IN REVERSE?

Unlike common wave downwind of high ground, the phenomenon we call bow wave occurs upwind where the whole atmosphere seems to pile up before surging over the crest.  Imagine a thick phone book flat on the floor against a wall. Push its outer edge toward the wall and the book will buckle upward. (Also, under the bend, beneath the wave crest, may churn strong rotor.) Bow wave is usually not strong or high enough to form clouds, but every year or so we do see as many as three lenticulars arrayed in series over flat ground upwind of mountains, like ordinary harmonics in reverse.One important distinction, while common waves remain relatively stationary, bow waves normally migrate. Beginning far upwind and growing as they drift toward the mountains, they finally collapse over the crest in a manner similar to surf.  (If our wall were no taller than the phone book’s thickness and we continued pushing, eventually the book would flop over the wall.) When this happens, a sailplane low in ridge lift may suddenly encounter rotor sink, or if higher up, wave sink — all flowing DOWN onto the windward slope. The only thing to do then is dive straight upwind, away from the hill and toward the next arriving lift. If the wind is moderate this should be easy, but stronger winds raise a peculiar safety problem.Dismiss the hill for a moment. Say you’re gliding at fifty knots into a twenty-knot headwind, ground speed thirty. Say your bird is old-school, designed to fly straight down with full spoilers, so you open ‘em up and nose over. Ground speed will increase of course, but only briefly. Keep nosing over and soon it will approach zero… because of that ambient wind. Then at full vertical your ground speed will amount to negative twenty!Does all this sound too hypothetical? Think again.Back to the hill, we were up on the main headwall of our local skyline where the top is narrow and very steep. Not far upwind of the crest and maybe five hundred feet above it, forty-seven knots indicated gave us zero sink and zero ground speed. That’s a dead nuts hover at 9000 MSL with 55 knots true airspeed, converted to statute, an ambient wind of 63 MPH!It was perfectly smooth, and quiet too at that speed, but knowing what lurked behind, it felt like a motorboat idling upstream at the very cusp of a waterfall. Then the sink of an advancing rotor grabbed us, meaning time to dive, and learn something I had never heard nor thought of.Several appalling seconds I held full forward stick waiting to gain airspeed — as we accelerated backward over the ridge! No joke.After a purgatory of free fall we’d accumulated the energy to level a bit, restore some forward progress and slam back across the ridge at safe height. But Holy Mo! If we’d responded any later or less aggressively we could have been sucked over and permanently down into horrors fearsome to contemplate.So even if you intend to never fly near such terrain, here’s a nugget that could save lives. Unless you’re intentionally running downwind, always carry at least a little more airspeed than the ambient wind so you can dive into it when necessary without actually going the wrong direction!