I've been a meteorologist for more than 35 years and in the last 15 years have specialized in microscale and mesoscale meteorology, especially doping the wind for wildland firefighters so they could stay out of the way of really large, fast-moving fires that are primarily moved by the wind.
Unless you're VERY skilled, figuring wind across a canyon is mostly a matter of luck. And even if you're very skilled, you won't get it right every time...sometimes not even most of the time.
Look at a small stream, how it moves quickly in the middle, slows near the sides and eddies where it actually makes contact with the bank. Then the eddies make contact with the slower water, causing turbulence, and the slower water creates friction with the faster water, which constantly moves the boundary between the areas of water moving at different speeds.
Now translate all of that into a vertical dimension rather than horizontal, switch the water for wind and you'll have SOME of the variability encountered in valley winds.
Kestrels are handy but they have their own limitations. Essentially, you have to accept that they are only measuring the wind they are exposed to - the wind at the height above the ground you're holding it, at the elevation on the hillside you're standing on, and only THAT hillside. It simply will not be the same in the center of the valley or on the other side of the valley.
Drawing a straight line across a valley to represent the path of a bullet, the center of the valley will generally have stronger winds because of less friction with the surface. Occasionally it will have much stronger winds because of the venturi effect that comes into play when wind is being funneled through a valley that is becoming narrower in the direction the wind is traveling.
If the valley is curving, you'll also get some rising motion on the outside edge, just like rushing water trying to make it around a tight bend in the river. We once had a nighttime backfire go exactly the wrong direction because down-valley winds picked up enough to push air up and over a cliffside that was directly above a bend in the river (the top of the cliff was where we lit our backfire).
Rising air will also play havoc with winds as the ground heats up in the afternoon. And since one side of the valley will almost always have different surface temps than the other side, influence on horizontally-moving wind will vary across the canyon, too.
Even when the wind is steady, friction with the surface will cause turbulence that builds steadily upward, turning your winds in multiple directions until it gets so far from the surface that it disengages - wherein the turbulence suddenly collapses and the wind becomes steady again. This steadiness only lasts until friction starts creating turbulence at ground-level again, building upwards from the surface in an ever-continuing cycle.
The motto of this story is simple: if you have still or very light winds, you might chance a long shot across a canyon and you will probably do well if you're a skilled shooter and if you don't wait long enough for conditions to change. But never assume the wind is doing anything you can count on - especially over long distances - and be ready for the consequences.
Last edited by czech1022; 04/21/16.
