We were gliding toward a big mountain in strong crosswind, expecting dependable ridge lift when we got there. A smaller hill lay a couple miles upwind, certain to undercut lift somewhere below, but we intended to remain safely above any of that.
Well below the crest we watched loaves of newly fallen snow tumble from pine boughs in random gusts and wander up-slope. Up on top, spindrift was wafting off the ridge in giant banners that diffused in the wind. Then … I did a blink-your-eyes double take. Exactly where we were headed, acres of wind-borne snow were going the wrong way, an airborne avalanche flowing rapidly down into a narrowing defile!
Entirely unseen but quickly becoming obvious, something was dumping big sink onto the windward flank of our mountain… We could actually see a descending curve bottom out in the flowing snow. Not what we were looking for, so we pulled a quick one-eighty and vamoosed.
What happened later I don’t remember, lots of normal stuff probably. But the question lingered, if that was rotor sink hundreds of feet higher than its source terrain far upwind, how’d it get there?
Recognition came ten years later via the ‘radical symbol’ for square root: √. No mathematical significance mind you, just the figure itself as if a prehistoric hieroglyph. Keep this shape in mind (only a whole lot bigger) as a geographical profile.
For those familiar with Crystal’s locale, we’re looking east through Vincent Gap. The little spike at left is Blue Ridge and Baden Powell is on the right, 1800 feet higher. Wind is 20+ from stage left, tumbling over the low peak, across the gap and up the bigger slope like a tide of lottery balls.
Here’s the thing. Each ball is an individual rotor tripped into motion by Blue Ridge, so from this perspective they’re all rolling clockwise, left to right. Therefore, even as all the rotors are pushed upslope, every one that contacts the hill does so with its down side first… Hence the airvalanche.
Without that flocking of loose snow on the pines we’d have flown straight into nasty sink and nowhere to go but down… One more reason, if you’re counting, to look out the front when you fly.