cross country.
While I do not advise beginners to fly on an off-angle ridge, beginners should nonetheless have a sense of what to expect if the wind angle shifts, for instance if the conditions unexpectedly change over the course of the day.
The more off-angle the wind is, the higher off the ridge you have to be.
As the angle shifts, the air becomes violent down on the treetops. There will be a turbulent wash as the wind angles over the mountain. By the time the angle is 45 degrees off, you typically have to be at least 300ft-400ft above the trees for the band to work well. Aside from being turbulent, the lift will typically be disorganized and weaker lower.
The more the angle shifts, the higher you must float along the ridge.
If the wind is on the weaker end of the spectrum (<20 knots) and/or the thermal activity is limited (think overcast above), the high band will get weaker.
If the wind is weak and/or the thermal activity is suppressed, watch out. This will weaken the high/float band. This is a really big problem! The lift band becomes compressed; it becomes only possible to float at best glide speed at 2000ft MSL or so and you cannot get higher or lower. It takes very little for this lift band to weaken to the point where it will no longer sustain you.
If the wind is weak/moderate and/or the thermals are not working well, do not count on an off-angle ridge.
Expect substantial sink when encountering thermals on the ridge.
Watch out for thermal suppression, especially when heading into a quartering headwind. Recognize that you will be approaching the thermals on a cross angle. This means you will need to traverse a long band of sink before you encounter the lift.
If the ridge band is not working well and the thermals are strong, you can easily get flushed down to ridge top. Moreover, this effect worsens as the wind angle increases. For example, if the wind angle on ridge top is 45 degrees off, but the thermal street above angles 55 degrees off at cloudbase, you may traverse 5-10 knot down sink over the course of a half mile! This amount of sink could be enough to knock a glider off a ridge, especially a low performance ship.
Watch your airspeed while down at ridge top.
When going upwind, your ground speed will be very low. However, when you turn downwind, your ground speed will be very high. When heading downwind, it is easy to let yourself get dangerously slow if you are not paying attention. The trees will be flashing by very quickly, giving you the illusion of flying a lot faster than you really are.
Certain ridges are tolerant of an off-angle wind; others are not.
Generally, ridges that are fairly high, straight, have few upwind obstructions, and few issues like gaps, bowls and spurs are more tolerant of an off-angle wind. Conversely, any of these issues will cause a ridge not to work in a given section if the angle is sufficiently off. A ridge with many issues may not work with a wind angle greater than 30 degrees off.
In summary, when the angle shifts beyond suggested tolerances (+- 30 degrees), the conditions become marginal. This makes flying in these conditions an expert endeavor. However, the lift does not necessarily go away. With practice and experience, you may still find a solid working ridge band. And if the conditions unexpectedly shift, you will know what to expect.
Soaring Economist 