unpredictable, and cloud-to-ground strikes are not limited to  areas below the cloud. Some strikes emanate from the side  of the Cb and travel horizontally for miles before turning  abruptly towards the ground. Inflight damage to gliders has  included burned control cables and blown-off canopies. In  some cases, strikes have caused little more than mild shock  and cosmetic damage. On the other extreme, a composite  training glider in Great Britain suffered a strike that caused  complete destruction of one wing; fortunately, both pilots  parachuted to safety. In that case, the glider was two or three  miles from the thunderstorm. Finally, ground launching,  especially with a metal cable, anywhere near a thunderstorm  should be avoided. Severe thunderstorms can sometimes spawn tornadoes, which  are rapidly spinning vortices, generally a few hundred to a few  thousand feet across. Winds can exceed 200 mph. Tornadoes  that do not reach the ground are called funnel clouds. By  definition, tornadoes form from severe thunderstorms and  should obviously be avoided on the ground or in the air. Slope or ridge soaring refers to using updrafts produced  by the mechanical lifting of air as it encounters the upwind  slope of a hill, ridge, or mountain. Slope soaring requires  two ingredients: elevated terrain and wind. Slope lift is the easiest lift source to visualize. When it  encounters topography, wind is deflected horizontally,  vertically, or in some combination of the two. Not all  topography produces good slope lift. Individual or isolated  hills do not produce slope lift because the wind tends to  deflect around the hill, rather than over it. A somewhat  broader hill with a windward face at least a mile or so long,  might produce some slope lift, but the lift is confined to a  small area. The best ridges for slope soaring are at least a  few miles long. Slope lift can extend to a maximum of two or three times  the ridge height. However, the pilot may be able to climb  only to ridge height. As a general rule, the higher the ridge  above the adjacent valley, the higher the glider pilot can  climb. Ridges only 100 or 200 feet high can produce slope  lift. The problem with very low ridges is maintaining safe  maneuvering altitude, as well as sufficient altitude to land  safely in the adjacent valley. Practically speaking, 500 to  1,000 feet above the adjacent valley is a minimum ridge  height.  [Figure 9-19] In addition to a ridge being long and high enough, the  windward slope needs to be steep enough as well. An ideal  slope is on the order of 1 to 4. Shallower slopes do not create  A ridge upstream can block the wind flow, so that no low- level flow occurs upwind of an otherwise promising ridge,  and no updraft. Additionally, if lee waves are produced by  an upstream ridge or mountain, slope lift can be enhanced  or destroyed, depending on the wavelength of the lee waves. Locally, the downdraft from a thermal just upwind of the  ridge can cancel the slope lift for a short distance. The  bottom line: never assume slope lift is present. Always have  an alternative. a vertical wind component strong enough to compensate for  the glider’s sink rate. Very steep, almost vertical slopes, on  the other hand, may not be ideal either. Such slopes create  slope lift, but can produce turbulent eddies along the lower  slope or anywhere close to the ridge itself. In such cases, only  the upper part of the slope may produce updrafts, although  steeper slopes do allow a quick escape to the adjacent valley.  [Figure 9-20] Figure 9-19.  Slope soaring. Figure 9-20.  Slope lift and eddy with near-vertical slope. Weather for Slope Soaring 9-14 Lift zone Best lift Eddy Lift zone