I had an experience shooting a bull at about ~ 450 yds across a canyon in Co some years back that impressed upon me greatly how the wind even with the best of gear and info could really ruin your day.
I found a bull at 8:00 am, slowly feeding up, broadside on a trail in front of a vertical cliff. The bull was feeding toward a copse of trees at the top no doubt to bed. The wind was at 90 degrees to my anticipated shot, left to right, at about 20 mph; at least that's what the forecast predicted. It as about a -20 wind chill too.
I hesitated briefly because of the wind but had a Volkswagen Bug-sized Boulder right in front of me and laying over my pack on it it was as good as it ever gets in the field.
I had some model of Leupold scope at the time with the B&C reticle with the 10 and 20 mph wind hash marks on my 340 at the time and placed the 20 mph hashmark of the appropriate vertical on the bull's chest and squeezed off.
There was a loud crack and dust flew from the rock wall behind the bull as it seemed to turn a 180 and then I thought I saw legs in the air just as it was getting behind another boulder.
I was shooting a 210-gr TSX at about 3050 fps if memory serves. Upon climbing down and then back up the other side of the canyon I found the bull with the bullet entrance being in the distal part of the neck not that far below the lower jaw. It broke his neck and killed him instantly then hit the rock wall with enough force to knock a big chunk out.
But the question, why was the hit a good two feet to the left and in the neck, was answered by realizing the wind "up there" on the bull's side of the canyon was just as strong as my side but going in the opposite direction, something indeterminable from that side because he was essentially on a rock slide with no tale-telling vegetation whipping about. The wind on both sides of the canyon in this instance effectively canceled each other out.
As there is anything from 15-20+ inches of horizontal movement at 400 yds with a 90 degree, 20 mph wind, with a whole hat full of cartridges, let alone these other kinds of complicating factors in the mountains, I take each circumstance individually and decide if it's an ethical shot for me or not but won't shoot past 400 with more than a 20 mph wind on an elk sized animal and preferably much less.
I didn't note if there was sun out or not, but if so, focusing your optics for mirage checks, even though 20mph is to much to tell speed, would at least let me know if i had variable wind.
With something in the back ground you have to assume there is a strong possiblity of wind shift or rollover effect close to a solid object.
